The Kaliningrad, West Russia earthquakes on the 21st of September 2004—Surprise events in a very low-seismicity area

The earthquakes in Kaliningrad, West Russia on the 21st of September 2004 were unexpected in a seismically quiet area. The main shock of magnitude mb = 5 was widely felt around the Baltic Sea. A comparison with some historic earthquakes in Northern Europe shows that its perceptibility area is smaller than that of the 1904 Oslo Graben earthquake of an estimated magnitude (ML) 5.4 but larger than those of the 1759 Kattegat and 1819 Lurøy earthquakes. The latter are claimed to have had magnitudes (MS) in the range of 5.7–6.0. An analysis of the Lg phase of the Kaliningrad earthquakes as recorded at a number of European stations accords only weakly with the macroseismic intensity pattern that shows fast attenuation towards west–northwest and southeast. The strike–slip focal mechanism of the main shock is discussed in the context of remnant glacial rebound stresses in generating present-day seismicity in N. Europe.