Tweede Kamer der Staten-Generaal

Tweede Kamer der Staten-Generaal

2

Vergaderjaar 2012–2013

33 529

Gaswinning Groningen-veld

Nr. 1

BRIEF VAN DE MINISTER VAN ECONOMISCHE ZAKEN Aan de Voorzitter van de Tweede Kamer der Staten-Generaal Den Haag, 25 januari 2013 Hierbij informeer ik uw Kamer over nieuwe inzichten met betrekking tot de effecten van gaswinning uit het Groningen-veld en de relatie met aardbevingen in de provincie Groningen. In deze brief ga ik in op de mogelijke gevolgen van deze inzichten voor de bewoners van het betreffende gebied en voor de Nederlandse samenleving als geheel. Tevens geef ik aan welke maatregelen ik in dit verband neem. 1. Aanleiding Op 16 augustus 2012 heeft bij het dorp Huizinge in de Groningse gemeente Loppersum een aardbeving plaatsgevonden. Deze beving – met een sterkte van 3,4 op de schaal van Richter – duurde langer en had een grotere energie-intensiteit dan voorgaande bevingen. De inwoners van Huizinge en omgeving hebben dit ook als zodanig ervaren. Het KNMI concludeerde in zijn analyse dat deze beving de krachtigste als gevolg van gaswinning in Groningen tot nu toe was. Ook het aantal meldingen over veroorzaakte schade was groter dan voorheen. Tot nu toe zijn er ruim 2500 schademeldingen bij de Nederlandse Aardolie Maatschappij (hierna: NAM), de exploitant van het Groningen-veld, binnengekomen. De beving op 16 augustus 2012 was aanleiding voor het Staatstoezicht op de Mijnen (hierna: SodM) om het verschijnsel aardbevingen als gevolg van de gaswinning in het Groningen-veld nader te onderzoeken. Vervolgens hebben ook het KNMI en de NAM onderzoeken uitgevoerd. Vanwege de complexiteit van de materie heeft in de afgelopen maanden intensief overleg plaatsgevonden tussen SodM, het KNMI en de NAM om de beschikbare kennis informatie en kennis te delen en te toetsen. 2. Bevindingen van de onderzoeken De onderzoeken hebben geresulteerd in een rapport van het KNMI, een rapport van het SodM, een brief van de NAM aan het SodM met voorgestelde maatregelen en op basis daarvan een advies van het SodM aan mij.

kst-33529-1 ISSN 0921 - 7371 ’s-Gravenhage 2013

Tweede Kamer, vergaderjaar 2012–2013, 33 529, nr. 1

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Deze documenten zijn als bijlagen bij deze brief gevoegd.1 De onderzoeken en bevindingen hebben uitsluitend betrekking op het Groningen-veld en niet op andere gasvelden in Nederland. De partijen die deze onderzoeken hebben verricht, hebben hierin ieder hun eigen rol en verantwoordelijkheden. Het SodM heeft als toezichthouder op mijnbouwactiviteiten, zoals gaswinning, de verantwoordelijkheid er op toe te zien dat dit op een veilige manier gebeurt voor mens en leefomgeving. Indien daartoe aanleiding bestaat, kan het SodM onderzoeken (doen) uitvoeren, bestuurlijke boetes opleggen dan wel mij adviseren over te treffen maatregelen. Het KNMI is de autoriteit op het gebied van seismologie en (onderzoek naar) aardbevingen, ongeacht waardoor deze worden veroorzaakt, natuurlijke activiteiten, gaswinning of andere oorzaken. De NAM dient als exploitant van het Groningen-veld de gaswinning uit te voeren conform wet- en regelgeving en conform de voorwaarden zoals neergelegd in de winningsvergunning en het bijbehorende winningsplan. Als minister van Economische Zaken ben ik verantwoordelijk voor beleid en regelgeving ten aanzien van gaswinning en tevens het bevoegd gezag voor de vergunningverlening voor mijnbouwactiviteiten, waaronder gaswinning. De belangrijkste bevindingen uit de onderzoeksrapporten zijn de volgende: • De jaarlijkse productie uit het Groningen-veld is sinds 2000 stapsgewijs toegenomen van 20 tot 30 miljard m3 naar 45 tot 50 miljard m3. Oorzaak is de in dezelfde periode teruggelopen productie uit de kleine velden van 50 miljard m3 naar 25 tot 30 miljard m3, welke is opgevangen door een verhoogde productie uit het Groningen-veld. Het afgelopen decennium is evenredig met de toenemende productie het aantal aardbevingen per jaar en daarmee ook het aantal krachtige aardbevingen in het Groningen-veld toegenomen. • Het is al langer bekend dat er een relatie bestaat tussen de gaswinning en de aardbevingen. Het KNMI ging er op basis van statistisch onderzoek van alle Nederlandse gasvelden steeds van uit dat bij aardbevingen als gevolg van gaswinning de maximale sterkte 3,9 op de schaal van Richter zou zijn. Wat betreft het Groningen-veld geeft het KNMI nu aan dat de maximaal mogelijke sterkte niet te schatten is op basis van historische gegevens van de aardbevingen in dit veld. Die sterkte kan dus ook hoger zijn. • Op basis van gerapporteerde aardbevingen bij gasvelden elders in de wereld verwacht het KNMI dat de maximale sterkte ergens tussen de 4 en 5 zal liggen. Geomechanisch en seismologisch onderzoek in het Groningen-veld kan uitwijzen met welke precieze maximale sterkte rekening gehouden dient te worden. • Wanneer uitgegaan wordt van een maximale sterkte van 5 op de schaal van Richter, is de komende twaalf maanden de kans op een beving van 3,9 of hoger naar verwachting 7 procent, aldus het SodM. Anders gezegd: er is dan een kans van één op veertien dat in deze periode zo’n beving zal plaatsvinden. 3. Maatregelen ten aanzien van het Groningen-veld Ik vind de nieuwe inzichten van het SodM, het KNMI en de NAM betekenisvol en neem deze serieus. Het is al enkele decennia duidelijk dat de winning van aardgas uit het Groningen-veld gepaard gaat met aardbevingen, maar het is nieuw dat rekening gehouden moet worden met de effecten van aardbevingen met een sterkte hoger dan 3,9 op de schaal van Richter. 1

Ter inzage gelegd bij het Centraal Informatiepunt Tweede Kamer


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Ik ben mij er terdege van bewust dat deze nieuwe inzichten leiden tot onzekerheid bij de mensen die in de nabijheid van het Groningen-veld wonen en met name in de gemeente Loppersum, waar zich de afgelopen jaren de sterkste aardbevingen voordeden. Daarom zijn twee soorten maatregelen overwogen: • Maatregelen gericht op het zoveel mogelijk voorkomen en beperken van schade. • Maatregelen die leiden tot het verminderen van de sterkte van de aardbevingen. 3.1 Voorkomen en beperken van schade Bewoners in het gebied van het Groningen-veld leven in de wetenschap dat zich aardbevingen als gevolg van gaswinning kunnen voordoen2. Nieuw is echter dat er nu mogelijk aardbevingen met een grotere sterkte dan eerder voorzien kunnen plaatsvinden. Het is van groot belang dat de effecten van eventueel sterkere bevingen voor bewoners in het gebied van het Groningen-veld zoveel mogelijk voorkomen en beperkt worden. Wanneer zich desondanks schade voordoet, moeten zij er op kunnen rekenen dat deze vergoed wordt. De NAM heeft in haar brief aan het SodM aangegeven de volgende maatregelen te zullen treffen: • De NAM zal in samenwerking met o.a. TNO de bewoners en huiseigenaren in Groningen assisteren bij het inschatten van de kwetsbaarheid van gebouwen bij aardbevingen met een hogere sterkte. • Wanneer de veiligheid van gebouwen ter discussie staat, zal de NAM specialistische kennis beschikbaar stellen en bijdragen aan eventueel noodzakelijke preventieve reparaties of versterkingen. Dit alles zal geschieden op basis van redelijkheid en nader uit te werken criteria. • Daarnaast zal de NAM in samenwerking met andere partijen, waaronder de veiligheidsregio, gerichte voorlichting geven over hoe te handelen in geval van een aardbeving. Het effect van deze maatregelen is dat preventief zwakke constructies worden geïnventariseerd en verstevigd, waardoor de schadelijke gevolgen van aardbevingen worden verminderd, en de veiligheid voor de inwoners wordt vergroot. Daarnaast worden de burgers beter geïnformeerd, zodat ze beter zijn voorbereid in geval zich een aardbeving voordoet. Ik onderschrijf deze maatregelen en heb vastgesteld dat het SodM al heeft aangegeven op de uitvoering te zullen toezien. In overleg met mij heeft de NAM alvast een bedrag van € 100 miljoen apart gezet om preventieve maatregelen aan gebouwen te treffen en zo schade bij bevingen te voorkomen of te beperken. Dit is een aanvulling op de reeds bestaande schadevergoedingsregelingen van de NAM in het geval van bevingen. 3.2 Verminderen van de sterkte van de aardbevingen Het SodM geeft in zijn advies aan hoe het aantal verwachte aardbevingen en daarmee ook het aantal sterkere aardbevingen verminderd kan worden. Specifiek adviseert het SodM mij daarom om de NAM er toe te bewegen «de gasproductie uit het Groningse gasveld zo snel mogelijk en zo veel als mogelijk en realistisch is, terug te brengen». Mijn besluitvorming over dit advies behoeft een zorgvuldige afweging van de diverse belangen en dient op zo volledig mogelijke informatie gebaseerd te zijn. 2

Ter illustratie is in de bijlage een kaartje opgenomen waarin de aardbevingen in het gebied rond het Groningen-veld in de periode 1996–2012 staan vermeld. Ter inzage gelegd bij het Centraal Informatiepunt Tweede Kamer


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Beschikbaarheid van Groningen-gas is van groot belang voor zowel Nederlandse als buitenlandse eindafnemers. In Nederland wordt dit gas gebruikt door bijna alle huishoudens, instellingen en bedrijven. Maar niet alleen Nederland is er van afhankelijk. Het Groningen-gas wordt ook gebruikt in Duitsland, België en Frankrijk. Vorig jaar werd uit het Groningen-veld 47 miljard m3 gas gewonnen, waarvan ongeveer de helft is afgezet in Nederland en de andere helft in het buitenland. Complicatie hierbij is dat het Groningen-gas laagcalorisch gas is, in tegenstelling tot het gas uit de kleine velden in ons land en het gas uit Noorwegen en Rusland. Verwarmingsketels en fornuizen die geschikt zijn voor laagcalorisch gas, kunnen niet overweg met hoogcalorisch gas en omgekeerd. Gezien de beschikbare faciliteiten om stikstof toe te voegen aan hoogcalorisch gas kan dit gas slechts in beperkte mate worden omgezet in laagcalorisch gas. Het Groningen-veld heeft daarmee een unieke plaats in de Noordwest-Europese gasvoorziening. De Groningse gaswinning kan niet binnen afzienbare tijd vervangen worden door gasimport of door andere maatregelen. Een verminderde beschikbaarheid van Groningen-gas heeft ernstige gevolgen voor de Nederlandse samenleving en voor de samenlevingen in de ons omringende landen. De verkoop van Groningen-gas heeft ook gevolgen voor de Rijksbegroting. Een vermindering van de jaarlijkse productie met 10 miljard m3 (ongeveer 20 procent) leidt bij de huidige gasprijs tot een tegenvaller op de Rijksbegroting van € 2,2 miljard per jaar, inclusief de vennootschapsbelasting. Zoals in paragraaf 2 beschreven komt hier nog bij dat geen volledig inzicht bestaat in de maximale sterkte van toekomstige aardbevingen in het Groningen-veld. Al met al maakt dit een besluit nu over beperking van de productie niet verantwoord. Eerst is nadere informatie nodig. Geomechanisch en seismologisch onderzoek zal moeten uitwijzen met welke maximale sterkte van aardbevingen in het Groningen-veld rekening dient te worden gehouden. De NAM is al met deze onderzoeken gestart en zij zullen versneld worden uitgevoerd. Gedurende de onderzoeken zullen onafhankelijke deskundigen deze onderzoeken nauwgezet volgen. Ook de resultaten zullen door onafhankelijke deskundigen worden beoordeeld. Ten behoeve van de onderzoeken zal het bestaande meetnet worden uitgebreid, waardoor meer en nauwkeuriger gegevens beschikbaar komen. De onderzoeksresultaten worden betrokken bij het opstellen van een gewijzigd winningsplan, dat de NAM op 1 december 2013 zal indienen en dat vervolgens door het SodM zal worden beoordeeld. De tijd tot 1 december 2013 kan tevens benut worden om te onderzoeken of het mogelijk is bij gelijkblijvende productie met alternatieve winningstechnieken het aantal en de maximale sterkte van aardbevingen te beperken. Ook voor dit onderzoek geldt dat de NAM het inmiddels in gang heeft gezet en versneld zal uitvoeren. Ik zal het SodM vragen om mij op basis van de resultaten van deze onderzoeken te adviseren over het gewijzigde winningsplan, waarna ik zal bezien of nadere maatregelen ten aanzien van de productie noodzakelijk zijn. 4. Conclusie De studies van het SodM, de NAM en het KNMI maken duidelijk dat de aanname dat de maximale sterkte van aardbevingen door gaswinning in het Groningen-veld 3,9 op de schaal van Richter zou zijn, niet langer houdbaar is. De kans bestaat dat er bevingen optreden met een sterkte hoger dan 3,9. Met welke sterkte nu rekening gehouden dient te worden


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moet blijken uit onderzoek. Om de effecten van sterkere bevingen voor bewoners te beperken zullen passende maatregelen worden genomen. Om tot een afgewogen besluit te kunnen komen over eventuele maatregelen om de kans op dergelijke bevingen te verminderen zullen nadere onderzoeken worden uitgevoerd. Ik zie erop toe dat de NAM de geschetste maatregelen neemt en de onderzoeken uitvoert c.q. laat uitvoeren. Mede op grond van de onderzoeksresultaten moet duidelijk worden of en zo ja welke aanvullende maatregelen mogelijk en wenselijk zijn. Ik zal uw Kamer van de voortgang op de hoogte houden. In dit kader bied ik ook aan om op korte termijn een technische briefing te laten verzorgen, waarbij medewerkers van het Ministerie van Economische Zaken, het SodM, de NAM en het KNMI aanwezig zullen zijn om vragen te beantwoorden. De minister van Economische Zaken, H.G.J. Kamp


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The August 16, 2012 earthquake near Huizinge (Groningen)

Bernard Dost and Dirk Kraaijpoel

KNMI, De Bilt January 2013

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Samenvatting In dit rapport worden de resultaten gepresenteerd van onderzoek door het KNMI naar de aardbeving van 16 augustus 2012 bij Huizinge, gemeente Loppersum, in de provincie Groningen. De locatie van de aardbeving is berekend met behulp van een lokaal snelheidsmodel van de ondergrond en lokale acceleratie data. Resultaat is een verplaatsing van ca 0.5 km naar het westen ten opzicht van de eerste analyse. De sterkte van de beving is geanalyseerd door de moment magnitude te berekenen. Deze komt uit op Mw= 3.6 met een onzekerheid van 0.1 magnitude units. De berekende lokale magnitude is 3.4 ± 0.1. De relatie tussen ML en Mw wordt nader onderzocht en kan leiden tot een bijstelling van de procedures voor de bepaling van de ML. In de verdere analyse wordt uitgegaan van een magnitude 3.6 voor dit event. Berekeningen geven aan dat de bron een gemiddelde beweging van 5 ± 3 cm omvatte langs een cirkelvormig breukoppervlak met een straal van 390 ± 110 m en een spanningsval van 25 ± 9 bar. Deze bepalingen gaan uit van het Brune model. Het mechanisme van de breuk is niet eenduidig te bepalen uit de polariteit van de geregistreerde golven. Onderzoek met behulp van golfvorminversie zal naar verwachting meer duidelijkheid geven. Meerdere S-golven zijn bij deze beving geregistreerd, hetgeen de duur van de sterk gevoelde beweging verlengd heeft. Dit is ook gemeld door de lokale bevolking. Data van het accelerometer netwerk in het Groningen veld heeft maximale versnellingen (PGA) gemeten tot een maximum van 85 cm/s2, of daarvan afgeleid 3.45 cm/s als maximale snelheid (PGV). Vergelijking met voor geïnduceerde bevingen afgeleide relaties tussen de kans op schade en de snelheid van de bodembeweging laat zien dat bij deze waarden een kans van 20-35% op schade bestaat. De gemeten PGA en PGV worden goed voorspeld door bestaande Ground Motion Prediction Equations (GMPE’s) die afgeleid zijn voor zwakke en ondiepe aardbevingen. Maximum intensiteit VI is berekend voor een beperkt gebied (< 4 km) rond het macroseismisch epicentrum, dat ca 2 km NE van het instrumentele epicentrum is gelegen. De toename van het aantal bevingen in het Groningen veld (190 events met een ML ≥ 1.5) maakt een update van de hazard berekeningen mogelijk. Gebaseerd op een breuk in de trend van de cumulatieve energie rond 2003, wordt de dataset opgedeeld in twee tijdsintervallen: 1991-2003 en 2003-2012. Analyse laat zien dat de frequentie-magnitude relatie in beide segmenten bepaald wordt door dezelfde Gutenberg-Richter (GR) b-waarde, maar een verschillende waarde voor de jaarlijkse hoeveelheid aardbevingen, de GR a-waarde. De hoeveelheid bevingen neemt toe en dit verschijnsel lijkt te correleren met de toegenomen productie. Het is niet mogelijk gebleken de maximaal mogelijke magnitude voor aardbevingen in het Groningen veld te schatten op basis van de statistiek. Dit wordt veroorzaakt door de specifieke vorm van de frequentiemagnitude relatie voor het veld en is mogelijk beïnvloed door de niet stationariteit van het proces. Verdere studies, waarbij geologische data en geomechanische modellen gebruikt worden, kunnen mogelijk extra informatie geven. Tenslotte is een vergelijking gemaakt met gas- en olievelden buiten Nederland en de daarin opgetreden geïnduceerde events. Maximale sterktes van bevingen, zoals in de literatuur vermeld, varieren van M= 4.2 tot 4.8. Hieruit wordt de conclusie getrokken dat niet verwacht wordt dat de maximaal mogelijke magnitude groter dan 5 zal worden. Maximale intensiteiten die behoren bij een ondiepe aardbeving met magnitude 4-5, zullen waarschijnlijk in de VI-VII range liggen.

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Summary This report presents the results of the analysis of the August 16, 2012 earthquake near Huizinge in the province of Groningen. The location of the event is refined by using a local velocity model and using additional data from the local acceleration network. The new location is shifted ca 500 m west of the original location. A moment magnitude was calculated and determined at Mw=3.6 ± 0.1, higher than the originally determined local magnitude of ML= 3.4 ± 0.1. The relation between ML and Mw is being investigated and a possible recalibration of the local magnitude may be the result. In the further analysis a magnitude 3.6 is used for this event. The source radius is estimated to be 390 m ± 110m, the stress-drop 25 bar ± 9 bar and the average displacement 5 cm ± 3 cm, all based on the assumption of a Brune model. No stable solution could be found for the mechanism of the event based on polarity data. Further research using waveform modeling techniques is expected to provide results. Multiple S-wave phases have been recorded, extending the duration of the strongest movement. This is in line with reports from the local people. The regional accelerometer network recorded peak ground acceleration (PGA) values up to 85 cm/s2, corresponding to a maximum peak ground velocity (PGV) of 3.45 cm/s. Comparison with damage probability curves for masonry structures, designed for induced seismicity, show a 20-35% probability of damage at these values. Recorded PGA and PGV values are well predicted by existing Ground Motion Prediction Equations, derived for small and shallow earthquakes. Maximum intensity of VI was detected in a limited region (< 4km) around the macroseismic epicenter, located ca 2 km NE of the instrumental epicenter. The extended dataset for the Groningen field ( 190 events of M≥ 1.5 ) allows an update of the hazard analysis. Based on cumulative energy trends, the dataset for Groningen was divided into two segments (1991-2003) and (2003-2012). Analysis shows that the frequency-magnitude relations are characterized by a similar value of the Gutenberg-Richter (GR) b-value, but do have a different seismicity rate. Seismicity rate increases with time and seems to coincide with increased production. The maximum probable magnitude, estimated only for all fields combined in the past, could not be estimated on the basis of the statistics alone for the Groningen field, due to the nature of the frequency-magnitude relation. Further study using geological information and geomechanical modeling may provide additional information. Based on a comparison with seismicity in hydrocarbon fields outside the Netherlands, where induced events were recorded up to a maximum of 4.2 < M < 4.8, we conclude that it is not expected that the maximum probable magnitude will exceed a magnitude 5. Intensities associated with a magnitude 4-5 event are expected to be in the VI-VII range.

