Earthquakes in the Northeastern Part of the Bohemian Massif Recorded by the MORC Station during the Period October March 1995

VìstníkÈeského .I1eolo.l1ickéhoústavu 72. 1. 1997

Earthquakes in the Northeastern Part of the Bohemian Massif Recorded by the MORC Station during the Period October 1994 -March 1995 DARJA

SKÁCELOVÁJ

-ZUZ4NA

SKÁCELOVÁ2

-JOSEF

HAVIØ2

lCeský geologický ústav, Klárov 3, 11821 Praha I 2Ústav fyziky Zemì, pøírodovìdecká fakulta, Masarykova Univerzita, Jeèná 29a, 612 46 Brno

A b str ac t. Seismic activity in the territory of the Clech Republicis currently monitoredby a numberof seismologicalstationsanda seismological network. Macroseismiceffects of earthquakesfrom the northeastempart of the BohemianMassif have alreadybeenmentionedin historicalliteratureas earthquakesoriginating in the area of Hrubý Jeseník,Opavaand Sternberk(Remeš1902,Koláèek 1921,Zátopek1939). Up to the year 1994,oRly isolated recentearthquakesin Moravia and Silesia were registeredby the Ostrava-OKC (with continuousanalogregistration)and the Vranov u Brna -VRAC stationsand oRly strong earthquakeswere registredby other stations,for examplean earthquakefrom the Opavaregion in June 1993 with a local magnitudeof 1.8 and 2.5 (Holub etal. 1994).The seismologicalstationMORC (Iatitude49°46'36.5", longitude 17°32'33"], with continuousdigital recording,wasrecentlypUlinto operationin theclose vicinity of MoravskýBerounin October1994(Firbasetal. 1994,1995). This sensitivestation,built in the frameof the internationalprojekt GEOPHON,wassituatedby designatthe centralareafor earthquakesfrom Opava, Hrubý Jeseníkand Sternberk,wherea seismologicalstationwasmissing.The MORC stationhasbecomeODeof the most important stationsmonitoring microseismicactivity and the first of al1from the NE part of the BohemianMassif (Havíø,Skácelová1996). A bs trakt. SeizllÙckáaktivita Ceské republikyje v souèasnédobì sledovánanìkolika seizmologickýllÙstanicemia sítìmi. Již v historických pramenechjsou popsánymakroseizllÙcképrojevy zemìtøesení,které vznikaly v oblastechHrubéhoJeseníku,Opavskaa Sternberska(Reme§1902, Koláèek 1921,Zátopek1939). Do roku 1994byly registroványpouzeizolovanétektonickéotøesyna Moravì a ve SlezskustanicemiOstrava-OKC (vybavenáanalogovýmkontinuálním záznamem)a stanicí Vranov u Brna-VRAC. Jen nìkterá silná zemìtøeseníbyla registrovánai ostatníllÙseizmologickými stanicellÙ,napø.otøesyz Opavskav èervnu 1993s lokálním magnitudem1.8 a 2.5 (Holub et al. 1994). Stanice MORC (Iatitude 49°46'36.5",longitude17"32'331 vybavenáširokopásmovýmdigitálním kontinuálnímzáznamembyla uvedenado provozuv øíjnuroku 1994v blízkosti MoravskéhoBerouna(Firbas etal. 1994,1995).Tato velmi citlivá stanice,vybudovanáv rámci mezinárodníhoprojektu GEOFON,byla zámìrnì umístìnado støeduseizllÙckyaktivních oblastí-Opavsko, Hrubý Jeseníka Sternbersko,kde nebyladosudumístìna žádnáseizmologickástanice. StaniceMORC se tak stalajednou dùležitých seizmologickýchstanicsledujícíjako první mikroseizmickouaktivitu severovýchodníhookraje Ceského masivu. Key words:

seismology,tectonic,micro-earthquakes, Moravia and Silesia,faults.

