abstract

Surface waves generated by the earthquake on October 25, 1976 in the Gulf of Finland and recorded by the North European long-period instruments in the distance range 1 to 10 degrees have been inverted in an attempt to find the fault mechanism. The resulting solution gives a very good fit between the theoretical and the observed surface waves. This intraplate earthquake was mainly of the strike-slip type, but contained a significant part of reverse dip-slip faulting, possibly amounting to up to half the strike slip. The seismic moment was in the range of 3 to 4 × 1022 dyne-cm, and the focal depth was 10 to 14 km. This earthquake was probably the largest in this area for at least 350 years. The body-wave magnitude was 4.5, which value is too low to give a teleseismic P-wave fault mechanism. However, the only clear teleseismic body-wave observation found by the author is in good agreement with the surface-wave solution, assuming the radius of the fault area to have been less than 1 km. This corresponds to a stress drop larger than 15 bars. The direction of compressional stress given by the fault solution was WNW-ESE, which is in agreement with the general stress pattern of Northern Europe proposed in several published studies. One of the two possible fault planes is in agreement with geological features of the source area.

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