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Contents Samenvatting Summary Introduction Data Location Magnitude Source mechanism and parameters Peak ground acceleration (PGA) and peak ground velocity (PGV) Damage and PGV Source duration Intensity Implication for hazard analysis Cumulative energy Frequency-magnitude relation Maximum magnitude

Conclusions References

2 3 5 5 6 8 10 13 15 16 17 19 19 20 21 24 25

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Introduction On August 16, 2012 an induced earthquake occurred in the north of the Netherlands near the village of Huizinge in the municipality of Loppersum. We consider the event being induced due to gas exploration from the Groningen field. The magnitude of the event was ML=3.4, calculated using data from the KNMI regional borehole network [1]. The strength of the earthquake is within its uncertainty comparable to the largest event in the region until present, but its effects at the surface were felt more strongly by the population. More than 2000 damage reports have been received by the company responsible for the gas production (NAM). In this document we present first results of a detailed analysis of the Huizinge event and discuss its impact with respect to the hazard analysis.

Data The Huizinge event was recorded by the regional KNMI borehole network, the regional accelerometer network and all additional seismic stations in the south of the Netherlands. European seismic stations reported the event at epicentral distances up to 800 km. Digital seismological data is in general freely available from global and regional networks and organizations are able to build their own virtual networks. The Orfeus Data Center, hosted at the KNMI, plays a key coordinating role in facilitating this European open data exchange. Data from the KNMI borehole network is available in real-time and feeds into automated location systems (Seiscomp). These systems are in development and the quality of automated locations are being assessed. Data from the accelerometer network are currently only available off-line. Recently a project started to update and expand the accelerometer network in the region and integrate the datastream into the real-time system.

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Location The event was rapidly identified by both the KNMI and major European data centers. However, rapid automatic locations were made publicly available by these centers using openly available real-time data. Notably these are the Geofon data center (http://geofon.gfz-potsdam.de/ ) and the European Mediterranean Seismological Center (EMSC) (http://www.emsc-csem.org ). Initial automatic locations and magnitude estimates have been later corrected by human interference. Those manual locations are shown in Figure 2.

Figure 2. Location of the Huizinge earthquake. The KNMI locations (1- regional model; 2- local model), Geofon location (3) and the EMSC location (4). Gas field are shown in light green, earthquakes as yellow circles, borehole stations by blue inverted triangles and accelerometers by blue squares. Faults at top of the reservoir are indicated by solid lines (data courtesy of NAM). KNMI location: Using a regional velocity model and data from the regional borehole network, the KNMI calculated the epicenter at: 53.353 N 6.665 E (in the national coordinate system: X: 240.017 and Y: 596.911). In the source inversion depth has been fixed at 3 km, which is the average depth of the gas fields in the region. Unfortunately, borehole station ENM, located to the North-west was not functioning at the time of the event. An improved epicenter location was obtained using a local model and including acceleration data from a network of 8 stations that are located within an epicenter distance of 2-10 km. This local model includes a high velocity (salt) layer and does not include a fixed Vp/Vs ratio, but applies the Castagna [14] relation to connect Vp to Vs velocity [2]. 6

This epicenter solution lies ca 500 m west of the epicenter obtained using the regional model without the acceleration data. Its parameters are: Origin time: 2012 08 16 20:30:33.32

Lat: 53.3547 Lon: 6.6571 Depth: 3 km X: 239.519 Y: 597.095

This epicenter location and depth estimate is our preferred hypocenter solution. It is located in the seismically most active part of the Groningen field and seems to be connected to a NW-SE trending fault on top of the reservoir (Figure 2). Accuracy of the location is estimated at 0.5 km in horizontal distance. Geofon location The delayed (manual) location of the Geofon center is: 53.38 N 6.53 E, with a depth at 10 km and a magnitude MLv= 3.9. Geofon uses data from a world wide network with a dense coverage in Europe. Station separation is at least 50-100 km and combined with an average earth model limits the accuracy of the location. EMSC location The delayed (manual) EMSC location is: 53.36 N 6.48E, with a depth of 10 km and a magnitude ML= 3.7. The EMSC uses mostly the same data as Geofon, but obtains additional data picks from regional networks. Both Geofon and EMSC used a standard depth of 10 km, indicating that the source is shallow. The dataset does not allow a more refined depth estimation. The location of both organizations is around 10 km west of the KNMI location.

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Magnitude For all induced events in the region, the determination of the local magnitude (ML) at the KNMI is part of the standard processing. For larger events in the Groningen field (ML>2.5) also moment magnitudes (Mw) have been determined. Within their error bounds both solutions (ML and Mw) are similar [1]. Evaluation and comparison of both types of magnitudes for the Huizinge event provides us with unique data to possibly recalibrate the local magnitude, a robust, simple and fast magnitude estimate, in relation to the more elaborate but generally more accurate, moment magnitude. Local magnitude (ML) In the standard KNMI analysis the local magnitude is determined, using the KNMI borehole network stations. The local magnitude is defined as ML=log Awa – log A0, where Awa is the maximum amplitude in mm recorded on a simulated Wood-Anderson seismograph and logA0 is the attenuation function. For the north of the Netherlands an attenuation relation was derived from measurements at borehole sensors at 200m depth [3]: log

1.33 log

0.00139

0.424

where r is the hypocenter distance in km. For each station Awa is measured as the average of the two horizontal components. An average of the 8 most reliable borehole data at a hypocenter distance < 50 km gave a value for the Huizinge event of ML= 3.4 ± 0.1.

Moment magnitude (Mw) The moment magnitude is defined as Mw= (log(M0)-9.1)/1.5 [5], where seismic moment M0 is estimated from the displacement spectra of the data. For a vertically heterogeneous model [6]: 4

/

/

/ ,

/

Ω

where is the density, V is the P- or S-wave velocity and the subscripts 0 and z indicate the values at the receiver and at source depth respectively. R is the hypocenter distance, Fs accounts for the freesurface amplification (Fs=2) and RΘ,Φ is the average radiation pattern coefficient (0.52 for P-waves and 0.63 for S-waves) and Ω0 is the low frequency spectral level. Kraaijpoel and Dost [2] determined source parameters for 4 earlier events in the Groningen field and showed a good correspondence between ML and Mw, using a shear velocity V0 = 700 m/s and a Vz= 2200 m/s at a source depth of 3 km. Densities are = 1960 kg m-3 and =2600 kg m-3. The raw data files are corrected for instrument response and absorption/scattering. The latter correction is of the form , with R hypocenter distance, Q the quality factor describing regional anelastic attenuation, the shear velocity, and a measure of the high frequency decay slope [7]. Q was calculated for previous events in the Groningen field and ranges from Q=20 for distances < 25 km to 60 at a distance >50 km. The measured values for in this region are between 0.02 and 0.05. 8

After correction for the high frequency attenuation, the S-wave displacement spectra were used to determine the low frequency spectral level Ω0 and angular corner frequency ωc, which will be used in the determination of source parameters. Results of the analysis provide a stable solution for the accelerometer recordings at an hypocenter distance of 4-10 km. The seismic moment is M0= 3.5 ± 0.9 E+14 Nm and the corresponding moment magnitude Mw= 3.6 ± 0.1. There is a difference of 0.2 magnitude units between the mean values of ML and Mw and although the error bars increase if restrictions on epicenter distances are relaxed, this difference is currently being investigated in detail. Both magnitude calculations are based on displacement data. For ML an additional Wood- Anderson filter is applied, that includes a 0.8 Hz high pass filter. For the larger events this filter’s corner frequency approaches the corner frequency of the events and its influence is part of ongoing research. The outcome may have implications for the procedure used to determine ML, but its effect is expected to be small (0.1-0.2 magnitude units). In summary, we consider the Mw = 3.6 ± 0.1 as the best magnitude estimate for the Huizinge earthquake. We will investigate closer the ML-MW relation to obtain a more robust and accurate rapid magnitude estimator for induced seismicity. However, we would like to note that earthquake magnitude estimates are inherently uncertain with 0.1 usually being the lowest possible uncertainty limit. Comparison with other magnitude estimates The magnitude reported by the EMSC, ML= 3.7, is within the error bounds similar to the Mw= 3.6 . The Geofon solution, MLv= 3.9, local magnitude derived from the vertical component, is too high, but may be explained by using stations that are located in only a small azimuth range. Unfortunately, both EMSC and Geofon do not publish error bars on their magnitude determinations.

9

Source mechanism and parameters Source mechanism Based on polarity information from the first onsets of the seismic waves and amplitude ratio’s between the P, SV and SH phases, no stable focal mechanism could yet be found. However, a previous event, ML = 2.7, 600m south east of the current event happened on April 14, 2009. For this event a focal mechanism could be determined, being a normal fault with a strike of 320o, a dip of 66o and a strike of -105o [1].

Figure 3, Source mechanism for the 2009, ML = 2.7 Huizinge event. On the left the SV radiation pattern is shown, on the right SH radiation pattern. Radiation is shown in a lower hemisphere projection. Positive polarity is indicated in red, negative in blue. Taking the 2009 event as an example, figure 3 shows the vertically (SV) and horizontally (SH) polarized shear wave radiation pattern. Due to a dipping fault surface and a small effect of the rake, the radiation pattern is complex. Finding a stable solution for the source mechanism is important for understanding the earthquake process and will be used in geomechanical modeling. It also may explain the patterns we see in the intensity values.

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Figure 4. Comparison of acceleration recorded in station Middelstum-1 for the April 14, 2009, ML = 2.7 (top three records) and the recent August 16, Huizinge event (bottom three records). From top to bottom: 090414-( 1=radial, 2=transverse, 3=vertical) and 120816- (4= radial, 5=transverse, 6= vertical).

Comparison between the two closely spaced events near Huizinge is expected to provide more insight in the possible mechanism. However, being at similar epicenter distances of 2-3 km, the difference in azimuth is around 20 degrees and the polarity of the P waves is very different. Remarkable is the double S pulse for the recent Huizinge event and deserves extra attention, as it extends the duration of the strong shaking. Waveform inversion, using a detailed velocity model is expected to assist in constraining the mechanism. Both topics are subject of ongoing research. Source parameters Source parameters, such as the seismic moment, stress-drop, radius of a circular fault model and the average displacement on the fault have been estimated based on Brune’s model [1]. Corner frequency Stress-drop Radius Average displacement [Hz] [bar] [m] [mm] Accelerometers 1.9 ± 0.6 24 ± 18 460 ± 130 60 ± 38 Boreholes 2.4 ± 0.4 25 ± 16 340 ± 50 48 ± 19 All 2.2 ± 0.5 25 ± 9 390 ± 110 53 ± 28 Table 1. Source parameters for the Huizinge earthquake

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As seen in Table 1, the accelerometers and borehole sensors give similar results. The event is characterized by a low corner frequency, around 2 Hz and a relatively high stress drop of 25 bars, compared to 17 bars for the ML = 3.5, 2006 event near Westeremden [1]. Source radius is comparable between both events, while the average displacement of the Huizinge 2012 event is 60% larger, thus resulting in the large stress drop estimate.

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Peak Ground Acceleration (PGA) and Peak Ground Velocity (PGV) For the assessment of damage, an estimate of peak ground acceleration (PGA) or peak ground velocity (PGV) is required [1]. PGV values are derived from the PGA measurements by applying a recursive filter that includes removal of the instrument response and conversion to velocity.

Figure 5. Maximum accelerations [a] and velocities [b] recorded by the KNMI accelerometer network. Indicated are maximum values, not average values. In b] PGV recorded in the transverse component is marked by “T”. The Huizinge event was recorded at 8 accelerometer stations at an epicenter distance between 2 and 18.5 km. The maximum horizontal peak ground acceleration (PGA) measured is 85 cm/s2, corresponding to a peak ground velocity of 3.45 cm/s (Table 2). Figure 5a shows the variation of PGA over the region with a west-east trending maximum value close to the epicenter. This maximum corresponds to the radial component, while in north-south direction the maximum is found on the transverse component. This difference is due to the source mechanism. station

PGA PGA PGA PGA z PGV PGV PGV Epic. dist rad trans hor rad trans hor [km] WSE 51.0 40.8 45.9 75 2.55 1.21 1.88 2.6 MID1 85.0 30.0 57.5 45.2 3.45 0.9 2.18 2.0 GARST 66.7 35.9 51.3 25.8 2.34 0.79 1.57 3.9 KANT 22.4 52.1 37.3 22.7 0.69 2.36 1.53 3.8 STDM 17.7 29.7 23.7 20.8 0.58 1.33 0.96 3.8 WIN 11.4 11.3 11.4 9.9 0.64 0.43 0.54 6.2 HKS 6.8 8.9 7.9 11.7 0.4 0.45 0.43 9.6 FRB2 4.0 7.1 5.6 5.1 0.1 0.11 0.11 18.5 Table2. Measured values of PGA and PGV in 8 accelerometer stations from figure 5.

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log(Peak ground acceleration [m/(s*s)])

2.1

1.8

1.5

1.2

0.9

0.6

0.3

0

5

10

15

20

Hypocenter distance [km]

Log(Peak ground velocity [cm/s])

1

0

−1

−2

−3

5

10

15

20

Hypocenter distance [km]

Figure 6. Measurements of PGA (top) and PGV (bottom) from the Huizinge event compared to models from [3, D04] in red and [4, D12] in blue. Uncertainties are indicated by dashed lines These measurements provide a unique opportunity to test Ground Motion Prediction Equations (GMPEs) that were derived in the past. Two models were selected: D04, a model based on measured data from small shallow events recorded in the Netherlands [3] and D12, a more recent model based on a larger dataset of similar events [4].

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For the PGA and PGV different definitions are used: 1] the maximum (peak) value on one of the horizontal components and 2] the geometric mean of the peaks of the two horizontal components. The latter is used in the GMPE’s listed. From the comparison of the data with predictions from the two models in figure 6, we conclude that for PGV model D04 fits the data within the error bars, while D12 underestimates the data. For the PGA measurements D12 is the better model, while D04 overestimates the data. The variance in the D12 model is large compared to D04, which might be caused by the different types of shallow seismicity from different regions used in the inversion.

Damage and PGV Based on measured accelerograms near Roswinkel, an assessment was made of the damage probability for different building classes as a function of PGV [10]. This relation between damage probability and PGV was used in an assessment of the maximum damage due to a M=3.9 earthquake near the Bergermeer gasfield [11] and may also act as a guideline for damage assessment of the Huizinge event. However, the signals recorded near Huizinge do show a different character, mainly due to the occurrence of two or three S-wave phases. Evaluation of response spectra, calculated from the accelerometer data, is required to further investigate the applicability of the aforementioned relation.

Figure 7. Damage probability (y-axis) versus PGV (x-value) for masonry buildings in a good state (blue), a bad state (purple) and monuments (red). Figure taken from [11]. The largest recorded PGV in the region is 34.5 mm/s, which is equivalent to a 20-35% probability of damage to masonry buildings. An evaluation of over 2000 damage reports in the region and comparison with the damage probability curves will be a good test-case for the applicability of the relation. 15

Source Duration

Figure 8. From top to bottom: accelerometer recording in station Middelstum-1 (1-3, radial, transverse and vertical)), Westeremden (4-6, rad, trans., vert.) and Stedum (7-9, rad, trans., vert.) for the August 16, 2012 Huizinge event. The accelerometer data from the Huizinge event shows in some stations multiple S-phases, resulting in longer duration of the strongest motion from 0.2 to 1.5 seconds (Figure 8). The P wave recorded in Westeremden shows a complex onset and it is unclear if this is a source (rupture) effect or a local effect. Waveform modeling will be used to investigate the complexity of the waveforms, looking for an explanation of both phenomena. The salt layer on top of the reservoir, acting as a high velocity layer, varies strongly in thickness nearby the epicenter. Multiple reflections in the salt may be candidates to explain this pattern of multiple phases. The effect of multiple S phases is not present in the 2009 event, as demonstrated in figure 4. In some other larger events in the region, a similar longer duration of the S-wave strong motion is reported in some of the stations, but certainly not all. Also these records will be used in the waveform modeling.

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Intensity The KNMI operates a Web site which provides an inquiry where people can contribute with their experiences during local earthquakes (http://www.knmi.nl/seismologie/seismoenquete.html ). These inquiries form the basis of an evaluation of intensities. Intensity data, describing the felt effects from the Huizinge event, are compared to measured PGA and PGV values in the region.

Figure 8. Community Internet intensities, for the 2012 Huizinge earthquake (epicenter marked by a star). Communities are based on the Dutch zip code system and averaged over 1 km2 areas. Cities are shown in grey. Incoming responses to the inquiries are processed using an approach developed at KNMI for induced earthquakes in the Netherlands, based on the Community Intensity Map approach [8]. Weights are assigned to the different categories in the inquiries and intensities calculated as a weighted sum. This method has been calibrated using a number of shallow induced events for which also a manual interpretation exists. Average values of Community Internet Intensities (CII) for the Huizinge event are shown in figure 8. Although many small communities exist in the region, people living in the larger cities, like Groningen south-east of the epicenter or Delfzijl to the east, dominate the response. Figure 9 shows the isoseims, obtained from the Community Internet Intensities in figure 8. Isoseisms are calculated using a kriging technique. In order to diminish the effect of spatial undersampling, a background intensity net was introduced. All processing was carried out using ArcMap software and the isoseisms are calibrated using comparison with manually interpreted isoseisms for past events. The isoseims fit well with a shallow source at 3 km depth. The Macroseismic epicenter, taken as the center of the intensity VI contour, is located north-east of the instrumental epicenter. This difference may be explained by the source mechanism. However, details in the intensity contours are also 17

influenced by the population density, e.g. to the south-west the effect of the city of Groningen is clearly visible and therefore one should be cautious with the interpretation of the contours. Intensity VI is measured in a limited region at < 4km from the macroseismic epicenter. EMS 98 1 2

Short description Not felt Scarcely felt

Additional info

Not felt by anyone Vibration is felt only by individual people at rest in houses, especially on upper floors of buildings. 3 Weak Felt indoors by few, people at rest feel swaying or light trembling, noticeable shaking of objects 4 Largely The earthquake is felt indoors by many people, outdoors by few. A few people observed are awakened. The level of vibration is possibly frightening. Windows, doors and dishes rattle. Hanging objects swing. No damage to buildings 5 Strong The earthquake is felt indoors by most, outdoors by many. Many sleeping people awake. A few run outdoors. Entire sections of all buildings tremble. Most objects swing considerably. China and glasses clatter together. The vibration is strong. Topheavy objects topple over. Doors and windows swing open or shut. 6 Slightly Felt by everyone indoors and by many to most outdoors. Many people in damaging buildings are frightened and run outdoors. Objects on walls fall. Slight damage to buildings; for example, fine cracks in plaster and small pieces of plaster fall 7 Damaging Most people are frightened and run outdoors. Furniture is shifted and many objects fall from shelves. Many buildings suffer slight to moderate damage. Cracks in walls; partial collapse of chimneys. Table 3. Overview of the descriptions belonging to the EMS98 intensity grades.