lntroduction Earthquake occurrences in Europe are linked to the recent stress field and movements along active seismic fault zanes. Dne of the most important factors affecting seismic active zanes in Central Europe is the continuous movement of the Alpine units. Strong earthquakes are most often connected with the active fault zone passing in a NE direction i.e. from the Eastern Alps to the Western Carpathians. Strong earthquakes originating in the area of the Vienna basin were very often observed in the whole area of southern Moravia and were considered to have originated in Moravia (Remeš 1902). In the area of the West Carpathians, the fau1tzone passing in the SW-NE direction from Bratislava to Žilina is also known for the common occurrence of earthquakes. Dne of the strongest earthquakes was observed in the area of Dobrá Voda (Malé Karpaty Mts.) in 1906, which reached an intensity of 90 (M.C.S.). The whole active zone passing from Vienna to Žilina forms an important line of recent movement of the Alpides. Another seismically active fault zone in Central Europe is situated in a NW-SE direction along the Marginal Sudetic Fault Ic/. Several strong earthquakes connected with faults of this direction are known in history, for

example an earthquake from the area of the Hronov-Poøíèí fault zone (1=7°) on 10.1.1901 (Woldøich J.N. 1901) or from the area of the Niemczy fault zone (1=7°, Paland) on 11.6.1895 (Pagaczewski 1972). Strong historical earthquakes from the area of Hrubý Jeseníkare probably connected with the Èervená Hora Saddle deep fault zone /II with a NS direction (with pronounced geophysica1 demonstration Èuta et al. 1964) and with faults in the Sudetic direction (Bìlá Id/, Klepáèov Ie/, Temenice/f/, and Bušín 19!-Fig. 1). The first reference to strong shocks in trus area is dated from 998 (Remeš 1902). The latest strong earthquake swarm from the area of the Hrubý Jeseník Mts. was registered by many seismological stations in autumn 1986 (Procházková 1988). Stations of the local Blahutovice seismological network (seismological network operated by Geofyzika Brno in the period 1984-1987) in the area of the Moravia Gateway recorded these earthquakes and they were localised in the area of the Klepáèov fault passing in a NW-SE direction, north of Šumperk. Earthquakes from the area of Opava probably have a connection with the Sudetic fault area as well. Some previous studies have concluded that earthquakes from trus area have hypocenters at the interlacing fault zone in NW-SE (for example Opava deep fault zone lb/) and W-E directed faults (Jeseníky margin fault Ii/) -see Fig 1. In the Osoblaha area, 49

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Bulletin ofthe Clech GeoloRicalSurvey72.1.1997

the Sudetic direction faults Ia! are indicated by a great descending gradient and there is a1sosome geological evidence of a great norma1-s1ipa1ongthese faults (Skácel 1980). In the southem part of the Opava basin, the existence of a sinking along the Jeseníky margin fault (Wilschowitz 1927, Kumpera 1994) was proved by borehole documentation. The latest strong earthquakes in this area were registered, especia11yby the Ostrava region stations (Holub et a1. 1994), in June 1993. The Sudetic direction, i.e. NWSE direction, was computed by experimental movement plane computation (Kaláb et a1.1995). Another significant active fault also having NW-SE direction is the Temenice fault in the area of the city of Štemberk, at the crossing with the Štemberk-Homí Benešov zone lm! in a NE-SW direction (Fig.l). The most frequent occurrence of neovulcanites is at the crossing of this zone and the sudetic direction fault zone (Buday et al. 1995). The oldest described earthquake was in February 1068, while other strong earthquakes were observed in 1635, 1843, 1863, and 1883 (Remeš 1902). Seismic activity in the contact area ofthe Silesicum and the Carpathian unit is connected with renewed faults in the Sudetic direction and with fault systems in the NW-SE direction, recently developed during Alpine folding. Weak earthquakes from these faults were registred only by the Blahutovice network stations in the period 1984-1987. These earthquakes were loca1ised in the area of Oderské Vrchy and were interpreted as recent seismic activity of the N-S direction faults (Firbas et a1. 1986). All above-mentioned historical earthquakes with an intensity ~ 1 on the ~.~.S. scale (value of (S:::3 2 macroselsmlC effect~ of. earth- ; ,;;\,:'.,\~~,: 3 quake) are summarlsed ln the """"" "

Table 1. Fig. 1 shows the tectonic schema with probable epicentres of historical earthquakes and earthquakes registred by the

