The exceptional earthquakes in Kaliningrad district, Russia on September 21, 2004

The earthquakes of magnitudes M_w 5.0 and 5.2 in the Kaliningrad enclave of Russia on September 21, 2004 were unexpected in a low-seismicity area. The earthquakes caused moderate damage in the Kaliningrad enclave, and smaller damage in northern Poland and in southern and western Lithuania. The largest earthquake was the strongest ever recorded instrumentally in the region, and it was felt at distances up to 800 km. In directions towards the west and south the perceptibility area is abruptly cut off by the Tornquist-Teisseyre Zone, the south-west margin of the East European Craton. The earthquakes are instrumentally located at depths 16-20 km under the central-northern part of the Sambia Peninsula in the Kaliningrad enclave. For these events it is noted that the macroseismic calculations of 10-19 km depths are in reasonable agreement. The source mechanism of the largest earthquake was determined to be a right lateral strike slip on a WNW-ESE near-vertical fault of orientation almost parallel to the Tornquist-Teisseyre Zone and to the north coast of the Sambia Peninsula. Based on available stress information it is interpreted that the underlying cause of the earthquakes is the absolute plate motion. Historical information is scanty. It is searched in an attempt to evaluate past seismic activity in the region, and to evaluate vulnerable weakness zones in the geological structures.