5km Figure 9. Isoseisms for the 2012 Huizinge event.

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Implicaations for hazard an nalysis The lastt update of the t hazard analysis a for the t region [1 1] included all a data until 2010. The Huizinge event allows us to extend thee dataset to o Septemberr 2012 and investigate the hazard d for the m detail, since s the dataset for this field now co ontains 190 eevents of MLL ≥ 1.5. Groningen field in more A new w public dataset on average annual production became recently available (www.nlog.nl/en/oillGas/oilGas.h html ) and could be used to compare c annual production to seismicitty.

Cumullative energy

Figure 10: Cumulativve square roo ot of the eneergy released d in the events in the Gro oningen field (blue) in GJoule compared c to average ann nual gas prod duction in Biillion Cubic Meter M (BCM).. Also shown n are the linear models m describ bing the cum mulative seism mic energy release. r We estim mated the cumulative seeismic energgy release with time for the period 1 1986-2012, following f the proccedures expllained in [1]. The increaase in the raate of seismic energy, wh hich was rep ported in [1], baseed on data fo or all gas fiellds for the peeriod 1986-2 2010 continu ues. Selectingg only the Groningen field data from thee database the increase in the rate of seism mic energy b becomes eve en more pronoun nced (Figure 10). The break in the en nergy curve correlates with w the pro oduction datta. The driving force behind the seismicitty is thought to be diifferential co ompaction [9]. Increaseed production since 20 001 may thereforre explain the increased rate of seeismic energgy release starting s around 2003. The T high production in the period 1990-1997 may have resulted d in relative low levels o of seismicity because not enough compacttion was avaailable at that time.

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As disussed in [1], the increased rate of seismicity implies a breakdown of the stationarity assumption in the seismic hazard assessment.

Frequency-magnitude relation

Figure 11. Annual cumulative frequency for two time periods (1991-2003 and 2003-2012). Seismicity rate (GR a- values) differs, but the b-values are equal within their error bounds. Following the methods described in [1], we calculated the frequency magnitude relation (FMR) for the Groningen field only. Since the assumption of stationarity over the total period where seismicity is observed in the Groningen field seems to be not valid any more, the data set is split-up in two periods: 1991-2003 and 2003-2012. Figure 11 shows the FMR for the two time periods. The FMR curves consists of a Gutenberg-Richter (GR) part, described by a linear relation, and a nonlinear part for the higher magnitudes. Parameters a , the seismicity rate, and b, the slope of the GR relation are calculated using a maximum likelihood methods (see [1] for references). For Groningen (1991-2003) the best result, using the method of Page is b= 1.08 ± 0.25, a= 2.33±0.37 and Mmax=3.1. This method is dependent on an assumption for Mmax, but this has not a large influence (e.g. with Mmax= 3.3: b=1.14 ± 0.29). Processing the data for Groningen for the period 1996-2003, where the magnitude of completeness is 1.5, leaving-out the observation of detected events in the period 1991-1996, gives as best estimate with Mmax= 3.1: b=1.41±0.41 and a=2.84±0.61. For this reduced dataset, the error in the b-values significantly increased and this demonstrates the importance of carefully constraining a sparse dataset.

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For the period 2003-2012 the curve is less well behaved, but contains 3 times more data and gives a best fit, using the same method, of b=1.09 ± 0.17, a=2.82 ± 0.25 at Mmax=3.9. The fit is best for the lower magnitude range and worse for the higher magnitudes. Using higher values for Mmax only results in a higher b-value, while a lower b-value would give a much better fit. We conclude from the analysis that both GR curves are characterized by a similar value of b, but at a different seismicity rate a. Since the b-value gives an indication of the mechanism behind the seismicity the fact that the change in behavior of the system is only due to the seismicity rate is reassuring.

Maximum magnitude Groningen field Since there is only information available on the statistics of seismicity for this region, no estimate of the maximum probable magnitude for the Groningen field could be made on the basis of estimated available fault surface or the reactivation potential of faults and the average slip from geomechanical modeling. Therefore, a Monte Carlo simulation [1] has been applied to the seismicity data from the Groningen field. In this simulation artificial datasets are generated by randomly varying the calculated annual cumulative frequencies within their error bounds and fit each dataset to a bounded frequencymagnitude relation. Result is a Poisson distribution of values for Mmax . In this procedure, values for Mmax >5.5 are discarded, being regarded unrealistic and an artifact of the method. Results for the Groningen field data show a flat probability density function, which could not be interpreted in terms of a Poisson distribution. Therefore, it was concluded that no reliable estimate can be given for the Mmax of the Groningen field alone. Until now, the assumption was that if we take the statistics of all fields, this would give a good indication of a Mmax for individual fields. This was a necessary step, since the dataset for individual fields was too small to draw conclusions. Only recently, this changed for the Groningen field. Other hydrocarbon fields in the Netherlands The Groningen dataset dominates the induced seismicity database for the Netherlands since 2003, therefore also the Mmax estimates from statistics for other fields may be questioned. In some instances, e.g. the Bergermeer field, independent information on available fault surface and estimates of average slip from geomechanical modeling are available[12][13]. These estimates confirm the Mmax=3.9 estimate from statistics. Since most of the smaller fields do contain faults of limited size, we do not see a reason to change the existing Mmax for the other fields.

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Hydrocarbon fields outside the Netherlands An alternative approach to estimate the maximum possible magnitude is to look for analogues in other areas with (similar types of) induced seismicity. Here we have to take care to make a distinction between triggered and induced seismicity. In triggered seismicity the stress perturbations that are released during an earthquake are primarily due to natural tectonic processes; the human activities merely trigger the earthquakes, perhaps sometime ahead of their natural occurrence. In induced seismicity the stress perturbations that are released during an earthquake are primarily induced by the human activities themselves, as is the case in Groningen. The field is located in an area that we regard as aseismic from a natural perspective. We therefore do not expect any triggered seismicity. Perhaps the closest analogue is found in the gas fields of Northern Germany. These fields are located in the same sedimentary sequence as the Groningen field, in a similar tectonic setting, albeit a bit deeper (4.5-5.0km). The Rotenburg event of October 20, 2004 took place in the direct vicinity of a gas field in production and had a moment magnitude of Mw = 4.4 (ML= 4.5). It is probably also an induced event [15]. Due to the limited number of events in Northern Germany a statistical analysis is not possible. The Ekofisk oilfield in the North Sea is located at a similar depth as the Groningen field in the sedimentary basin of the Dutch Central Graben. The largest earthquake in the surroundings of the Ekofisk field was the May 7, 2001 event with moment magnitude Mw 4.1-4.4 [16]. Although the area is not completely void of natural seismicity this largest event was most probably induced, although its nature was quite different from the events in Groningen. The Ekofisk event took place in the overburden on a very large sub-horizontal plane with small displacement. The subsidence induced by the oil recovery is in the order of meters and the event followed an unintended water injection. The Lacq gas field in the South of France has a history of more than 1000 micro-earthquakes with local magnitudes up to Ml 4.2 [17,18,19]. The depth is similar to Groningen, but sedimentary and tectonic context is quite different, with a less compacting chalk reservoir and the vicinity of the Pyrenees. An event of short-period-body-wave magnitude mbLg 4.3, on April 9, 1993, was probably induced by the gas extraction from the Fashing field in South-Central Texas [20]. The field is located in a limestone reservoir under an anticlinal structure at depth similar to the Groningen field. In the same area on October 20, 2011 an event took place with magnitude 4.8, as reported in the public media. For this event we have not found a scientific publication. Higher magnitude events have also been observed in the neighborhood of hydrocarbon reservoirs (e.g., Gazli 1984, Ml 7.2; Coalinga 1983, Ml 6.7), however these have been identified as natural or triggered rather than induced. In recent years overviews of induced seismicity in hydrocarbon fields are published [21,22] and for human induced events [23]. The latter includes both triggered and induced earthquakes. In [21] the author remarks “Induced seismicity in hydrocarbon fields is typically small to moderate (ML ≤ 4.5)”.

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To conclude, the magnitudes of the largest induced events in hydrocarbon reservoirs, as reported in the scientific literature, remain, if rounded upwards, below ML = 5.0. One has to keep in mind, however, that the comparison is made for hydrocarbon fields in different geological settings and tectonic regions. Also, enough existing fault surface should be available to accommodate the movement of a larger event. Consequences for Groningen Based on statistics only, no reliable estimate could be obtained of a maximum probable earthquake for the Groningen field. Further research using additional information from geology and geomechanical modeling is expected to provide additional constraints on the possible value of the Mmax. Until this information is available, we estimate an conservative upper limit for Mmax at magnitude 5.0. An estimate of the intensity that corresponds to a Mmax = 5.0 is difficult to assess. Magnitudeintensity relations have a large standard deviation. A rough estimate will be that intensities will reach an intensity VII (Table 3).

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Conclusions The earthquake on August 16, 2012, near Huizinge (Groningen) is the largest induced earthquake ever recorded in the Netherlands related to hydrocarbon production. A detailed analysis shows 1. The Huizinge event is located in the area of highest activity in the Groningen field within the community of Loppersum. 2. The moment magnitude of the event is Mw=3.6 ± 0.1, which is larger than the original estimate of the local magnitude ML = 3.4 ± 0.1. The ML-Mw relation is subject of ongoing research. 3. The source is characterized by a radius of 390 ± 110 m, an average displacement of 5 ± 3 cm and a stress-drop of 25 ± 9 bar. Polarity inversion did not provide a stable mechanism yet, waveform modeling is expected to provide results. 4. Accelerometer recordings at epicenter distances of 2-10 km show values of PGA up to 85 cm/s2 and PGV up to 3.45 cm/s. At these values a damage probability of 20-35% exists for masonry structures, although this relation is valid only for events of short duration. It is shown that Ground Motion Prediction Equations derived for small shallow earthquakes predict the measurements. 5. Multiple S-wave phases have been recorded, extending the duration of the strong motion. This longer duration was also observed by local people. 6. Maximum intensity VI is observed in a limited region (<4 km) around the macroseismic epicenter. 7. For the Groningen field an update of the hazard analysis is carried out, including an estimate of the maximum probable magnitude. Based on cumulative energy calculations the dataset was divided into two segments (1991-2003) and (2003-2012). Both segments are characterized by a constant b-value, but different seismicity rate. Comparison with average annual production data shows a correlation between seismicity rate and production. 8. A value for Mmax could not be derived for the Groningen field, due to the nature of the frequency-magnitude relation. 9. A literature study of induced seismicity in other hydrocarbon fields indicate maximum recorded events between ML = 4.2 (Lacq) and M = 4.8 (Fashing). Following only these observations we do not expect larger events to occur in the Groningen field, i.e. a maximum probable magnitude below ML = 5.0.

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References [1] Dost, B., F. Goutbeek, T. van Eck and D. Kraaijpoel, 2012, Monitoring induced seismicity in the North of the Netherlands: status report 2010, KNMI Scientific report; WR 2012-03. [2] Kraaijpoel, D. and B. Dost, 2012, Implications of salt-related propagation and mode conversion effects on the analysis of induced seismicity, Journal of Seismology, DOI 10.1007/s10950-012-9309-4 [3] Dost, B., T. Van Eck and H. Haak, 2004, Scaling of peak ground acceleration and peak ground velocity recorded in the Netherlands, Bollettino di Geofisica Teorica ed Appliocata, 45,3: 153-168. [4] Douglas, J., B. Edwards, V. Convertito, N/ Sharma, A. Tramelli, D. Kraaijpoel, B.M. Cabrera, N. Maercklin and G. De Natale, 2013, Predicting ground motion from induced earthquakes in geothermal areas, submitted to Bul. Seism. Soc. Am., 2012. [5] Hanks, T.C. and H. Kanamori, 1979, A Moment Magnitude Scale, Journal of Geophys. Research, 84: 2348-2350. [6] Aki, K. and P.G. Richards, 1980, Quantitative Seismology, theory and methods, W.H. Freeman and company, San Francisco, 932 pp. [7] Anderson, J.G. and S. Hough, 1984, A model for the shape of the Fourier amplitude spectrum of acceleration at high frequencies, Bulletin of the Seismological Society of America, 74: 1969-1994. [8] Wald, D.J., V. Quitoriano, L.A. Dengler, and J.W. Dewey, 1999, Utilization of the Internet for Rapid Community Intensity Maps, Seismological Research Letters, 70: 680-697. [9] Dost, B. and H.W. Haak, 2007, Natural and induced seismicity, in Th. E. Wong, D.A.J. Batjes, and J. de Jager. Eds., Geology of the Netherlands, Royal Netherlands Academy of arts and Sciences, 223229. [10] Van staalduinen, P.C. en C.P.W. Geurts, 1998, De relatie tussen schade aan gebouwen en lichte, ondiepe aardbevingen in Nederland: inventarisatie, TNO rapport 97-CON-R1523-1, 98pp. [11] Van Kanten-Roos, W., B. Dost, A.C.W.M. Vrouwenvelder and T. van Eck, 2011, Maximale schade door geinduceerde aardbevingen: inventarisatie van studies met toepassingen op Bergermeer. TNOKNMI report (also published as appendix in [1]). [12] Logan, J.M., N.G. Higgs, J.W. Rudnicki, 1997, Seismicity risk assessment of a possible gas storage project in the Bergermeer field, Bergen concession, Report to BP, 137pp. [13] Muntendam-Bos, A.G., B.B.T. Wassing, C.R. Geel, M. Louh and K van Thienen-Visser, 2008, Bergermeer Seismicity Study, TNO report 2008-U-R1071/B, 95pp.

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[14] Castagna, J.P., Batzle, M.L. and Eastwood, R.L., 1985, Relationships between compressional wave and shear-wave velocities in clastic silicate rocks: Geophysics, 50, 571-581. [15] Dahm, T., F. Krueger, K. Stammler, K. Klinge, R. Kind, K. Wylegalla and JR Grasso, 2007, The 2004 Mw 4.4 Rotenburg, Northern Germany, Earthquake and Its Possible Relationship with Gas Recovery , Bull. Seism. Soc. Am., 97: 691-704, DOI: 10.1785/0120050149. [16] Ottemoeller, L., H.H. Nielsen, K. Atakan, J. Braunmiller, and J. Havskov, 2005, The 7 May 2001 induced seismic event in the Ekofisk oil field, North Sea, J. Geoph. Res., 110, B10301, doi: 10.1029/2004JB003374. [17] Maury, V., J.R.Grasso and G. Wittlinger, 1990, Lacq Gas Field (France): Monitoring of induced Subsidence and Seismicity Consequences on Gas Production and Field Operation., Society of Petroleum Engineers (SPE), SPE 20887. [18] Feignier, B. and J.R. Grasso, 1990, Seismicity Induced by Gas Production: I. Correlation of Focal Mechanisms and Dome Structure, Pure and Applied Geophysics, 134: 405-426. [19] Grasso, JR., 1992, Mechanics of seismic instabilities induced by the recovery of hydrocarbons, Pure appl. Geophysics, 139:507-534. [20] Davis, S.D., P. A. Nyffenegger and C. Frohlich, 1995, The 9 April 1993 Earthquake in SouthCentral Texas: Was It Induced by Fluid Withdrawal? Bull. Seism. Soc. Am., 85: 1888-1895. [21] Suckale, J., 2009, Induced seismicity in hydrocarbon fields, Advances in geophysics, 51: 55-106 [22] Suckale, J., 2010, Moderate-to-large seismicity induced by hydrocarbon production, The Leading Edge, 29: 310-319 [23] Klose, C. D., 2013, Mechanical and statistical evidence of the causality of human-made mass shifts on the Earth’s upper crust and the occurrence of earthquakes, Journal of Seismology, 17:109135 DOI 10.1007/s10950-012-0321-8.

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Reassessment of the probability of higher magnitude earthquakes in the Groningen gas field Including a position statement by KNMI by Mevr. Dr. A.G. Muntendam-Bos and Dr. J.A. de Waal

16 January 2013 State Supervision of Mines

Confidential Final Report, dd. 16-01-2013

Position Statement of KNMI with regard to the report: “Reassessment of the probability of future higher magnitude earthquakes in the Groningen gas field”, dated January 16, 2013, by the State Survey of Mines In this Statement we declare our position with regard to the conclusions of the Report. It should be mentioned that during the preparation of the Report, SSM has frequently consulted and shared drafts with KNMI. The Report presents the results of an SSM analysis of the seismicity in the Groningen field (GF) based on the seismic catalogue data as provided by KNMI in the public domain. Notable differences with earlier analyses by KNMI (e.g., Dost et al., 2012, which has a broader scope) are the stronger focus on the GF in isolation, and the attempt to establish a computational model for the relation between gas production and seismicity. The SSM analysis addresses descriptive statistics of the past seismicity, as well as predictions of (the statistics of) future seismicity. The predictions involve two kinds of extrapolation: (a) extrapolation in time, and (b) extrapolation in magnitude. The descriptive statistics primarily concern (i) the evaluation of the seismicity rate, the number of events in a certain time window (say, a year) above a certain threshold magnitude, and (ii) the characterization of the relative frequencies of events of different magnitudes within a population of events. The extrapolation in time concerns the seismicity rate. The Report suggests extrapolation -or prediction --, using a computational model that expresses seismicity rates as a function of cumulative and annual gas production (Equation 4). The proposed model gives a history match according to the authors’ criteria and is subsequently used to predict seismicity rates for several production scenarios. The extrapolation in magnitude concerns higher, still scarce or unobserved magnitudes. The Report suggests extrapolation using the assumption of the classical Gutenberg-Richter relation bounded by an undetermined maximum magnitude. The extrapolated GutenbergRichter relation is combined with the extrapolated seismicity rates to predict probabilities for the occurrence of events exceeding certain magnitudes. With regard to the descriptive statistics KNMI supports the conclusions (1-3) of the Report, based on our own research, concerning (1) the increase in the annual number of earthquakes in the GF, (2) the inability to estimate a Mmax for the GF using earthquake statistics and (3) Mmax > 3.9 cannot be excluded based on seismicity data only. We conclude that: • The seismicity rate of the Groningen field has been increasing significantly since the onset of seismicity • The seismicity of the Groningen field has not been stationary over time • The distribution of the current catalogue of past events in Groningen is well described by a Gutenberg-Richter relation with a b-value of around 1.0, a typical value for natural and induced earthquakes. • The distribution of magnitudes does not show evidence for a maximum magnitude.

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Confidential Final Report, dd. 16-01-2013 With regard to the extrapolation in time KNMI takes the position that the model proposed by SSM is speculative and should be better motivated and tested. KNMI is therefore not able to give full support to conclusions 5-8 of the Report, dealing with inferences of the proposed preliminary model. However, as a first attempt the model gives some directions and both the SSM and NAM model agree that the annual number of earthquakes depend on cumulative production. Cumulative production is responsible for compaction and we agree that differential compaction is most likely the driving force behind seismicity in the field. With regard to the extrapolation in magnitude KNMI takes the position that the bounded Gutenberg-Richter model is a reasonable model to predict the relative frequencies of higher, unobserved magnitudes. However, it should be clear that this model is an assumption. Other types of relative frequency-magnitude distributions may also be envisioned. KNMI supports conclusion 4 of the Report with the additional qualifier that it is based on the assumption of a bounded Gutenberg-Richter model for all magnitudes above the magnitude of completeness. The percentages mentioned depend on that assumption. Since we do not know the Mmax, these conclusions are only used as examples.

