Focal mechanism of the Kaliningrad earthquake of 21 September 2004 based on waveform inversion using a limited number of stations

The focal-mechanism solution for the second shock of the Kaliningrad earthquake on 21 September 2004 (13:32 UTC) with a magnitude of Mw 5.2 was obtained using the waveform inversion (WFI) method. The method was used with the aim of its subsequent application as a discriminator in the East Baltic region for recognizing the genesis of seismic events based on data from a limited number of stations. The WFI method was tested by broadband channels. The results of focal-mechanism solution (strike = 119°; dip = 73°; rake = –163°) allowed (1) to state the source mechanism as a right-lateral strike-slip, (2) to estimate the optimal source depth equal to 3.0 km, and (3) to estimate the parameters of the compression axes (Paz = 340°; Ppl = 29°), tension (Taz = 252°; Tpl = 1°) and the axis coinciding with the intersection of two nodal planes (Baz = 162°; Bpl = 66°). These results are in satisfactory agreement with the results of the leading seismological agencies. Seismotectonic analysis showed that the epicentre of the earthquake is located inside the structure formed by the Yantarnensk fault zone and the zone of the Bakalinsk ruptured flexure. The WFI method showed its potential use as a discriminator of the genesis of seismic events.