Pozorování zemětřesení v kaliningradské enklávě a okolí
Earthquakes of moderate size (M=4.4 and M=5.0) shocked Kaliningrad district of Russian Federation on 21 September 2004. A seismic trembling was felt throughout all territory of Lithuania. These extremely rear and unusual phenomena have provoked great concern of mass media and public. Geological Survey of Lithuania analysed data of international seismic agencies, processed data of Seismic Monitoring System (SMS) of Ignalina Nuclear Power Plant (INPP) and spread information within time of a few hours.
Necessity of collection of macroseismic data was realized shortly. Reports and notes provided by mass media were used as initial source of data. A lot of testimonies and comments were sent by people from different parts the country to the Internet portal ?Delfi? (www.delfi.lt). Some of these testimonies were used as macroseismic data as well. A large number of data covering majority of territory of Lithuania was provided by the Department of Civil Defence. Ordinary questionnaires, personal communication and site visiting were employed quite a lot.
All macroseismic data was grouped according to administrative division of the country for each seismic event. Then intensities for each location referring to EMS-98 scale were evaluated, geographical co-ordinates defined and maps of intensities compiled (fig.1, 2). Trebling of higher intensities was observed in the Western part of Lithuania. Seismic waves of the second (more powerful earthquake) generated shaking of intensities IV-V in cities Klaipëda, Palanga and Kretinga. Intensities of III points were assigned to Vilnius and Panevëţys, III ? IV ? to Kaunas and Điauliai, IV points to Alytus. Trembling was felt scarcely (I ? II pints) in the Northeastern parts of the country (fig. 2). The first earthquake produced similar level of shaking in the Western Lithuania, while intensities in other locations appeared to be lower by one point comparing to intensities of the second event (fig. 1). People were frightened and were running out of the buildings, cracks in some walls and windowpanes were formed at locations of stronger shaking in Lithuania.
Macroseismic data collected in Lithuania was presented at a regional workshop in Tartu. Participants from different countries of the Baltic region have correlated their data and decided to compile macroseismic maps of whole region.
Currently there are four seismic stations operating in Lithuania. They are owned by INPP and located in the Northeastern part of Lithuania. This departmental seismic network is not capable to cover whole territory of the country with uniform resolution. The fact that no quarry blast (distance to INPP is ~200 km) was recorded by SMS of INPP supports this idea. It is obvious the system of seismological observations is not sufficient while, as Kaliningrad events have shown, earthquakes of moderate size could shake any part of the Baltic region. At least a few more seismic stations should be installed in Lithuania in order to monitor small seismic activity and potentially seismogenic zones could be defined. Data collected by wider seismic network and additional knowledge of tectonics, geodynamics and hydrology should provide a basis assessing seismic hazard of the Lithuania.
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