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Nederlandstalige conclusies Dit rapport beschrijft een analyse van aardbevingsdata uit het Groningen veld door SodM waarin het te verwachten jaarlijkse aantal aardbevingen (de seismiciteit) wordt gekoppeld aan productie en productiesnelheid. Dat geeft een betere beschrijving van het seismische gedrag van Groningen tot nu toe en andere voorspellingen voor de toekomst. De belangrijkste conclusies op basis van de SodM analyse zijn: 1. Het jaarlijkse aantal aardbevingen en de energie die daarbij vrijkomt nemen toe en daarmee voor Groningen ook de kans op het optreden van aardbevingen met hogere magnitude. 2. Een Monte Carlo analyse toont aan dat het niet mogelijk is om op basis van de seismische data van het Groningen veld een waarde voor Mmax te bepalen, anders dan dat de waarde daarvan boven de 3,6 ligt. Dat betekent niet dat er geen bovengrens is. 3. Hogere waarden voor Mmax kunnen op voorhand niet worden uitgesloten zonder aanvullende schattingen op basis van niet-seismische methodes zoals geomechanische berekeningen. Zulke data is momenteel niet beschikbaar voor Groningen. 4. Omdat op dit moment geen uitspraak kan worden gedaan over Mmax is de verwachtingswaarde voor de kans op een aardbeving met een magnitude van 3,9 of hoger in Groningen niet nauwkeurig te bepalen. Gedurende de komende 12 maanden is de verwachtingswaarde voor die kans in het ongunstigste geval (uitgaande van een Mmax van 6,0) ongeveer 7,6%. Bij een Mmax van 5,0 is dat ongeveer 7 %, bij een Mmax van 4,5 ongeveer 5,8 %. Bij een Mmax van 3,9 wordt de verwachtingswaarde 0%. De verwachtingswaarde voor de kans op een aardbeving met magnitude van 4.5 of hoger gedurende de komende 12 maanden ligt tussen 0 en 2%. 5. Er is een voorlopige versie van een vergelijking gevonden die, binnen de te verwachten intrinsieke statistische fluctuaties, het jaarlijkse aantal aardbevingen met magnitude M≥1,5 en de variaties daarin – voorspelt op basis van de cumulatieve productie en de productiesnelheid. Die vergelijking is gerelateerd aan een (rate type) compactie model waarmee het waargenomen niet-lineaire compactiegedrag van het Groningen veld goed wordt beschreven. De gevonden vergelijking suggereert dat de mate van vertraging in de bodemdaling de seismiciteit bepaalt. 6. SodM heeft op basis daarvan een aanpak ontwikkeld voor de beschrijving van het waargenomen seismische gedrag van het Groningen veld. De b-waarde uit de GutenbergRichter relatie voor Groningen wordt daarin gecombineerd met de bovengenoemde vergelijking en een aanname voor de maximaal mogelijke magnitude Mmax. De op basis van deze aanpak berekende (veranderingen in) de seismiciteit in Groningen zijn in overeenstemming met de waarnemingen. Dezelfde aanpak kan worden gebruikt om de waarschijnlijkheid te berekenen voor het optreden van een aardbeving boven een gegeven magnitude voor een tijdsperiode in de toekomst. 7. De verwachtingswaarde voor de kans op een aardbeving met een grotere magnitude (M≥3,9) kan op termijn van enkele jaren met ongeveer een factor twee worden verlaagd door de jaarlijkse productie uit het Groningen veld in een keer te verlagen met een factor twee ten opzichte van de huidige productiesnelheid van ca. 50 miljard normal kubieke meter gas per jaar, gevolgd door een geleidelijke verdere afname. Een significante verwachtingswaarde voor de kans op een aardbeving met een grotere magnitude blijft ook dan bestaan. 8. Op basis van de de gevonden relatie tussen het jaarlijks aantal aardbevingen, de productie en de productiesnelheid zou de productiesnelheid tot ca. 12 normal BCM/jaar verlaagd moeten worden om het risico op aardbevingen te minimaliseren. Het is daarom mogelijk dat bij die productiesnelheid na enkele jaren vrijwel geen aardbevingen met een magnitude ≥ 1.5 meer zouden optreden in het Groningen veld. 4


Executive summary A higher than predicted annual frequency of earthquakes with a magnitude equal or above 3.0 has led to an independent assessment by State Supervision of Mines (SSM) of the available Groningen earthquake data and the applied analysis methods. The occurrence of the highest magnitude earthquake thus far, near Huizinge in August 2012, with a moment magnitude of 3.6 gave further impetus. In the re-assessment SSM has limited the analysis to the earthquake data from the Groningen field only. The Groningen field shows an increasing number of earthquakes over time, as reported in [1]. As a result, the expectation value for the probability for higher magnitude earthquakes has increased significantly for Groningen. Firm conclusions on this could only be drawn recently given the inherent statistical uncertainty resulting from the initially much more limited number of earthquakes and the fact that a clear increase only started around 2003. Annual gas production increased from 20 Billion normal cubic meter (normal BCM) in 2000 to a level around 50 billion normal BCM in 2011. In the same period the annual number of registered earthquakes with a magnitude of 1.5 or higher increased from on average 4 per year during the period 1991-2002 to 28 earthquakes in 2011. Superimposed on this longer term trend, increases and decreases in the annual gas production are followed by increases and decreases in the annual number of earthquakes with a delay of approximately a year. The effect of the increasing cumulative production can be separated from the effect of the changing annual production using a preliminary version of an equation related to a (rate type) compaction model that can be used to describe the observed non-linear compaction behaviour of the Groningen field [2,3,4]. The thus calculated annual number of earthquakes agrees, within the intrinsic statistical uncertainty, with the historically observed variation in the Groningen seismicity between 1964 and 2012. This suggests that the seismicity level is linked to the amount of subsidence delay. Note that this is still work in progress. The SSM analysis confirms previous preliminary analysis on Groningen data [1] on the fact that earthquakes with a magnitude equal to or above 2.5 are approximately ten times less probable then earthquakes with a magnitude equal to or above 1.5, independent of the total number of earthquakes in a given period (e.g. in a given year)1. Earthquakes with a magnitude equal to or above 3.5 are again approximately ten times less probable. This behaviour is expected to continue for higher magnitude earthquakes that have not yet taken place in Groningen, although bounded by the maximum magnitude that can occur. Based on the data from all fields in the Netherlands for the full period since 1996, a maximum probable magnitude of 3.9 was calculated during an earlier study [1]. An SSM Monte Carlo analysis on the seismicity data from Groningen only now shows that little can be said about the maximum possible magnitude in Groningen other than that it can have any value above 3.6. Perhaps that non-seismic methods can be applied to obtain estimates for the maximum possible magnitude. This could include estimates based on the maximum percentage of the stored elastic energy that can be released in a single earthquake. Or an upper limit based on an analysis of the distribution and size of faults present in the field. At the moment such results are not available for Groningen. Using the total number of seismic events in a given period and making an assumption on the maximum possible magnitude, the probability for earthquakes equal to or above a given other 1

Hence, for every 10 tremors with a magnitude equal to or above1.5 there is on average one tremor with a magnitude equal to or above 2.5.

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Confidential Final Report, dd. 16-01-2013 magnitude can be calculated for that given period. Doing so, the historic seismic behaviour of the Groningen field is reproduced within the intrinsic statistical uncertainty. For the coming 20 earthquakes (approximately the number of earthquakes with M≥1.5 expected during the next 12 months) this approach results in a worst case expectation value for the probability for an earthquake with a magnitude equal or above 3.9 of around 7.6 %. In this calculation a value of 6.0 is imposed for Mmax. If Mmax would be 5.0 the expectation value for the probability becomes 7 % and 5.8 % for an Mmax of 4,5. For an Mmax of 3.9 the expectation value for the probability becomes 0%. The expectation value for the probability of a magnitude 4.5 or larger earthquake to occur within the next twelve months is between 0 and 2%. Combining the derived (preliminary) relation to compute the annual number of earthquakes on the basis of both cumulative and annual production with the above approach, the seismicity to be expected under various Groningen production scenario’s can be calculated. Results suggest that the expectation value for the annual number of earthquakes of magnitude M ≥ 1.5 might be decreased by approximately a factor of two, by decreasing the annual production rate by a factor two compared to the current production rate of some 50 billion normal cubic meter/year (normal BCM) followed by further reductions. The expectation value for the number of larger magnitude earthquakes then will also halve. However, under this scenario a significant expectation probability for larger magnitude earthquakes will remain (typically 2-5 % for an M≥4.5 during the next 4 years). Based on the derived (preliminary) relation between annual number of earthquakes and production, production rates would have to be lowered to values around 12 normal BCM/year in order to achieve minimal risk. It is therefore possible that at this production rate almost no earthquakes with magnitudes ≥1.5 would occur after a number of years.

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Contents Position Statement of KNMI with regard to the report: “Reassessment of the probability of future higher magnitude earthquakes in the Groningen gas field”, dated January 16, 2013, by the State Survey of Mines...........................2 Nederlandstalige conclusies...............................................................................4 Executive summary............................................................................................5 Contents .............................................................................................................7 Introduction........................................................................................................8 Induced Seismicity (variation) in Groningen...................................................10 General observations................................................................................... 10 Energy release............................................................................................. 11 Statistical analysis....................................................................................... 11 Seismicity and magnitudes for Groningen.......................................................14 The Gutenberg-Richter law......................................................................... 14 Implications for Groningen......................................................................... 15 Monte Carlo derivation of BGR parameters............................................... 15 Discussion ................................................................................................... 17 Relation between production and seismicity ...................................................18 Consequences for future earthquakes in Groningen ........................................23 Expectation probability for larger magnitude earthquakes due to already realised production...................................................................................... 23 Predicted earthquakes in Groningen under different production scenarios 24 Conclusions......................................................................................................28 References........................................................................................................29 Appendix A: October 8th SSM technical assement..........................................30 Appendix B: Summary of peer review workshop outcomes ...........................34

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Introduction The Groningen field, the largest gas field of Europe, has been in production since 1964. In 1991, the first production-induced earthquake with a local magnitude Ml of 2.4 was recorded at Middelstum. To date, over 585 induced earthquakes have been related to gas production from this field. Most earthquakes have been of a small magnitude (Ml <1.5), while some 200 earthquakes had magnitudes Ml ≥1.5. Initially, the detection capabilities of the seismic network were limited. Since the installation of 8 borehole stations in 1995, a detection threshold of Ml ≥1.5 has been achieved for the whole of the Groningen field [1]. Until recently there were no indications for differences between the local magnitude Ml and the moment magnitude Mw (which better represents the released energy) for the induced earthquakes in Groningen. In August 2012, the largest magnitude earthquake so far occurred near Huizinge with a local magnitude moment Ml of 3.4 and a moment magnitude Mw of 3.6. The damage caused by this earthquake was extensive compared to previous earthquakes of comparable magnitude, though not of a structural nature. This time over 2000 damage claims were submitted to the operator NAM. The event raised general concern on the level of acceptability of damage caused by induced earthquakes and led to questions whether earthquakes with even larger magnitudes, possibly causing structural damage to property, could occur in the future. Preliminary analysis made by the KNMI on the Huizinge earthquake (personal communication, 2012) shows that the Huizinge 3.6 earthquake was recorded as a multiple pulse event of longer duration. A multiple earthquake source causing this phenomenon could be excluded, however more extensive investigation into the origin of the multiple is ongoing. In order to address the questions raised and in order to investigate whether or not mitigating measures are feasible, State Supervision of Mines (SSM) commenced an independent analysis on the Groningen seismicity dataset. The analysis was made on public data only: http://www.knmi.nl/seismologie/geinduceerde-bevingen-nl. First results were shared with KNMI on the 11th of September 2012. Further developments were shared with KNMI, TNO-AGE and NAM during meetings on the 21st of September, the 8th of October and the 10th of October of 2012. During the meeting on the 8th of October a starting point conceptual model and a proposed way forward were presented by SSM (see Appendix A). Results as arrived at by early November were put forward for peer review during an expert workshop on the 8th and 9th of November 2012. A summary of the workshop outcomes is given in Appendix B. Subsequently early December 2012 an updated report taking into account the results from the peer review was submitted to KNMI for a second review. This report summarizes the results of the analysis made by SSM, including the additional work carried out in response to the peer review and the later changes made in response to the KNMI review.

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1992

1995

2000

2005

2010

2012

Figure 1: Spatial distribution of earthquakes over the Groningen gas field through time. The colour coding of the dots indicates the magnitude class: yellow 1.5≤M≤2.0, orange 2.0<M≤3.0, red M>3.0. The red lines indicate the contours of the subsidence bowl as observed in 2008.

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Induced Seismicity (variation) in Groningen General observations Figure 1 shows the location of earthquakes of magnitudes 1.5 and larger through time in roughly 5-year intervals. The area where the seismicity is occurring has been increasing, with two distinct areas: the area around Middelstum and the area towards the south-west of the field. The area around Middelstum coincides with the deepest part of the subsidence bowl caused by the Groningen gas production. Both areas correspond with areas of higher average porosity while lower porosity zones around the southern production clusters show little seismicity. This suggests a link between seismicity and reservoir compaction (which is higher in higher porosity zones). The number of earthquakes of a certain magnitude against time is shown in Figure 2. For all magnitude classes (e.g. M ≥1.5) the number of earthquakes is increasing almost linearly on a log-normal scale with time. The steep incline in the number of earthquakes prior to 1996 is due to the incompleteness of the dataset for earthquakes with magnitudes below 2.5. This means that earthquakes of lower magnitudes close to the network stations were recorded, but earthquakes at greater distances were not detected. Since 1996, the network threshold over the whole of the Groningen field is magnitude 1.5. Hence, as of that moment all earthquakes of magnitude larger or equal to 1.5 that occur within or close to the Groningen field will have been detected by the seismic stations.

1000 >=1,5 >=2,0 >=2,5

Number of events

>=3,0 100

>=3,5

10

1 aug-87

jan-93

jul-98

jan-04

jul-09

dec-14

time

Figure 2: Number of earthquakes equal or larger than a particular threshold magnitude plotted against time of occurrence. The number of earthquakes is increasing almost linearly in this log-normal figure. Notice also the increasing density of earthquakes with time, especially for the classes up to M≥2.5.

Of particular importance is the observation that prior to 2003 earthquakes with magnitude ≥3.0 were absent, whereas since that time they have occurred approximately once every 1.3 years. Based on extrapolation (see Figure 2), the occurrence of an earthquake with a

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Confidential Final Report, dd. 16-01-2013 magnitude 3.0 or greater would have been likely at least since 1998. Extrapolation of the statistics also suggests that unnoticed earthquakes with magnitudes above 1.5 are likely to have taken place prior to 1990. For the magnitude classes up to M ≥2.5 a clear increase in the density of earthquakes through time can be observed. This implies that the frequency at which an earthquake of this class occurs is increasing. A similar increase is plausible for the higher magnitudes

Energy release The cumulative seismic energy released is shown together with the cumulative production in Figure 3. In the cumulative production, the annual cycle of low production in summer and increased production in winter is clearly visible. Figure 3 also shows the increase in annual production since 2003. The cumulative seismic energy that was released by the earthquakes clearly shows the higher magnitude earthquakes occurring since 2003. With each magnitude point increase the energy release of an earthquake increases by a factor of 30. Thus higher magnitude earthquakes release the most energy. The increased energy release by the higher magnitude earthquakes (M≥3.0) introduces a break in the trend of energy release prior to 2003. This result is consistent with the analysis presented in reference[1]. 70

2100 Cumulative Energy

2000

Cumulative Production

cumulative E (GJ)

1900 50

1800

40

1700

30

1600 1500

20

1400 10 0 1990

Cumulative Q (MMm3)

60

1300 1200 1995

2000

2005

2010

Figure 3: Cumulative seismic energy release and cumulative production through time. The higher magnitude earthquakes (M≥3.0) release the most energy (10 times more than a magnitude 2.5 earthquake), which introduces the steps observed in the figure.

Statistical analysis It is important to test the observations made in the previous sections on statistical significance: 1) the frequency of magnitude ≥ 3.0 earthquakes since 2003, and 2) the increase in the number of earthquakes with M≥1.5. Statistical significance is tested by deriving Poisson confidence intervals for particular equal time periods. In order for two Poisson distributions to be statistically significantly different, the number of observed earthquakes in one particular time period needs to be outside the confidence interval of the number of observed earthquakes in the other, equally long, time period. We adopt a 99% confidence level for the confidence intervals.

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Confidential Final Report, dd. 16-01-2013 1) Frequency of magnitude ≥ 3.0 earthquakes since 2003 In order to test whether the frequency of the magnitude ≥ 3.0 earthquakes is feasible within a Poisson distribution which shows no prior seismicity at that magnitude level, we adopt two 10 year time periods: 1993-2002 and 2003-2012. During the 1993-2002 10-year period no earthquakes of magnitude ≥ 3.0 were observed. The exact confidence interval corresponding to a 99% confidence level for this time period is 0 to 5.3 earthquakes. During the following 10year period (2003-2012) 7 magnitude ≥ 3.0 earthquakes were observed. This is well outside the 99% confidence interval. The exact confidence interval corresponding to a 99% confidence level for the latter time period is 2.0 to 17.1 earthquakes. Hence, the frequency of the magnitude ≥ 3.0 earthquakes since 2003 is statistically significantly different from the previous period at a 99% confidence level. 2) Increase in number of earthquakes with M≥1.5 Similarly, the increase in the number of earthquakes with M≥1.5 can be statistically tested. In the 1996-2002 time period 32 earthquakes of magnitude ≥1.5, 9 of magnitude ≥2.0, and none of magnitude ≥3.0 occurred. In the 2006-2012 time period 121 earthquakes of magnitude ≥1.5, 36 of magnitude ≥2.0, and 5 of magnitude ≥3.0 were detected. The confidence intervals for the two periods for these magnitude classes are given in Table 1. For all magnitudes the number of earthquakes in the period 1996-2002 are outside the confidence interval for the period 20062012 (at a 99% confidence level and only just for magnitude 3.0). Table 1: Confidence intervals derived for the number of earthquakes of magnitudes ≥ M for the periods 1996-2002 and 2006-2012. M

1996-2002 number of confidence earthquake interval s

2006-2012 number of confidence earthquake interval s

1.5

32

19.3-49.6

121

94.5-152.3

2.0

9

3.1-20.0

36

22.4-54.5

3.0

0

0-5.3

5

1.1-14.1

Based on the tests above, it can be concluded that the seismicity in the Groningen field is nonstationary in time. At the 99% confidence level the increase in the number of earthquakes is statistically significant. Spatial separation of seismicity At the peer review workshop NAM suggested that two distinct spatial areas of seismicity (around the town of Middelstum and in the south-west of the field) should be regarded separately. Figure 4 shows the number of earthquakes for both spatial areas separately. The conclusions drawn above on the total dataset remain valid for both areas. However, seismicity in the south-west seems to be increasing less rapidly compared to the central area. This might be related to lower average porosities and hence lower compaction in the south-west area. Average pressure drop for the two areas seems very similar.

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100

100

>=1,5

>=1,5

>=2,0

>=2,0

>=2,5

>=2,5

>=3,0

Number of events

Number of events

1000

>=3,5

10

1 aug-87

jan-93

jul-98

jan-04

jul-09

10

1 aug-87

dec-14

jan-93

time

jul-98

jan-04

jul-09

dec-14

time

Figure 4: Number of earthquakes larger than a particular threshold magnitude plotted against time of occurrence. Left figure represents the seismicity at the area around the town of Middelstum, the right figure represents the seismicity in the south-west of the field. The number of earthquakes in both areas is increasing with time as is the frequency of occurrence.