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I(M. Date Re ion ~, 998 Hrubý JesenfKI Feb.l068 Sternberk 1356 Hrubý Jesenfk 1358 Hrubý JesenÍKI 2.7.1635 Sternberk 1.10.1762 Hrubý Jeseník 27.2.1768 Hrubý JesenfkI 1843 Sternberk IV 1863 Sternberk Oct.1883 Sternberk V 11.6.1895 Niemcza VII VII 10.1.1901 Hronov IX 10.1.1906 Dobrá Voda 24.7.1935 Hrubý Jesenfk V Aug.-Sep. 1986 Hrubý Jesenfk Aug.1993

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Seismologicalstations in Moravia and Silesia Seismologicalstationsoperatedby the IPE were located taking into accounthistoricalobservationsof the seismic activity in Moravia andSilesiaso asto bestdetectand analyse seismic activity at the NE margin of the BohemianMassif in the contactareawith the Alpine units in cooperationwith other seismic stations in the Czech Republic (seeFig. 2) and neighbouringcountries(Polish Academyof the Science,ZAMG Wien). The MORC station is situated in the center of the geological unit Silesicum.The neareststationsare the OKC (42 km) and the SPF network (56 km) in the eastand the VRAC (85 km), the JAVC (100 km) and the KRUC (115 km) in the south-see Fig. 2. StationOKC is equippedwith continuousanalogregistrationwith short-perioddigital registration for selectedevents.The SPF network (Frenštátpod Radhoštìm)usesthe trigger regime (selectionof strong events)of monitoring and it recordsonly strongearthquakes from the NE part of BohemianMassif. The KRUC station (Moravský Krumlov) registerssome of earthquakes from Silesia (the distanceis 115 km) and the JAVC

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view earthquakesoriginating in the eastemand northeastem margin of the BohemianMassif show that obvious that seismicactivity in this areais primarily connectedto the Peripieninlineamentand the Sudetic direction faults (Schenket al. 1986,Schenkováet al. 1980).

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station(Velká,Javorina)only monitors strongearthquakes by stationMORC from the NE part of BohemianMassif. (the weak quakes are damped on the border of the The dataof both (VRAC and MORC) stationsareprocesBohernianMassif and the WestCarpathian).The VRAC sing together. Station VRAC and MORC are equipped stationregistersa greatpart of the earthquakesmonitored with broadbandcontinuousdigital registration,with a three componentseismometer.In the period from October 1994to March 1996,togetherwith the MORC station,a Table 2: Earthquaes from Va1ašské Meziøíèíarea numberof earthquakesfrom the NE part of the Bohemian MassifwereregisteredattheVRAC station(5 events)and KRUC station(2 events)-for examplethe local earthquake in Fig. 3. Positionof the MORC stationimprovedmonitoring of the seismicactivity in the areaof Nízký JeseníkMts. and the Opavaregionand,togetherwith the VRAC stationdata, also enablesbetter interpretationof the earthquakes originating in the northernpart of the Boskovicefurrow. The most importanteventsaresummarisedin the encTable 3: Earthquakes from Silesia and Moravia losed list of earthquakes(Table2 and 3). Local magnitude is computedby formule in the documentby Scherbaum and Stoll (1983).

Tectonic eartbquakesin Moravia and Silesia recorded by MORC in tbe period October 1994-Marcb 1996 The area o• the southernmargin o• the Nízký Jeseník Mts. During 16 months of operation, the MORC station registered severa1 tectonic earthquakes from the southern margin of Nízký Jeseník. A1together 6 events were loca1ised into the epicentral area about 16-18 km north of the city of Šternberk at the crossing zone of the Šternberk-Horní Benešov fault zone in the NW-SE direction and the prolongation of the Temenice fault direction. The strongest of these 6 earthquakes (26.1.1996) had the local magnitude 0.9 and was also recorded by the VRAC and KRUC stations. The latest earthquake was registered on 10.3.1996 and was localised at a more southern area than preceding ones, 12. I-

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(Firbas et aJ.1987).Other events from the sameepicentraldistance were localised into the southern marginof the OpavaBasin. This areais situatedclose to the Jeseníkyfault and it was alsothe epicentralarea of severalevents registered in 1993. The maximum local magnitude of these events recorded by the MORC stationwas 1.1 and their signals wererecordedby the VRAC station as well.