Figure 5 provides the comparison of the annual production with the annual number of earthquakes for the two areas separately. 70 annual production (bcm) / annual number of events M>=1.5

SW 60 50

Centraal annual production

40 30 20 10 0 -10 1985

1990

1995

2000

2005

2010

2015

time

Figure 5: Both the annual production and the annual number of earthquakes with a magnitude of 1.5 or larger are shown against time.

For both spatial areas the statistical significance of the increase in number of earthquakes of magnitudes ≥ 1.5 was examined. The results are given in Table 2. For both spatial areas the conclusion holds that at the 99% confidence level the increase in the number of earthquakes is statistically significant. Table 2: Confidence intervals derived for the number of earthquakes of magnitudes ≥ 1.5 in both spatial areas for the periods 1996-2002 and 2006-2012.

1996-2002

region Central SW

2006-2012

number of Confidence number of Confidence earthquakes interval earthquakes interval 28 16.2-44.7 91 68.3-118.6 4 0.7-12.6 26 14.6-42.3

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Seismicity and magnitudes for Groningen The Gutenberg-Richter law The Gutenberg-Richter law(GR) is an empirical relation between the magnitude M of some seismic event, and N(M), the number of earthquakes with magnitudes higher than M. In 1944, Beno Gutenberg and Charles Francis Richter [10,11] proposed the following linear relationship: (1)

log10 N(M) = -b M + a

where N(M) is the number of earthquakes having a magnitude ≥ M, and a and b are constants for a fixed data set. The constant, b, describes how the number of earthquakes in the zone varies for different magnitudes (it is the negative of the slope of the GR relationship). Instead of using the number of earthquakes it is common practice to use the frequency of occurrence, also named Frequency-Magnitude Relation (FMR). The relation (1) stills holds, however N(M) is now the number of earthquakes which occur in a given area and time period, with a magnitude ≥ M. The constant a is subsequently a measure of the level of seismicity, while the constant b remains the same for both relations. The GR and FMR relations are consistent with earthquake sources having a constant stress drop and thus being self-similar. There is a tendency for the slope of the FMR and GR to decrease for smaller magnitude earthquakes. This effect is described as "roll-off" of the FMR and GR. It was assumed that many low-magnitude earthquakes are missed because fewer stations detect and record them [12] . However, some modern models of earthquake dynamics have roll-off as a natural consequence of the model without the need for the feature to be inserted arbitrarily [14,15]. In addition, if a system is finite in size this may impose a maximum possible magnitude. If such a maximum possible magnitude exists, the selfsimilarity will also break-down for the larger magnitude earthquakes. In order to account for both these phenomena, a modification of the GR was derived, which accounts for both a minimum (Mmin) and maximum (Mmax) magnitude. The modified GR is often called the Bounded Gutenberg-Richter relationship (BGR) [16]: e-β(M-Mmin)-e-β(Mmax-Mmin) α-βMmin _____________________________

N(M)=e

(2)

-β(Mmax-Mmin)

1- e

where α = aln (10 ) and β = bln (10 ). As for the GR, the BGR is valid for both the number of earthquakes with magnitudes equal to or higher than M, as for the frequency of earthquakes which occur in a given area and time period, with a magnitude ≥ M. The main assumption in the derivation of the above relations is a constant level of seismicity through time. If the level of seismicity would change over time, the a-value would no longer be a constant but a function of time. The FMR is sensitive to non-stationarity since frequencies computed over a long time period during which the level of seismicity changes will deviate significantly from frequencies during smaller time periods. For instance, if seismicity rates are decreasing during a 10 year-period, the frequency in the first few years will be significantly higher than for the last few years, whereas the FMR for the complete period will give the average frequency. This will cause a deviation in the a-value. The GR and BGR can be normalised by the total number of earthquakes in the given area during any time period: 14

Confidential Final Report, dd. 16-01-2013 N(M)=Ntot 10-bM

(3)

a

where Ntot=10 , the total number of earthquakes. The normalisation removes the time dependent information and different GR curves and their b-values can be more easily compared. For a given b-value the probability for the occurrence of an earthquake with a particular magnitude will depend only on the total number of earthquakes in a period.

Implications for Groningen As shown in the previous chapter, the induced seismicity of Groningen is non-stationary with time: no detected seismicity (M≥2.5) prior to 1991, M≥3.0 occurring since 2003 with approximately annual frequency, increasing annual seismicity since 2003 (M≥1.5) and an increasing energy release since 2003. KNMI [17] has independently investigated the influence of the non-stationarity on the parameters of the BFMR. The calculation of a- and bvalues were carried out using a maximum likelihood method. For the Groningen data in the time period 1991-2003, the best result is b= 1.08 ± 0.25, a= 2.33±0.37 and Mmax=3.1. For the period 2003-2012 the curve is less well behaved, but contains 3 times more data, and gives a best fit, using the same method, of b=1.09 ± 0.17, a=2.82 ± 0.25 at Mmax=3.9. The fit is best for the lower magnitude range and worse for the higher magnitudes (Figure 6). KNMI concludes that the b-value for both datasets is equal within the error bounds and that the avalue, the seismicity rate, increased from 2.33 to 2.82. In addition, the maximum magnitude has increased from 3.1 to 3.9.

Figure 6: Annual cumulative frequency for two time periods (1991-2003 and 2003-2012). Seismicity rate (GR a- values) and Maximum possible magnitudes differ, but the b-values are equal within their error bounds.

Monte Carlo derivation of BGR parameters In order to derive all possible combinations of the parameters a, b and Mmax honouring the seismicity data of Groningen within a 1-sigma uncertainty, a Monte Carlo simulation was performed. A total of 100.000 realisations were generated by randomly extracting values of a, b and Mmax from normal distributions for each. The experiment was done twice (both 15

Confidential Final Report, dd. 16-01-2013 100.000 realisations) initially for a large parameter range and subsequently for a smaller parameter range. The normal distribution of Mmax was limited on the low side by the maximum magnitude observed, since it is not physically feasible for an earthquake to occur within a given area which has a magnitude above the maximum possible magnitude feasible. For completeness, the analysis was done for 1) the BFMR for the full period 1996-2012, 2) the BFMR for the period 2003-2012, and 3) the normalized BGR for the full period 19962012.

Figure 7: Scatterplots showing all realisations that comply with the data within a 1-sigma uncertainty. The color indicates the Mmax of the realisation. The left figure shows the results of analysis 1 (BFMR, full period), the middle for analysis 2 (BFMR, 2003-2012), and the right for analysis 3 (normalized BGR, full period). The mean values are: analysis 1) a=2.53±0.04, b=0.98±0.02, and Mmax=5.28±0.99; analysis 2) a=2.61±0.05, b=0.93±0.03, and Mmax=5.29±0.98; analysis 3) a=1.47±0.04, b=0.99±0.02, and Mmax=5.29±1.0.

All realizations that comply with the data within 1-sigma Poisson distribution uncertainty were accepted as a possible parameter combinations of the BFMR or BGR models. These realizations are shown in Figure 7. The subsequent probability distributions are shown in Figure 8. These show that the seismicity data of all three analysis are non-discriminative for the maximum possible magnitude.

Figure 8: Comparison of the estimation of the maximum possible magnitude for analysis 1 (left), analysis 2 (middle), and analysis 3 (right). The slight irregularity of the probability distribution is caused by under-sampling of the full model space (despite the 100.000 realisations). The irregularity becomes less with increasing amount of realisations.

In order to show that the method applied produces very similar results as the maximum likelihood method followed by KNMI [1], the analysis has also been performed on a dataset comprising all induced seismicity in the Netherlands between 1986 and 2010. The results are shown in Figure 9. Though the probabilities for the maximum possible magnitude between 3.8 and 4.5 are somewhat larger than found by the KNMI, the result leads to a very similar interpretation. The larger probabilities are partly caused by the method used and particularly by the fact that no maximum possible magnitudes below 3.6 are accepted, since already several magnitude 3.5 earthquakes have been observed in the time period.

16


Figure 9: Scatterplot showing all a-,b- and Mmax values feasible for all induced earthquakes in the Netherlands between 1986 and 2010 (left) and the estimation of the maximum possible magnitude (right).

The fact that from the dataset of all induced earthquakes an indication for a maximum possible magnitude can be drawn, while the Groningen induced seismicity does not indicate any maximum possible magnitude implies that deriving conclusions for individual fields on the basis of an analysis of induced seismicity from multiple fields is problematic. However, due to data scarcity the precision of the analysis of the Groningen data only was previously to low to draw conclusion, though the accuracy improved. With the increased datasets for Groningen only, the precision is now such that the improved accuracy no longer goes at the expense of the precision. However, for other fields in the Netherlands, this intrinsic trade-off is still valid, hence conclusions drawn for these fields based on the general all induced seismicity analysis should be treated with care.

Discussion The Monte Carlo analysis shows that the b-value of the Gutenberg-Richter relation for all analysis is approximately -1 and confirms a stationary magnitude distribution. This means that earthquakes with a magnitude equal to or above 2.5 are approximately ten times less probable then earthquakes with a magnitude equal to or above 1.5, independent of the total number of earthquakes in a given period (e.g. in a given year)2. Earthquakes with a magnitude equal to or above 3.5 are again approximately ten times less probable. As the data is non-discriminative for the maximum possible magnitude, this behaviour may continue for higher magnitude earthquakes that have not yet taken place in Groningen. Hence, assuming no maximum possible magnitude and a probability for a magnitude 1.5 earthquake of 100%, the probability for a magnitude 2.5 earthquake would be 10%, the probability for a magnitude 3.5 1%, the probability for a magnitude 4.0 0,3%, and the probability for a magnitude 4.5 0.1% or 10 in 100, 1 in 100, 3 in 1000 and 1 in 1000, respectively. Imposing a maximum possible magnitude reduces the probability for the higher magnitude earthquakes slightly. E.g. imposing a Mmax of 5.0, reduces the probabilities for a single earthquake of magnitudes 3.5, 4.0 and 4.5 to 0.96%, 0.28% and 0.07%, respectively. Based on the data from all fields in the Netherlands for the full period since 1996, a maximum probable magnitude of 3.9 was calculated. The fact that from the dataset of all induced earthquakes an indication for a maximum possible magnitude can be drawn, while the Groningen induced seismicity is non-discriminative for a maximum possible magnitude implies that deriving conclusions for individual fields on the basis of an analysis of the combined seismicity from a number of fields can be problematic. 2

Hence, for every 10 tremors with a magnitude equal to or above1.5 there is on average one tremor with a magnitude equal to or above 2.5.

17


Relation between production and seismicity Figure 10 shows the production and seismicity since 1996. The production shows a clear annual cycle of low production in summer and increased production in winter. In addition, the increase in annual seismicity is clearly visible, with higher magnitude earthquakes occurring later in time (as of 2003). Of particular interest is the observation that higher magnitude earthquakes seem to occur 6-9 months after the peak winter production period. 8

5 ,0 4 ,5

7

3 ,5 5

3 ,0

4

2 ,5

magnitude

maandelijkse productie (normal bcm)

4 ,0 6

2 ,0

3

1 ,5 2 1 ,0 1

0 ,5

0 1996

0 ,0 1998

2000

2002

ma a n d e lijks e p ro d u c tie

2004

2006

2008

2010

2012

g e r e g is tre e r d e a a r d b e v in g e n me t ma g n itu d e >=1 .5

Figure 10: The monthly production since 1996 and the detected seismicity versus time. A clear summer/winter cycle can be seen in the production, as well as an increase in annual seismicity. Of particular interest is the observation that higher magnitude earthquakes seem to occur with a delay of 69 months following a winter peak production period.

In order to determine a possible relation between production and seismicity, Figure 11 shows both the annual production (in normal BCM) and the annual number of earthquakes of magnitudes 1.5 and higher. The annual production rates have been decreasing between 1996 and 2001. From 2001 the annual production shows an increasing trend, reaching 50 normal BCM in 2010, with only a relatively low production of 23 normal BCM over the winter of 2006/2007. The annual number of earthquakes with M≥1.5 is reasonably steady up to 2002. From 2003 onwards the annual number of earthquakes with M≥1.5 is also increasing. Note that the low production over 1-7-2006/1-7-2007 was followed by a low annual number of earthquakes in the year 1-7-2007/1-7-2008. An attempt has been made to find a conceptual model and an equation relating the annual number of earthquakes with magnitude equal to or above 1.5 (the “seismicity”) to production history. From a physics point of view it is hypothesised that the total amount of (differential) compaction due to cumulative production and production rate plays a key role. The model and equation should corroborate the historically observed seismicity within the intrinsic statistical uncertainty: • • •

no seismicity prior to 1986 more or les constant seismicity at 3 – 5 earthquakes/year between 1993 and 2003 increasing seismicity for the years thereafter

18


60 annual production annual seismcity

Annual number of events M>=1.5 Annual production (BCM/yr)

50

40

30

20

10

1-7-2012

1-7-2011

1-7-2010

1-7-2009

1-7-2008

1-7-2007

1-7-2006

1-7-2005

1-7-2004

1-7-2003

1-7-2002

1-7-2001

1-7-2000

1-7-1999

1-7-1998

1-7-1997

1-7-1996

1-7-1995

1-7-1994

1-7-1993

1-7-1992

1-7-1991

1-7-1990

1-7-1989

0

Figure 11: Both the annual production and the annual number of earthquakes (periods ranging JulyJuly) with magnitude of 1.5 or higher are shown against time.

Figure 12: Illustration of the effect of the rate-type compaction model on the subsidence and its implications for the seismicity. The rate-type compaction model introduces an initial delay in the compaction in response to the pressure depletion. This leads to less subsidence than predicted on the basis of a linear compaction model, the so called delayed compaction. The subsidence thus predicted agrees well with the observed subsidence at the deepest point of the Groningen subsidence bowl. After a so called “transition zone” the amount of compaction in response to the additional pressure depletion equals the linear compaction response, hence the two subsidence lines become close to parallel. The onset of seismicity of magnitudes 1.5 and higher is estimated at the end of the transition zone, which equals the total production in 1984 with a 1 year delay, hence as off 1985.

This has led to a model and a preliminary version of an equation in which the annual number of seismic earthquakes is linked to the observed non-linear compaction behaviour of the Groningen reservoir rock [2,3]. To do so, use is made of a rate type compaction model formalism as described in references [4,5,6,7]. An alternative NAM model describing the 19

Confidential Final Report, dd. 16-01-2013 non-linear Groningen compaction/subsidence behaviour in terms of a characteristic response time [3] is likely to predict similar behaviour. Increasing depletion and changes in depletion rate (caused by changes in production rate) in both models lead to a delayed (strain) response of the reservoir rock (Figure 12). Next the assumption is made that the seismicity is proportional to the amount of delayed (inelastic) strain. On the basis of the rate type compaction model formalism this then yields the following equation: Nj (M≥1.5) = C x (Qcumj-1 - Qcumref) x [ (Qdotj-1/Qdotref)b - 1]

(4)

with C Nj (M≥1.5) Qcumj-1 Qcumref Qdotj-1 Qdotref b

proportionality constant (normal BCM-1) annual number of earthquakes with M≥1.5 in year j cumulative production in year j-1 (July to July) (normal BCM) cumulative production (normal BCM) at the start of seismiciteit production rate in year j-1 (July to July) (normal BCM/year) production rate in (normal BCM/year) below which no earthquakes with M≥1.5 occur rate sensitivity constant (0.015 for Rotliegend sandstone [4])

Fitting equation (4) by adjusting Qdotref suggests that a production rate of 12 normal BCM/year will result in 0 -1 earthquakes/year with a magnitude equal to or above magnitude 1.5. The first earthquake observed within the Groningen field was in December 1991 at a reservoir depletion of 145 bar. The earthquake had a local magnitude of 2.4 (Figure 12). At the time of the event, the seismic network was very sparse and its detection limit was magnitude 2.5 and higher. At later stages during the Groningen seismicity history, earthquakes of local magnitude 1.5 have occurred at depletions as low as approximately 122 bar (Figure 13). This threshold corresponds to the end of the transition zone predicted by the rate-type compaction model. In addition, this depletion threshold also agrees reasonably with the findings of [8], where a depletion threshold of 112 bar was found for all gas depletion induced seismicity in the Netherlands. It is therefore reasonable to assume that earthquakes with magnitudes of 1.5-2.5 have been occurring prior to the first earthquake detected and that the depletion threshold equals this 122 bar. Hence, Qcumref for earthquakes of magnitude 1.5 or higher in the Groningen field corresponds to the cumulative production at 122 bar depletion (~1000 BCM; Figure 12). The 1-year delay between production and seismicity was derived from a statistical cross-correlation analysis on the data in Figure 11. The delay could be related to the time it takes a pressure drop to travel from the production clusters to the central area of the Groningen field where many of the earthquakes occur. The proportionality constant C plays a similar role as the seismogenic index in [9]. For the Groningen field C turns out to be approximately equal to one. The explanation for this is probably that Qdotref is used to calibrate equation (4) to the observed seismicity rates. Hence, equation (4) effectively contains only 1 free adjustable parameter for fitting to the data. Applying equation (4) to the historical Groningen production gives calculated seismicity rates that correspond to the observed numbers within the intrinsic statistical uncertainty as shown in Figure 14.

20


σz,eff=422 bar Hence, dP=122 bar

Figure 13: The earthquakes of local magnitude greater or equal to 1.5 commence to occur at effective stresses of 422 bar corresponding to a depletion of 122 bar. This corresponds to the end of the transition zone of the rate type compaction model. 35

observed 30

predicted predicted due to already realised production

Nm > 1,5

25

20

15

10

5

0 7-5-1990

31-1-1993

28-10-1995

24-7-1998

19-4-2001

14-1-2004

10-10-2006

6-7-2009

1-4-2012

27-12-2014

Figure 14: Modelled expectation value for the annual number of earthquakes based on equation (2) for the historic Groningen production. The error bars provide the confidence intervals of the predicted number of earthquakes based on a 95% confidence interval. The historically observed annual number of earthquakes is given in orange. The green point represents the prediction for the period 1-7-2012/1-72013 due to the already realised production in the period 1-7-2011/1-7-2012 with its confidence interval corresponding to a 95% confidence level. Note that the number of observed earthquakes is incomplete for the years preceding 1996.

Equation (4) even suggests that for each given magnitude M there could exist a production rate Qdotref below which no earthquakes above that magnitude will occur. The lower M, the lower the corresponding production rate will be. Hence, there might be a production rate dependent magnitude which acts as a bounding maximum magnitude in the BGR when deriving the probability for an earthquake with a particular magnitude to occur based on the 21

Confidential Final Report, dd. 16-01-2013 computed expectation number of annual earthquakes with M≥1.5. Based on the seismicity data, the following speculative relationship between such a bounding maximum magnitude and the production rate is guestimated: M’max= (Qdotj-1/Qdotref)*1.5

(5)

Equation (5) implies that at a rate twice the reference rate, no earthquakes above magnitude 3 will occur. As indicated previously, there have been two intervals in the production history during which the annual production was lower than 25 normal BCM/yr while seismicity was occurring: 1-7-1999/1-7-2002, and 1-7-2006/1-7-2007 (see Figure 11). Equation (5) would imply that during the periods with production less than 25 BCM/yr no earthquakes with magnitudes larger than approximately 3.0 could occur This is consistent with the data as shown in Table 3. Taking into account the one year delay, the observed maximum magnitude in the corresponding seismicity periods are consistently lower than the predicted bounding maximum magnitude. This is despite the fact that on the basis of an unbounded GR and the expectation amount of earthquakes with M≥1.5 predicted on the basis of equation (2), a significant expectation value for the probability of earthquakes with magnitudes larger than M’max are calculated (up to 90%). Table 3: Comparison between the bounding maximum magnitude derived from equation (5) and the maximum magnitude detected during 2 periods in which the annual production rate has been less than 25 normal BCM/yr. The observed maximum magnitude is consistently lower than the predicted bounding maximum magnitude despite the fact that on the basis of an unbounded GR and the amount of seismicity with M≥1.5 predicted significant expectation values for probabilities of earthquakes with magnitudes larger than M’max exist (up to 90%). period

Annual production (BCM/yr)

M’max based on equation 3

1-7-1999/1-7-2002 1-7-2006/1-7-2007

22.2-24.0 23.5

2.7-3.0 2.9

Maximum magnitude detected in period +1 year delay 2.2 2.2

In conclusion, the preliminary equations derived in this section suggest that the expectation value for the probability for an earthquake with a particular magnitude to occur is determined by 1) the expectation value for the seismicity rate derived from the predicted expectation number of earthquakes as computed by equation (4), 2) the slope (b-value) of the BGR determining the variation of the number of earthquakes of different magnitudes (for Groningen b equals 1) and 3) a production rate dependent bounding maximum magnitude as guestimated by equation (5). A strong word of warning needs to be given here. The work on the relation between seismicity and delayed compaction via cumulative production and production rate is still in progress. In addition, the number of data points is limited resulting in significant intrinsic statistical uncertainties. Equations (4) and in particular equation (5) need to be treated with care when using them to predict future seismicity. Given its speculative nature equation (5) has not been used for the further analysis presented in this report.

22


Consequences for future earthquakes in Groningen The expectation probability for an earthquake of a particular magnitude to occur e.g. in the next year depends on the derived b-value of the BGR relation, the assumed value for the maximum possible magnitude and the expectation value of next year’s number of earthquakes with magnitude 1.5 or higher. Predictions for the expectation value of the number of earthquakes of M≥1.5 are derived from equation (4). The assumption is that future seismicity follows a BGR, also for higher magnitudes but with an as yet unknown value of Mmax. Given the recent production rates and the production level expected for the coming years, equation (5) is not relevant in this analysis (a production rate of 50 normal BCM per year would lead to a M’max of 6.3). In a previous section, it was demonstrated that no maximum possible magnitude can be derived on the basis of the Groningen seismicity data. This does not imply that such a maximum value does not exist. In fact, it is highly likely that there is such a maximum, despite the fact that it cannot be derived from the Groningen earthquake data. Perhaps that non-seismic methods can be applied to obtain estimates for the maximum possible magnitude. This could include estimates based on the maximum percentage of the stored elastic energy that can be released in a single earthquake. Or an upper limit based on an analysis of the distribution and size of faults present in the field. At the moment such results are not available for Groningen. According to KNMI [17], a recent analysis of all known gas production induced earthquakes globally, shows that no induced seismic earthquakes of magnitudes larger than 5.0 have been reported so far. Based on the b value of 1 derived from the Groningen dataset, the expectation value for the probability of earthquakes at such a magnitude level in Groningen is low. This is because the expectation value of the total number of earthquakes with M≥1.5 expected to occur during the total Groningen field life is estimated to be well below a thousand.

Expectation probability for larger magnitude earthquakes due to already realised production Figure 15 shows the relation between the expectation value for the probability and maximum possible magnitude for an earthquake of magnitude of 3.9 or higher and 4.5 or higher, respectively. The calculation is based on the expectation value for the number of earthquakes as predicted by equation (4) due to the already realised production rate between July 2011 and July 2012 and the cumulative production in July 2012, which is 20 earthquakes of magnitude 1.5 or higher (green dot in Figure 14).

23


8 M >= 3.9

Probability (%)

7

M >= 4.5

6 5 4 3 2 1 0 3

3,5

4

4,5

5

5,5

6

6,5

Magnitude Figure 15: Expectation values for the probability of an earthquake of magnitude 3.9 or higher and 4.5 or higher, respectively, occurring in 2013 as a function of imposed maximum possible magnitude. The calculation is based on the expectation number of earthquakes predicted by equation (2) due to the already realised production rate between July 2011 and July 2012 and the cumulative production in July 2012, which results in an expectation value of 20 earthquakes of magnitude 1.5 or higher.

The expectation value for the probability for an earthquake with magnitude 3.9 or higher increases is 0 for imposed maximum possible magnitudes of 3.7, 3.8 and 3.9, which corresponds to the implicit assumption in the double bounded GR that it is not possible for an earthquakes to have a magnitude larger than the maximum possible magnitude. It increases up to a worst case expectation value for the probability (at Mmax=6.0) of 7.6% that one of the next 20 earthquakes will have a magnitude >3.9. For an imposed Mmax of 5.0 the expectation value for the probability for an earthquake with a magnitude equal to or above 3.9 in the next 20 seismic earthquakes in Groningen becomes 7 % and 5.8 % for an imposed Mmax of 4.5. The expectation value for the probability for an earthquake with a local magnitude of 4.5 or higher ranges from 0 (for Mmax =3.7-4.5) to almost 2% (at Mmax=6.0). For an imposed Mmax of 5.0 the expectation value for the probability is 1.4%.

Predicted earthquakes in Groningen under different production scenarios As described in the introduction, the August 2012 earthquake, with a moment magnitude of 3.6, had the largest magnitude so far. The damage caused by this earthquake was extensive compared to previous earthquakes of comparable magnitude, though not of a structural nature. The earthquake raised general concern on the level of acceptability of damage caused by induced earthquakes and led to questions whether earthquakes with even larger magnitudes, possibly causing structural damage to property, could occur in the future. The results of the analysis described in this report show a distinct possibility that larger magnitude earthquakes (M≥ 3.9) may occur, with an expectation value for the probability of up to 7.6 % for the next 20 seismic earthquakes. Hence, the question is raised whether or not the occurrence of such earthquakes might be mitigated by reducing production rates. Even though extensive further research is required to fully comprehend the mechanism and physics of the occurrence of seismic earthquakes, the preliminary results described in this report have been used to derive estimates of the number of earthquakes expected for a

24

Confidential Final Report, dd. 16-01-2013 number of different production scenarios which may be used to justify precautionary measures (under the precautionary principle) while further research is executed. 35

history predicted

prediction 50 bcm

Prediction 40 bcm

Prediction 30 bcm

Prediction 20 bcm

Observed

Prediction 10 bcm

already realised production

30

25

20

15

10

5

0 1-1-2004

31-122004

1-1-2006 1-1-2007 2-1-2008 1-1-2009 2-1-2010 2-1-2011 3-1-2012 2-1-2013 3-1-2014 3-1-2015 4-1-2016

Figure 16: Predicted annual expectation number of earthquakes based on the relation given in equation (2) for both the historic seismicity (dark blue) and five possible production scenario’s for the years 1-72012/1-7-2013 – 1-7-2014/1-7-2015 giving seismicity for the years 1-7-2013/1-7-2014 – 1-7-2015/1-7-2016. The error bars provide the confidence intervals of the predicted expectation number of earthquakes based on a 95% confidence interval. The historically observed annual number of earthquakes is given in orange.

The expectation number of annual earthquakes predicted by equation (4) for five level production scenario’s at different annual production rates is given in Figure 16. All scenario’s, except the level production at 10 bcm/yr, show an increase in annual expectation number of earthquakes during the 3 year period modelled. However, both the annual expectation number of earthquakes and its increase with time are distinctly lower for lower production rates. The scenario with a level production rate of 10 normal BCM shows no annual expectation number of earthquakes with time, as equation (5) predicts the absence of seismicity of magnitude equal or above 1.5 below a rate of 12 normal BCM/year. However, since the occurrence of seismicity follows a Poisson’s distribution, up to 4 earthquakes per year may still occur (within a 95% confidence level interval). Based on the annual expectation number of earthquakes the expectation value for the probability (%) for an earthquake with a magnitude larger than a particular magnitude M can be computed. Table 4 shows the expectation values for the probability (%) for the five scenario’s of an earthquake with a magnitude larger than 4.0, 4.5 and 5.0, respectively, to occur given an imposed maximum possible magnitude of 4.5, 5 and 6, respectively, on the basis of the total expectation number of earthquakes predicted by the scenario’s in the Groningen field for the next 4 years (1-7-2012/1-7-2016). In the computation equation (5) has not been incorporated, hence no rate dependent maximum bounding magnitude was imposed3. The highest expectation values for the probability are obtained for the highest production scenario. The lower the constant annual production level, the lower the expectation values for the probability for larger magnitude earthquakes. 3

The rate dependent maximum magnitude was not included in the expectation probability calculations since the equation is still speculative and needs further substantiation prior to its use in the expectation probability calculations.

25


Table 5 shows the same expectation values for the probability (%) for the year 1-7-2013/17-2014. As for the total annual expectation number of earthquakes, the expectation value for the probability for a larger magnitude earthquake to occur next year decreases by a factor of two, after the annual production rate is decreased by a factor of two for a twelve month period including a full winter period. As in Table 4 no rate dependent maximum bounding magnitude was applied NAM proposes a simple linear relation between cumulative production and expectation value for total number of earthquakes for the period since 2001: N (M≥1.5) = 0.32 x Qcum

(3)

With some (unspecified) delay between N and Qcum. This relation under-predicts the recently observed high annual number of earthquakes: 15 earthquakes predicted vs. 24 observed in the period 1-7-2011/1-7-2012. However, the effect of the annual production rate is comparable to that predicted by equation (4). a decrease in the annual production rate by a factor of two decreases the predicted expectation value for the annual number of earthquakes for a twelve month period by a factor of two. Hence, the expectation value for the probability for a higher magnitude earthquake in this time period is also decreased by a factor two. 100 prediction SSM relation 1 year 90

prediction SSM relation next 4 years prediction NAM relation 1 year

Number of events predicted

80

prediction NAM relation next 4 years 70 60 50 40 30 20 10 0 0

10

20 30 40 annual production rate (BCM)

50

60

Figure 17: Comparison between the expectation number of earthquakes predicted by the NAM linear correlation and the SSM rate type compaction model based equation.

26


Table 4: Expectation value for the probability (%) for an earthquake with a magnitude larger than 4.0, 4.5 and 5.0, respectively, to occur given an imposed maximum possible magnitude of 4.5, 5 and 6, respectively, provided the total expecation number of earthquakes (Nm(2012-2016)) in the Groningen field for the next 4 years is given by one of the seven production scenario’s considered for the Groningen field. The numbers in brackets correspond to the confidence intervals at a 95% confidence level. Earthquakes with magnitudes in excess of the maximum magnitude are not feasible, hence their expectation value for the probability is 0%. Mmax=4,5 scenario 50 bcm 40 bcm 30 bcm 20 bcm

93 (75-113) 80 (63-100) 65 (50-83) 45 (32-60)

P(0,M>4, 2016) 18 (15-22) 16 (13-19) 13 (10-16) 9.2 (6.7-12)

10 bcm

20 (12-31)

4.2 (2.6-6.5)

Nm (2012-2016)

Mmax=5

Mmax=6

P(0,M>4.5, 2016) 0.0 0.0 0.0 0.0

P(0,M>5, 2016) 0.0 0.0 0.0 0.0

P(0,M>4, 2016) 23 (19-28) 20 (16-24) 17 (13-21) 12 (8.7-14)

P(0,M>4.5, 2016) 6.1 (5.0-7.4) 5.3 (4.2-6.6) 4.3 (3.3-5.5) 3.0 (2.1-3.6)

P(0,M>5, 2016) 0.0 0.0 0.0 0.0

P(0,M>4, 2016) 25 (21-30) 22 (18-24) 18 (14-23) 13 (9.5- 17)

P(0,M>4.5, 2016) 8.6 (7.0-10) 7.4 (5.9-8.2) 6.1 (4.7-7.7) 4.3 (3.0-5.6)

P(0,M>5, 2016) 2.6 (2.1-3.1) 2.2 (1.7-2.5) 1.8 (1.4-2.3) 1.3 (0.9-1.7)

0.0

0.0

5.5 (3.4-8.4)

1.4 (0.8-2.1)

0.0

6.0 (3.7-9.2)

1.9 (1.1-2.9)

0.6 (0.3-0.9)

Table 5: Expectation value for the probability (%) for an earthquake with a magnitude larger than 4.0, 4.5 and 5.0, respectively, to occur given an imposed maximum possible magnitude of 4.5, 5 and 6, respectively, provided the expectation number of earthquakes (Nm (2013-2014)) in the Groningen field in the year 1-72013/1-7-2014 is given by one of the seven production scenario’s considered for the Groningen field. The numbers in brackets correspond to the confidence intervals at a 95% confidence level. Earthquakes with magnitudes in excess of the maximum magnitude are not feasible, hence their expectation value for the probability is 0%. Mmax=4,5 scenario 50 bcm 40 bcm 30 bcm 20 bcm

23 (15-35) 19 (11-30) 14 (8-23) 8 (3-14)

P(0,M>4, 2014) 4.8 (3.2-7.3) 4.0 (2.4-6.2) 3.0 (1.7-4.8) 1.7 (0.6-3.0)

10 bcm

0 (0-4)

0 (0-0.9)

Nm (2013-2014)

Mmax=5

Mmax=6

P(0,M>4.5, 2014) 0.0 0.0 0.0 0.0

P(0,M>5, 2014) 0.0 0.0 0.0 0.0

P(0,M>4, 2014) 6.3 (4.2-9.5) 5.2 (3.0-8,1) 3.9 (2.3-6.3) 2.3 (0.9-3.9)

P(0,M>4.5, 2014) 1.6 (1.0-2.4) 1.3 (0.7-2.0) 1.0 (0.5-1.6) 0.5 (0.2-1.0)

P(0,M>5, 2014) 0.0 0.0 0.0 0.0

P(0,M>4, 2014) 6.9 (4.6-10) 5.7 (3.4-8.9) 4.3 (2.4-6.9) 2.4 (0.9-4.3)

P(0,M>4.5, 2014) 2.2 (1.4-3.3) 1.8 (1.0-2.9) 1.3 (0.8-2.2) 0.8 (0.3-1.3)

P(0,M>5, 2014) 0.7 (0.4-1.0) 0,5 (0.3-0.9) 0.4 (0.2-0.7) 0.2 (0.1-0.4)

0.0

0.0

0 (0-1.1)

0 (0-0.3)

0.0

0 (0-1.2)

0 (0-0.4)

0 (0-0.1)

27


Conclusions 1.

2.

3. 4.

5.

6.

7.

8.

In the Groningen field the annual number of gas production induced earthquakes and their released energy are increasing with time. For Groningen this leads to a higher expectation value for the probability for the occurrence of higher magnitude earthquakes. A Monte Carlo analysis shows that it is not possible to determine a value for Mmax on the basis of the Groningen seismicity data other then that its value is above 3.6. This does not imply that an upper bound does not exist. Mmax values above 3.9 cannot be excluded without additional estimates based on non-seismic methods. These are not available for Groningen. As Mmax for Groningen cannot be determined at the moment, the probability for an earthquake with magnitude 3.9 or higher to occur during the next twelve months is poorly defined. The worst case expectation value for the probability imposing an Mmax of 6.0 is approximately 7.6%. For an imposed Mmax of 5.0 this becomes 7 %, 5.8 % for an imposed Mmax of 4.5 and 0 % for an imposed Mmax of 3.9. The expectation value for the probability for an earthquake with magnitude 4.5 or higher during the next 12 months is between 0 and 2%. A preliminary version of an equation has been found that predicts the expectation number of annual earthquakes with a magnitude equal to or above 1.5 - and its variation over time - in terms of cumulative production and production rate. The equation is related to a (rate type) compaction model that can be used to properly describe the observed non-linear compaction behaviour of the Groningen field. On this basis SSM has developed an approach that predicts the observed seismic behaviour of the Groningen field within the intrinsic statistical fluctuations. The b-value derived from the Gutenberg Richter relationship for the Groningen field (b = -1) is combined with the above equation and an assumption on the value of the maximum possible magnitude Mmax in Groningen. The same approach can be used to calculate the expectation value for the probability for the occurrence of an earthquake above a given magnitude during a given time period in the future. The expectation value for the probability for a larger magnitude earthquake (M>3.9) might be decreased by approximately a factor of two, by decreasing the annual production rate by a factor of two compared to the current production rate of around 50 normal BCM per year, followed by a gradual decline. Even then a significant expectation value for the probability for a larger magnitude earthquake remains. Based on the derived preliminary version of the relation between the annual expectation number of earthquakes and the production, the production rate would have to be lowered to values around 12 BCM/year in order to achieve minimal risk. It is therefore possible that at this production rate almost no earthquakes with magnitudes ≥1.5 would occur after a number of years.

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References [1]

Dost, B., Goutbeek, F., van Eck, T., Kraaijpoel, D., 2012. Monitoring induced seismicity in the North of the Netherlands: status report 2010, KNMI scientific report; WR 2012-03

[2]

NAM, 2010. Bodemdaling door Aardgaswinning, NAM velden in Groningen, Friesland en het noorden van Drenthe, NAM B.V. EP201006302236: 43 pp

[3]

NAM, 2012, Ketelaar, V.B.H., Van der Veen, W. & Doornhof, D., 2011. Monitoring effecten van bodemdaling op Ameland-Oost, evaluatie na 23 jaar gaswinning. In: Publicatie Begeleidingscommissie Monitoring Bodemdaling Ameland, oktober 2011: 9-29

[4]

De Waal, J.A., 1986. On the rate type compaction behaviour of sandstone reservoir rock. PhD dissertation, Delft University of Technology, The Netherlands: 166 pp.

[5]

Bjerrum, L., 1967. Engineering geology of Norwegian normally consolidated marine clays as related to the settlements of buildings. Geotechnique 17: 81-118

[6]

Kolymbas, D., 1977. A rate-dependent constitutive equation for soils. Mechanical Research Communications, 4: 367-372

[7] Den Haan, E.J., 1994. Vertical compression of soils. PhD dissertation, Delft University Press: 96 pp. [8]

Van Thienen-Visser, K., Nepveu, M. en Hettelaar, J., 2012. Deterministische hazard analyse voor geïnduceerde seismiciteit in Nederland, TNO-rapport 2012 R10198,

[9] Shapiro, A.A., 2010. Seismogenic index and magnitude probability of earthquakes induced during reservoir fluid stimulations, The Leading Edge, 29(3):304 [10] Gutenberg, R., and C.F. Richter, 1944. Frequency of earthquakes in California, Bulletin of the Seismological Society of America, 34, 185-188 [11] Gutenberg, B and C.F. Richter, 1954. Seismicity of the Earth and Associated Phenomena, 2nd ed. (Princeton, N.J.: Princeton University Press, 1954). [12] Abercrombie, R.E., and J.N. Brune, 1994. Evidence for a constant b-value above magnitude 0 in the southern San Andreas, San Jacinto, and San Miguel fault zones and at the Long Valley caldera, California, Geophys. Res. Lett., 21 (15), 1647-1650 [14] Bhattacharya, P., Chakrabarti, B.K., Kamal, and Samanta, D., 2009. "Fractal models of earthquake dynamics", Heinz Georg Schuster (ed), Reviews of Nonlinear Dynamics and Complexity, pp. 107–150 V.2, Wiley-VCH, ISBN 3-527-40850-9 [15] Pelletier, J.D., 2000. "Spring-block models of seismicity: review and analysis of a structurally heterogeneous model coupled to the viscous asthenosphere" Geocomplexity and the Physics of Earthquakes, American Geophysical Union, ISBN 0-87590-978-7 [16] Kramer, S. L., 1996. Geotechnical earthquake engineering, Prentice-Hall [17] Dost, B. en Kraaijpoel, D., 2013. The august 16, 2012 earthquake near Huizinge (Groningen), KNMI scientific report. [18] Presentation: “Induced Seismicity”, NAM, presented 18th December 2012, shared 21st December 2012.