Tectonicactivity in the contact areaofthe BohemianMassifand the Carpathianswas observedin 1994.On 9.8.1994,an earthquake swarm from the areabetween Hranice na Moravì and Valašské Meziøíèí was recorded. The MORC station(it was in test-measurement)registered 9 earthquakes during 6 hours (tab.3). The two strongest earthquakes had local magnitudesof 1.1 and 1.5andwerealsorecordedby the SPF network (Kaláb 1994).

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Fig.3Signalofthe localearthquake 10.3.1996 onthestationsMORC,VRACandKRUC. in the vicinity of the city of Olomouc. The direct prolongation of the Bušiny fault, the so called Olomouc-PøeTav fault, passesclo se to this area. The local magnitude of this earthquake was 0.7 and it was also recorded by the VRAC and KRUC stations (Fig. 3).

The areaof OderskéVrchy and the area of the southernmarginof the OpavaBasin The MORC stationalsoregistered4 eventsoriginating about22-26km from the station.Basedon the polarization ana1ysis,ODeof theseeventswas identified as a weak tectonic earthquakefrom the area of OderskéVrchy, in 52

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Seismic activity of the nort-

hern part of the Boskovice furrow has been registered since 1993, when the VRAC station's continuous recording started. During the 3 years of operation of the VRAC station, 4 tectonic earthquakes from the vicinity of Jevíèko and Moravská llebová were identified. Two events of the earthquake swarm from the area of Jevíèko in January 1993 had a local magnitude of 1.8 and 1.9 (Havíø, pazdírková 1995) and they were also recorded by the DPC station, by the stations of the SPF and by the Temelín network stations in south Bohemia (see Fig.2), which allowed for a very good localisation. These earthquakes originated in the area at the crossing of the Konice-Nectava faults and the northern prolongation of the eastern margin of the Boskovice furrow.

53

Vìstnfk Ceského~eolo~jckéhoústavu72, 1, 1997

A weak earthquake from the area of Moravská 1øebováwasregisteredby the MORC station.This earthquakewaslocalisedto the Konice-Nectavafault zone,bul in its more NW part.

Conclusion Earthquakes in the northern part of the Bohemian Massif are linked to the crossings of the Variscian fault systems in the NNE-SSW direction and the Sudetic faults in the NW-SE direction, especially in the area of Hrubý Jeseník (the Èervená Hora Saddle deep fault zone and the Bìlá-Klepáèov fault system crossing area) and to the north of Šternberk (the Šternberk-Horní Benešov fault zone and the Temenice fault crossing area). In the Opava area, earthquakes are tied to the Sudetic direction faults (Opava deep fault zone) and to the W-E direction faults (Jeseníky margin fault). Weak seismic activity in the area of Oderské Vrchy and Valašské Meziøíèí (the crossing area of the Sudetic faults with the fault systems lhal were newly developed during Alpine folding) was recorded as well. Earthquakes originating in some of the above described areas were mentioned in historical sources and they are currently being monitored by sensitive seismological stations with continuous recording which enables detection of even very weak earthquakes. This monitoring brings unique information about the seismic activity in the region lhal can be used for detailed studies and geological interpretation. The felicitous position and up-to-date equipment of the MORC station ensured its overriding importance in the area of the NE Bohemian Massif. During its operation earthquakes were recorded from areas known for their historical seismic activity as well as from areas where earthquakes had not been registered up to 1994 -see Fig. 1 (Firbas et al. 1996). The MORC station enables the more precise location of earthquakes in cooperation with other stations in the Ostrava region and southern Moravia. Localisation of the newly registered seismic activity from the area of the northern part of the Boskovice furrow, which, up to now, had only been interpreted by the VRAC station data, is a good example. The registration and interpretation of tectonic earthquakes can be a great contribution to the knowledge ofrecent motions along fault zanes and as a follow up to the current geological survey ofthe NE part or the Bohernian

Massif. Acknowledgements: The authors wish to thank the Grant Agency of the Clech Republic (project number 205/93/2143)and GeoforschungsZentrum Potsdam,which financially supportedthis project.

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JgX, No.25. Zátopek, A. (1939): Zemìtøesení v severní èásti Zemì Moravskoslezské 24.èervence 1935. -III. Roèenka Stát. ústav. geofys., 1939, Praha.