29


Appendix A: October 8th SSM technical assement Summary of technical assessment as presented by SSM on the 8th of October to KNMI, TNOAGE and the NAM SSM observations: 1. Gas production induced tremors in the Groningen field have been observed since the early nineties. Since 1996 completeness of the recording network has been achieved for magnitudes above 1.5 (be it with limited redundancy). 2. No tremors have been observed in the Groningen gas field prior to 1991, at that time the average reservoir pressure had dropped by some 150 bar. 3. Lower magnitude tremors (e.g. below 2.0) might well have occurred prior to 1991. 4. On the 16th of august 2012, the highest magnitude Groningen gas production induced tremor to date took place near Huizinge. It had a moment magnitude of 3.6. 5. Pressure differences within the field were significant during the early production period, subsequently they were strongly reduced, recently they are increasing again. 6. Pressure differences in the field are calculated using subsurface models and production data. Experience (e.g. 4D seismic elsewhere) demonstrates that uncertainties are usually way larger then initially considered possible (we start to believe our own models beyond reason). In particular the effects of faults not seen on seismic, fault transmissibility, barriers, baffles and thief zones can be large. 7. The Frequency Magnitude analysis applied by KNMI assumes an underlying stationary process. This is usually valid for tectonically driven seismicity but questionable for gas production induced seismicity. 8. Differential compaction over faults with unfavourable geometries is the likely engine behind the induced seismicity in Groningen. The induced stresses build up as a result of differential compaction and are locally (partially?) released when tremors occur. 9. As cumulative production from the Groningen field increases, the strength of the engine behind the induced seismicity increases in strength over time until a steady state situation is realised with more or less equal amounts of build-up and release of differential stresses. 10. It is not clear that such a pseudo steady state has been arrived at, the tremor data suggests this in not yet the case. 11. There has been a steady non-linear increase in the annual number of tremors since 1991. This is true for the total number of tremors and also for the different magnitude classes. 12. There has been an increase in the released seismic energy over time with a break around 2003 and possibly another break around 2012. 13. Production rates in Groningen have varied considerably over time. 14. So far these non-stationary aspects have not been taken into account in the seismic risk analysis. This could have a significant effect and needs to be sorted out. An example of not accounting for such effects is seen when cumulative annual frequencies are derived for two different time windows during the Groningen field life. 15. Not accounting for non-stationary effects could explain the observed curvature at higher magnitudes in the Frequency-Magnitude plot for the full Groningen production history. The curvature would then not be related to a maximum possible tremor magnitude. 16. There has been a marked increase in the frequency of tremors with a magnitude above 3.0. Before 2003 tremors of such magnitude were not observed. Since 2003 they have occurred almost annually. 17. There are clear indications that variations in the production rate have an large influence on the number of tremors observed in the following year. The data suggest that there is a delay of between one and two years between a change in production rate and its impact on the tremor frequency. In particular acceleration and deceleration seem to play a significant role. 18. Based on the data available to date it cannot be excluded that tremors with magnitudes higher than the previously estimated maximum of 3.7/3.9 can occur in the future.

30

Confidential Final Report, dd. 16-01-2013 What needs to be solved / can possibly be solved / cannot be solved: 1. Full deterministic prediction of the induced seismicity based on modelling or monitoring is not considered possible. Any predictions will remain of a statistical nature, at best providing the probability/frequency of tremors of a given magnitude as a function of time. 2. It might be possible to incorporate the effects of increasing seismicity over time as cumulative production increases. Possibly a model can be developed to calculate the impact of production, production rate, pressure differences etc. on these probabilities. Potentially this could include the effects on the likely maximum magnitude to be expected during the field life and the period shortly thereafter. 3. Will the frequency of tremors continue to increase as production of the Groningen field continues? This seems likely given the observations. It also suggests similar increases for the different magnitude classes. 4. What is the maximum magnitude that could occur in the future? A clear answer cannot be provided at the moment. Such a maximum could be linked to the maximum energy available if the tremors are fully induced without impact of local tectonics. The magnitude can also be limited by the maximum size of the largest fault present in the ensemble of affected faults. The maximum ride slip could be different for compaction-induced tremors compared to tectonically driven events. The tremor data cannot be used to exclude the possibility of future tremors with magnitudes above 3.7 / 3.9. 5. Is the apparent effect of production rate changes on seismic frequencies not of a statistical nature? If not, can it be quantified and captured in a model? SSM proposed starting point conceptual model: Based on the data available and preliminary analysis carried out on this data SSM propose a starting point conceptual model for the induced seismicity in Groningen. It goes as follows: 1. Differential compaction over faults with unfavourable geometries provides the engine for the induced seismicity. 2. The (traditional) Gutenberg Richter relationship/model remains valid throughout field life to describe the relative probability of tremors as a function of magnitude at any given moment in time (for the relative probabilities at each particular moment in time). Background is the fact that the number of faults and their (assumed log-normal Gaussian) distribution does not change over the production time period. And that all faults simultaneously feel the effects of the increasing production -> increasing pressure drop -> increasing (differential) compaction. 3. No tremors are initially observed, simply because there is not enough differential compaction during the early production period to generate observable events. This effect is further enhanced by the non-linear compaction behaviour of the Groningen reservoir, further reducing compaction during early field life (De Waal et al, 2012). Given the very low number of low magnitude tremors at this stage (if any), the probability for higher magnitude events at the time was virtually zero (and none were actually observed). 4. The increasing strength of the engine over time implies that faults that slip at later stages occurs when more differential compaction has accumulated. This is enhanced by the time dependent compaction behaviour resulting in larger amounts of (differential) compaction per unit of production during later field life. This explains why magnitudes increase over time. Or actually why the total number of annual tremors increases and therefore via Gutenberg Richter also the absolute probability for higher magnitude events. 5. In that respect the observation that boundary faults have not yet generated observable tremors could be a concern. Alternatively it could be that induced stresses from differential compaction can relax non-seismically at boundary faults e.g. due to the presence of salt. 6. The total number of tremors in a given year (the seismicity level) varies over time. This is firstly caused by the increasing differential compaction over unfavourable fault geometries as cumulative production and hence compaction increase over time. Using the Gutenberg Richter model to calculate/predict annual frequencies is not valid if not correcting for this effect.

31

Confidential Final Report, dd. 16-01-2013 7.

8.

Secondly, changes in production rate during the field production history will have an effect, be it solely from the speed with which the “movie” is played. E.g. when increasing the production rate threefold, it can be expected that the annual number of tremors will also triple. This will not increase the total number of tremors of a given magnitude over the total production period as the increased production rate will shorten the field production period proportionally. But not accounting for this “movie frame-rate” effect will cause significant differences between observed frequencies during high production rate periods and predicted frequencies, when these predictions are based on data from a preceding low production rate period. Thirdly, the available data suggests a significant effect of changes in the production rate above and beyond the “frame-rate” effect. The physical background could be that differential stresses building up due to increasing differential compaction might be able to relax micro-seismically or non-seismically when build up rates are slow and hence more time for relaxation is available. At higher production rates there would not be enough time for the non-seismic relaxation mechanisms to reduce the stresses significantly, causing the tremors to “hang” for longer periods and resulting in higher magnitude event when they eventually go. Alternatively or in addition, higher deformation rates results in increased friction angles over the fault zones, enhancing the process (e.g. Dieterich 1987, Runia 1983).

Summarising: 1. Differential (time dependent) compaction over unfavourable fault geometries is the engine driving the seismicity 2. Gutenberg Richter remains valid to describe relative frequencies for tremors with different magnitudes at a particular given moment in time 3. The total number of events per unit of time or per unit of production increases with increasing total cumulative production 4. The number of events can increase or decrease at a given time due to the “frame-rate” effect and a relaxation-mechanism related loading rate effect 5. In particular accelerations and decelerations seem to correlate very well with changes in seismicity 6. There is a delay between changes in production rate and the impact on the tremor frequencies 7. At the total number of tremors increases with time, so does the probability for larger magnitude events and hence they start to occur 8. All these effects need to be taken into account when predicting tremor frequencies 9. Whether of not there is a maximum magnitude for the induced tremors remains unresolved at this stage Proposed way forward: 1. Investigate if there are measures that can already be taken now to prevent or reduce the risk for and the magnitude of induced seismicity in Groningen. 2. Realise that short term measures could also worsen things as a result of incomplete understanding. An example is where existing pressure differences within the field could in some cases have a stabilising effect. On the other hand unjustified postponement of actions also poses risks. 3. Using available data and knowledge investigate short term (3 months?) what can be concluded on the induced seismic behaviour of Groningen. Investigate the potential dependence on production, production rate, production rate changes, reservoir pressure, reservoir pressure differences, stress (changes), (time-dependent) reservoir compaction, geometry, time etc. 4. Test the validity of the proposed SSM conceptual model against these results. 5. From the above derive any conclusions that can be made with respect to the induced seismicity to be expected in the future (frequencies and magnitudes).

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Confidential Final Report, dd. 16-01-2013 6. 7. 8. 9.

10.

Extend the modelling work to assess the impact of different types of tremors, of different duration and at different magnitude levels on different types of buildings. Repeat the assessment of potential measures once the results of 2-4 are available. Increase monitoring of the Groningen seismicity both near surface and at reservoir level. Investigate possible links between the time dependence in the Groningen subsidence behaviour and the observed thresholds in seismicity. In this context look at the potential merits of using rate and state type constitutive models to describe the compaction and seismic behaviour of the Groningen reservoir. Investigate the feasibility of the proposed SSM conceptual model, improve or modify the model over time as appropriate.

We should start thinking about: 1. What could be done now to reduce the tremor and the risk they create? 2. How must the present seismic hazard risk analysis for Groningen be updated to account for the effects of increasing production and production rate (frame-rate effect)? 3. What about the effect of changes in loading rate observed above and on top of that? 4. What work needs to be done to advise on the December Winningsplan? 5. What data is required for that and when can NAM provide that data? 6. Do we need a “Hand on the Tap” type of approach for Groningen? References 1. Dieterich, J.H., 1978. Time-dependent friction in rocks and the mechanics of stick-slip, Pure Applied Geophysics, 116, pp 790 - 806 2. Runia A, 1983, Slip Instability and State Variable Friction Laws; Journal of Geophysical Research, Volume 88, No. B12, pp 10359 – 10370 3. Waal, de et al, 2012, The effective subsidence capacity concept: How to assure that subsidence in the Wadden Sea remains within defined limits? Netherlands Journal of Geology, special Wadden sea issue, Netherlands Journal of Geosciences, No 91 – Vol. 3, pp 385 - 399

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Appendix B: Summary of peer review workshop outcomes A peer review workshop was held on the 8th and 9th of November 2012 with experts from Shell, NAM, TNO-AGE, KNMI and SSM. The objective was to review the work presented in the first few chapters of this report. The outcome of the peer review is presented in the table below. 1

2

3

4

SodM position prior to workshop

Workshop outcomes

SodM position after the workshop

Both the annual rate and the maximum magnitude of tremors in Groningen are increasing.

No agreement could be reached. Further statistical testing was recommended.

Both the annual rate and the maximum magnitude of tremors in Groningen are increasing.

The area where most of the tremors occur is expanding and corresponds to the area where the largest subsidence occurs.

The area in the Groningen field where most of the seismicity occurred corresponds to the area where the largest pressure drop and-or pressure gradients occurred.

Seismicity in Groningen increases with increasing cumulative production.

Most experts agree that the Groningen seismicity is not a stationary process in time. Some feel it needs to be statistically tested The data suggests a probable relation between production The data suggests a probable relation between annual production and annual number of events (at a 14% significance level with a rate and seismicity (at a 20% significance level). 0-3 year timelag-window).

5

Groningen seismicity is not a stationary process.

6

The varying Groningen seismicity is not taken into account in a (Gutenberg Richter) annual frequency analysis which only applies to stationary seismicity processes.

Most experts agree that the Groningen seismicity is not a stationary process in time. Some feel it needs to be statistically tested. An analysis for the maximum probable magnitude based purely on the Groningen seismicity data has so far not been done due to the small number of earthquakes.

SodM supports statistical testing. In our opinion it is unlikely that the results will change our position. The area where most of the tremors occur corresponds to the area with the largest compaction/subsidence. The largest pressure drop corresponds in general with the largest subsidence. In hindsight the area around the southern production clusters is a clear exception with large pressure drops and few tremors. Also there seem to be two maximum compaction areas, both reflected in the seismicity. Hence the move back to our original position. Seismicity in Groningen increases with increasing cumulative production. No change in position, most experts agreed. The data suggests a probable relation between annual production and annual number of events. No change in our position as it was agreed at the workshop that there is only a one-in-seven chance that the relationship found is coincidental. There is a 75% chance that the timelag is one year. Groningen seismicity is not a stationary process. No change in position, most experts agreed. The varying Groningen seismicity cannot be taken into account in the KNMI annual frequency-magnitude relationship which only applies for a stationary seismicity process. KNMI prefers to use the term “annual frequency-magnitude relationship”. Minor changes, no analysis refuting the SodM position was presented.

34

Confidential Final Report, dd. 16-01-2013 7 8

9 10

11

12

This leads to deviations in the calculated annual frequencies, in particular for higher magnitudes. The downward curvature in the calculated Gutenberg Richter annual frequency relation suggesting a maximum possible magnitude of 3,7 / 3,9 is an artefact of the analysis method. The alternative approach applied by SodM is not sensitive to varying seismicity levels in time. Results show a constant ratio (b-value -1) between tremors of different magnitudes, independent of seismicity levels or time. Each unit increase in magnitude reduces the probability by a factor of 10: This is valid for all Groningen tremors, including the largest magnitude events. While its existence at some level is likely, a maximum magnitude thus cannot be derived from the available Groningen seismic data. Its minimum value is 3,9 and probably above 4,5.

13

Conclusions on Mmax on the basis of statistics from multiple fields is problematic.

14

There is a 5-10% probability of a magnitude 3.9 event occurring in the next year.

Not challenged The data of all fields in the Netherlands has been used to derive a maximum probable magnitude. The result of the current analysis indicates a 10-15% probability that the maximum magnitude is above 3.9*. Not challenged, but some experts not convinced. Not challenged, agreed by all experts.

This leads to deviations in the calculated annual frequencies, in particular for higher magnitudes. The downward curvature in the calculated annual frequency-magnitude relationship suggesting a maximum possible magnitude of 3,7 / 3,9 is caused by the deviations in the calculated annual frequencies. Change in wording to better clarify our position. The alternative approach applied by SodM is not sensitive to varying seismicity levels in time. Results show a constant ratio (b-value -1) between tremors of different magnitudes, independent of seismicity levels or time.

Not challenged, agreed by all experts.

Each unit increase in magnitude reduces the probability by a factor of 10. This is valid for all Groningen tremors, including the largest magnitude events.

The magnitude and validity of the largest probable event need to be reviewed by KNMI in the light of the latest data, using the Monte Carlo method as in previous studies within the framework of internationally accepted methods of probabilistic seismic hazard assessment. This work should be peer reviewed by independent experts. Until results of this analysis are available no seismologically based statements on the maximum probable magnitude for Groningen should be made. Derivation of a maximum probable magnitude for a specific field on the basis of statistics from multiple fields is intrinsically problematic.

Until results of the KNMI Monte Carlo analysis are available no seismologically based statements on the maximum probable magnitude for Groningen should be made. SodM agrees to add a Monte Carlo analysis** but expect limited impact. Preliminary Monte Carlo analysis by SodM confirms seismologically based statements on the maximum probable magnitude cannot be made***. Derivation of a maximum probable magnitude for a specific field on the basis of statistics from multiple fields is intrinsically problematic. Essentially a re-wording of the earlier position. There is a 5-10% probability of a magnitude 3.9 event occurring in the next year.

No agreement could be reached

No analysis was presented refuting the SodM analysis. The position was strengthened by the workshop agreement that no statements should be made on a maximum probable magnitude. The present analysis could not be made reliably at an earlier stage given a then still more limited dataset and the required statistical significance.

15

The present analysis could not be made reliably at an Not challenged earlier stage given a then still more limited dataset and the required statistical significance. * 2010 KNMI, using data from all fields in the Netherlands ** The Monte Carlo analysis must honour the varying seismicity levels. Otherwise deviations in the calculated annual frequencies will occur again, invalidating conclusions. *** All workshop experts agreed that the benefits of constraining seismological analyses using geomechanics should be investigated.

35

Nederlandse Aardolie Maatschappij B.V.

Schepersmaat 2 Postbus 28000 9400 HH ASSEN Telefoon: (0592) 369111 Telefax : (0592) 362200

www.nam.nl

Staatstoezicht op de Mijnen t.a.v. de Inspecteur-generaal de hoogedelgestrenge heer J.W. de Jong M.Eng Postbus 24037 2490 AA Den Haag

Uw ref:

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Onze ref: EP201301221641

Assen,

21 januari 2013

Onderwerp: Actualisatie seismologische inzichten Groningenveld

Geachte heer De Jong, In aanvulling op NAM’s brief aan de Minister van 21 december 2012 (met kenmerk 201212205148) wil ik U graag nader berichten over de seismologische inzichten rond de maximale magnitude van aardbevingen in het Groningen veld. NAM heeft maatregelen genomen en zal verdere maatregelen nemen die naar onze mening redelijkerwijs verwacht kunnen worden met als doel de schade door aardbevingen zo veel mogelijk te beperken, zoals nader uiteengezet in deze brief. Dit betreft zowel maatregelen om op de langere termijn aardbevingen te voorkomen of te beperken als korte termijn maatregelen om schade als gevolg van aardbevingen te voorkomen of te beperken. De bevingen op 15 en 16 augustus hebben nogmaals duidelijk gemaakt dat wij te maken hebben met een serieuze zaak. Uit de vele emoties die zijn geuit blijkt dat inwoners zich onveilig hebben gevoeld in hun eigen leefomgeving. Vanzelfsprekend betreuren wij dat ten zeerste en heeft dit geleid tot een veel persoonlijkere aanpak van bijvoorbeeld de schadeafhandeling. Wij realiseren ons dat de recente nieuwe inzichten en de aanstaande periode van nieuwe studies tot meer vragen en mogelijk ook tot een gevoel van onveiligheid zullen leiden. De NAM zal zich dan ook maximaal inspannen de hierboven genoemde passende maatregelen te nemen en om met de de bevolking in de regio te communiceren, naar hen te luisteren en hen te voorzien van relevante informatie. Nieuwe inzichten 1

In eerder verschenen rapporten zijn de geïnduceerde aardbevingen op Nederlands grondgebied geanalyseerd op basis van de totale set historische aardbevingen, dus inclusief andere gasvelden dan het Groningen veld. Deze analyses gaven aan dat de maximale magnitude M=3,9 zou zijn, met een relatief beperkte onzekerheidsmarge rond deze waarde.

1

Onder meer: KNMI, Monitoring Induced Seismicity in the North of the Netherlands: status report 2010, juli 2012. Nederlandse Aardolie Maatschappij B.V. is statutair gevestigd te 's-Gravenhage - Handelsregister no. 04008869

Op basis van een recente statistische analyse van de historische aardbevingen in alleen het 2 Groningen veld (in tegenstelling tot de totale set) en een externe review daarop is NAM van mening dat: 1. het op basis van alleen statistische analyse op de relatief beperkte hoeveelheid data van het Groningen veld niet mogelijk is een betrouwbare inschatting te maken van een maximale magnitude, en er een kans van meer dan 50% is dat er gedurende de resterende Groningen productieperiode, die nog meer dan 50 jaar zal duren, één of meer aardbevingen zullen optreden met een sterkte van meer dan 3,9, en 2. er verder onderzoek nodig is naar de kans van het optreden van aardbevingen met hogere magnitudes, de maximaal te verwachten magnitude, en de hierbij te verwachten schade contouren. NAM voert verdere studies uit met het doel om in de loop van 2013 meer duidelijkheid te krijgen over de kans op sterkere aardbevingen in het Groningen veld. De NAM heeft voorinzage gekregen in de samenvatting van het KNMI-rapport over de recente Huizinge aardbeving. Het KNMI concludeert op basis van een nadere analyse van de historische aardbevingen in het Groningen veld dat “Het niet mogelijk is gebleken de maximaal mogelijke magnitude voor aardbevingen in het Groningen veld te schatten op basis van de statistiek” en concludeert verder dat “Maximale sterktes van bevingen, zoals in de literatuur vermeld, varieren van M=4.2 tot 4.8. Hieruit wordt de conclusie getrokken dat niet verwacht wordt dat de maximaal mogelijke magnitude groter dan 5 zal worden. Maximale intensiteiten die behoren bij een ondiepe aardbeving met magnitude 4-5, zullen waarschijnlijk in de VI-VII range liggen.” Maatregelen om aardbevingen en schade door aardbevingen zoveel mogelijk te voorkomen en te beperken In de afgelopen jaren heeft NAM conform haar maatschappelijke en wettelijke verantwoordelijkheden een aantal maatregelen (A) genomen in het kader van het optreden van aardbevingen in het Groningen veld. Deze maatregelen zijn hieronder beschreven en bevatten onder meer een uitgebreid data acquisitie- en studieprogramma. Het doel van dit programma is om beter inzicht te krijgen in de kans op het optreden van sterkere aardbevingen en maatregelen te identificeren om deze te beperken zoals bijvoorbeeld het optimaliseren van het reservoir management. Vanwege de nieuwe inzichten ten aanzien van de onzekerheid in de maximale magnitude van toekomstige aardbevingen zal NAM dit data acquisitie en studieprogramma inzake aardbevingen in het Groningen veld versnellen (B) met als doel het inzicht in de aardbevingen te verbeteren en zo mogelijk aanvullende maatregelen te identificeren, maar ook al direct voorzorgsmaatregelen nemen die redelijkerwijs genomen kunnen worden om schade door toekomstige aardbevingen te voorkomen of te verlichten (C). A. Maatregelen afgelopen jaren NAM heeft de afgelopen jaren een aantal maatregelen getroffen: -

NAM heeft midden 2012 een gedetailleerde update van het structureel model van het Groningen veld afgerond met circa 1800 breuken en een beschrijving van de onzekerheden van het ondergrondse akoestische snelheidsmodel. Hiermee is de basis gelegd voor verder geomechanisch en seismologisch onderzoek naar de oorzaken en mogelijke maatregelen in het kader van aardbevingen.

2

Studie is uitgevoerd door Shell Projects & Technology, review door Prof. Ian Main (University of Edinburgh) and Prof. Julian Bommer (Imperial College London). 2

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Naar aanleiding van de Huizinge beving (gemeente Loppersum) heeft NAM de procedures voor de schadeafhandeling in overleg met belanghebbenden verbeterd met meer optionaliteit voor claimanten en een meer persoonlijke en duurzame benadering van schadeafhandeling (onder meer door middel van contactpersonen). De meer duurzame reparaties (evenals ruime bekendheid met de schadeafhandeling door NAM) hebben er mede toe geleid dat het aantal claims beduidend is toegenomen en dat er nog steeds claims ingediend worden. NAM blijft in gesprek met belanghebbenden om de procedures en de toegankelijkheid waar nodig verder te verbeteren. Hierbij verwijs ik ook naar de nieuwe website www.namplatform.nl. NAM is in samenwerking met KNMI begin 2012 gestart met de uitbreiding van het seismische meetnet van het Groningen veld. NAM heeft in de afgelopen maanden de benodigde fondsen voor additionele versnellingsmeters beschikbaar gemaakt aan KNMI. Om de locatie van de hypocentra beter te kunnen bepalen gaat NAM aanvullende metingen doen in de in april 2013 te boren Borgsweer put en zullen in Q2 2013 op verschillende dieptes geofoons geïnstalleerd worden in de Zeerijp put gelegen nabij Loppersum. Verder zal NAM een innovatief passief seismisch netwerk laten installeren om met hogere resolutie te kunnen meten. De specificaties hiervoor zullen op korte termijn afgerond worden waarna de aanbesteding rond de zomer zal plaatsvinden.

B) Versnellen van het data acquisitie- en studieprogramma om zo spoedig mogelijk beter inzicht te krijgen in het optreden van grotere aardbevingen en welke maatregelen redelijkerwijs genomen kunnen worden om deze te voorkomen NAM’s versnelde data acquisitie- en studieprogramma (“Study and Data Acquisition Plan for Induced Seismicity in Groningen”, November 2012) is met u afgestemd en heeft u als een bijlage bij onze genoemde brief van 21 december 2012 aan de Minister ontvangen. NAM zal het seismologisch en geomechanisch onderzoek naar de kans op het optreden van grotere magnitude aardbevingen in het Groningen veld zo snel mogelijk uitvoeren, in de loop van 2013 de verdere resultaten bespreken met experts en waar nodig laten verifiëren door externe instituten. Het tijdspad voor de studies is vastgelegd in genoemd “Study and Data Acquisition Plan for Induced Seismicity in Groningen”. Doel van dit studiewerk is: 1. 2.

Zo spoedig mogelijk beter inzicht te krijgen in de kans op het optreden van sterkere aardbevingen en de mogelijke schade. Het identificeren van aanvullende maatregelen die redelijkerwijs genomen kunnen worden door NAM om schade door toekomstige aardbevingen zoveel mogelijk te voorkomen en te verlichten. Dit kunnen maatregelen zijn om de magnitude/frequentie van aardbevingen te beperken (ondergrond), bijvoorbeeld door middel van lokale drukegalisatie, en/of gerichte maatregelen om schade als gevolg van aardbevingen te voorkomen (aan de oppervlakte).

C) Voorzorgsmaatregelen Vanwege de nieuwe inzichten ten aanzien van de maximale magnitude van toekomstige aardbevingen en de tijd die gemoeid is met het versnelde data acquisitie- en studieprogramma zal NAM die aanvullende voorzorgsmaatregelen nemen die redelijkerwijs genomen kunnen worden om schade door toekomstige aardbevingen te voorkomen en te verlichten. De maatregelen tonen onderling sterke samenhang. Een conceptueel plaatje van de onderliggende risico analyse is gegeven in figuur 1.

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C.1) Beperken van frequentie / magnitude van aardbevingen NAM zal compactie en differentiële compactie rond breuksystemen, voor zover dit redelijkerwijs mogelijk en effectief is, inbrengen in het reservoir management van het Groningen veld met het doel de frequentie/magnitude van aardbevingen te beperken. Dit is een van de doelstellingen van het voornoemde “Study and Data Acquisition Plan for Induced Seismicity in Groningen”. Voor deze analyse zijn een aantal stappen nodig:

1. Het beter lokaliseren van de aardbevingen in het Groningen veld. Hiertoe worden het seismische 3

meetnet en het ondergrondse akoestische snelheidsmodel verbeterd. Afgerond Q4 2013 . 2. Het uitvoeren van geomechanische studies om gebieden met een hogere aardbevingsgevoeligheid te identificeren, en vervolgens het dynamisch modelleren van reservoir management concepten en het vaststellen van de invloed hiervan op compactie en differentiële compactie. Het stabilliseren of reduceren van drukverschillen over breuken heeft mogelijk een effect. Afhankelijk van de studieresultaten zou een optimalisatie van het productiepatroon en mogelijk additionele putten en productieinstallaties meegenomen worden in de optimalisatie van het reservoir 4 management. Eerste resultaten Q4 2013 . Gezien de complexiteit van het mechanisme achter het optreden van aardbevingen en de aanwezigheid van omvangrijke breuksystemen in het Groningen veld (1800 gemodelleerde breuken) is op dit moment nog niet te voorspellen hoe effectief deze maatregel zal zijn hoewel de verwachting is dat dit beperkt zal zijn. C.2) ‘Field-wide’ productiemaatregelen Aardbevingen zijn in het Groningen veld onlosmakelijk verbonden met productie. Dit verband is toegelicht in bijlage A. NAM heeft bezien wat op basis van de huidige inzichten het effect zal zijn van het volledig insluiten van het veld en het gedeeltelijk beperken van de productie. Alleen de theoretische optie om het veld volledig in te sluiten zal op termijn leiden tot het stoppen van bevingen. Dit zou een vergaand effect hebben op de gasmarkt en leveringszekerheid van Noordwest Europa. Het gedeeltelijk beperken van de productie zal op termijn de statistische kans per jaar op aardbevingen, en daarmee ook de kans per jaar op een sterkere aardbeving proportioneel met productie doen afnemen. Bij een beperking van de jaarlijkste productie zullen de aardbevingen die statistisch gezien in dat jaar zouden optreden verspreid worden over de tijd die het kost om datzelfde volume te produceren. Een productiebeperking zal de kans op een sterkere aardbeving niet wegnemen, de sterkte van mogelijke aardbevingen niet verminderen en de kans gedurende de resterende Groningen productie niet doen afnemen bij gelijkblijvend reservoir management. NAM vindt een productiebeperking geen goede voorzorgsmaatregel omdat de kans op een krachtige aardbeving er niet mee wordt weggenomen (niet substantieel verlaagd wordt) en en het in die zin dus niet effectief en niet evenwichtig is. Een substantiële productiebeperking zal een vergaand effect hebben op de gasmarkt en leveringszekerheid van Noordwest Europa. Omdat aardbevingen nog steeds zullen optreden zou ten onrechte de verwachting gewekt worden dat het optreden van aardbevingen hiermee zou kunnen worden voorkomen.

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Deze tijdslijn wordt mede bepaald door installatie van het eerder genoemde passieve meetnetwerk en de metingen in de Borgsweer put en meet apparatuur in de Zeerijp put. 4 Deze tijdslijn is gebaseerd op een aantal studies die sterk samenhangen zoals in meer detail gedefinieerd in het “Study and Data Acquisition Plan for Groningen Seismicity”. 4

Zoals eerder in deze brief beschreven zal NAM kijken naar optimalisatie van het reservoir management, onder andere het stabiliseren van drukverschillen over breuken. Het field-wide egaliseren van productie zal hier echter niet aan bij kunnen dragen. De productielocaties in het gebied rondom Loppersum (waar de meeste compactie en aardbevingen plaats vinden) worden conform het huidige productiebeleid al relatief constant in de tijd geproduceerd en we zien daardoor weinig drukverschillen in dat gebied als gevolg van productiefluctuaties. Daarbij is de invloed van drukveranderingen door de sterkere productiefluctuaties in verder weg gelegen productielocaties gering omdat deze meerdere jaren nodig hebben om het relevante gebied in het noorden te bereiken. NAM ziet op dit moment geen relatie tussen productiefluctuaties binnen het jaar en het optreden van aardbevingen. Een egalisering van productie over het gehele veld is daarom volgens NAM geen effectieve maatregel. C.3) Voorkomen van schade NAM zal uit voorzorg daarom de volgende maatregelen nemen om schade als gevolg van toekomstige aardbevingen zoveel mogelijk te voorkomen of te beperken, omdat aardbevingen zullen blijven optreden: 3. Het (in overleg met de overheid) in algemene zin informeren van de bevolking in het relevante gebied over de mogelijke gebouwenschade en hoe te handelen in geval van een aardbeving: In Q1 2013. 4. Validatie van de bestaande informatie over aardbevingsveiligheid in relatie tot kwetsbaarheid van gebouwen op basis van de nieuwe inzichten opgedaan uit interpretatie van de schadegevallen als gevolg van de Huizinge aardbeving: Afgerond Q2 2013. 5. Het installeren van tiltmeters/versnellingsmeters op referentiegebouwen: Afgerond Q2 2013 (ook opgenomen in het “Study and Data Acquisition Plan for Groningen Seismicity”). 6. Het ontwikkelen van een gebouwenschade scenario op basis van hogere magnitudes. NAM zal hierbij ook kijken naar de relevante Europese codes: Afgerond Q4 2013. 7. Het in samenwerking met TNO en andere betrokken partijen assisteren van bewoners en eigenaren om de kwetsbaarheid van gebouwen bij aardbevingen met een hogere magnitude te kunnen inschatten, op basis van locatie en bouwkundige eigenschappen, primair vanuit het oogpunt van veiligheid. NAM zal desgevraagd specialistische kennis bieden in geval van twijfel, en in redelijkheid (op basis van nader vast te stellen criteria) bijdragen aan eventuele noodzakelijke preventieve reparaties of versterkingen: vanaf Q2 2013 (Loppersum), vanaf Q4 2013 (overige gebieden, afhankelijk van inzichten risico-contouren). C.4) Behandeling van schade Ook in de toekomst zullen aardbevingen schade tot gevolg hebben. NAM zal uit voorzorg de volgende maatregelen nemen om te zorgen voor een betere respons bij toekomstige aardbevingen: 8. Overleg opstarten met de Veiligheidsregio over Emergency Response scenario’s bij sterkere aardbevingen: In Q1 2013. 9. Het verbeteren van NAM’s ‘earthquake response procedures’ (waaronder begrepen een zorgvuldige en snellere communicatie met belanghebbenden, en het opstellen van een protocol samen met o.a. het KNMI en relevante provincies en gemeenten): Afgerond Q1 2013.

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De reeds door NAM in gang gezette en de hierboven aangegeven aanvullende maatregelen (gebaseerd op de nadere studies) krijgen voorts een plaats in het gewijzigde winningsplan voor Groningen, welke wij in de loop van 2013 in procedure zullen brengen, zodra de lopende studies voldoende houvast bieden. Afsluiting Ik hoop hiermee een overzicht te hebben gegeven van de voortgang van NAM’s inzichten en de maatregelen die NAM heeft genomen en nog zal gaan nemen. Ik wil graag benadrukken dat dit alles voor NAM de hoogst mogelijke prioriteit en aandacht heeft. Indien gewenst ben ik gaarne bereid om een en ander nader toe te lichten. NAM zal u verder op de hoogte houden van relevante ontwikkelingen en de voortgang.

Hoogachtend, Nederlandse Aardolie Maatschappij B.V.

dr. L.E.C. van de Leemput Directeur

Bijlagen: Figuur 1 (risico-analyse) en bijlage A (aardbevingen per jaar vs productie per jaar)

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Figuur 1: Risico-analyse

Risico analyse geïnduceerde aardbevingen in het Groningen veld

Sterke aardbeving

Schade door aardbeving

Gesteente compactie door gaswinning in combinatie met een seismisch gevoelig breuksysteem

Aardbevingen voorkomen/beperken: • Volledig insluiten: vergaand effect op gasmarkt en leveringszekerheid • Reservoir Management: werkt alleen na de nodige metingen en studies

Schade voorkomen/beperken • Bevolking informeren • Kwetsbare gebouwen versterken

Financiële schade Verlies draagvlak voor gasproductie Zorgen onder bevolking

Mitigeren gevolgen • Goede schadeafhandeling • Overleg veiligheidsregio • Verbeteren procedures • Gericht social investment

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Bijlage A: Aardbevingen per jaar vs productie per jaar NAM baseert haar studies en beleid ten aanzien van de Groningen aardbevingen op de door KNMI geregistreerde aardbevingen met een magnitude groter dan of gelijk aan 1,5 op de schaal van Richter. Dit zijn de aardbevingen waarvan met het huidige meetnet het epicentrum en de magnitude goed bepaald kunnen worden. De meeste van deze bevingen worden niet gevoeld aan de oppervlakte. Het aantal aardbevingen per jaar is de afgelopen 10 jaar opgelopen proportioneel met de toegenomen productie per jaar, met ongeveer een jaar vertraging. De toename van de productie uit het Groningen veld is te zien in de linker figuur hieronder. Deze toename in de Groningen productie is het gevolg van de afgenomen productie uit Kleine Velden (rechter figuur). De gemiddelde productie uit het Groningen veld is beperkt door het door de Minister op grond van de Gaswet bepaalde maximum dat door GasTerra gemiddeld per jaar verkocht mag worden voor de periode 2006-2020. Hierdoor zal de Groningen productie per jaar naar verwachting tot 2015 tussen 45 en 50 bedragen en daarna 3 geleidelijk afnemen naar zo’n 40 miljard m in 2020. Na 2020 zal het veld definitief een eindfase ingaan en de productie jaarlijks snel verder afnemen vanwege de teruglopende productiecapaciteit. In 3 2030 zal de productie al gedaald zijn onder de 15 miljard m . Het aantal aardbevingen per jaar zal hier proportioneel mee afnemen. Een andere fasering van de productie uit het Groningen veld (bijvoorbeeld door een beperking van de jaarproductie) zal naar verwachting een evenredig effect hebben op de fasering van de te verwachten aardbevingen (met ongeveer een jaar vertraging). Dit geldt op precies dezelfde manier voor sterkere aardbevingen. De maximaal te verwachten magnitude zal er niet door veranderen. Kleine velden & Groningen volumes 100 90 80 70 60 50 40 30 20 10 0

Groningen

Kleine Velden 1963 1967 1971 1975 1979 1983 1987 1991 1995 1999 2003 2007 2011 2015 2019 2023 2027 2031

miljard m3 (GE)

Productietoename afgelopen 10 jaar

1963 1967 1971 1975 1979 1983 1987 1991 1995 1999 2003 2007 2011 2015 2019 2023 2027 2031 2035

miljard m3 (GE)

Jaarlijkse Groningen volumes 90 80 70 60 50 40 30 20 10 0

Figuur 1 & 2: Jaarlijkse Groningen volumes en Kleine Velden context (bron: NAM & nlog.nl): de sterk toegenomen Groningen productie van 2000-2010 (linker plaatje) moet in context gezien worden van de afgenomen Kleine Velden productie (rechter plaatje).

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OVERZICHTSKAART AARDBEVINGEN 1996 - 2012

±

!

! ! Roodeschool

! ! Warffum

Adorp

!

Uithuizen

Usquert

!

! !

! !! !

Spijk

Bierum ! ! ! ! Godlinze ! ! !! ! !! ! ! !! ! ! !!! ! ! ! ! !!! ! !!! ! Leermens ! ! ! !! Middelstum ! ! ! ! ! ! ! ! ! ! ! !!!! ! ! ! ! Delfzijl Onderdendam ! ! !!! !Loppersum ! ! ! ! ! ! Tjamsweer ! Appingedam !! ! ! ! !!! !! ! ! ! ! !! ! ! ! Amsweer ! ! ! ! !! ! !!! ! ! ! !! ! ! Ten Post ! !! ! ! !!! ! ! ! Tjuchem ! !! ! ! !! ! Ten Boer! !! ! ! ! ! ! De Paauwen !! ! ! Siddeburen! !! ! ! ! Nieuwolda ! Lageland ! ! !! ! ! ! ! GRONINGEN Nw-Scheemda ! ! ! ! Froombosch ! ! ! !! !Kolham Scheemda ! ! ! ! ! !!! Hoogezand !Sappemeer

Winschoten

!!

VEENDAM

Legenda

! Magnitude 3.0 < M <= 3.5 ! ! Magnitude 2.5 < M <= 3.0 ! Magnitude 2.0 < M <= 2.5 ! !Magnitude 1.5 < M <= 2.0 Gasveld Groningen

!

Oude Pekela

!

! !

!

Termunten

!! ! ! !

!

Wildervank

0 1,250 2,500 5,000 7,500 Meters Onstwedde Tekeningnr. : Datum : 24-01-2013 EP201301222505004

Tweede Kamer der Staten-Generaal

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