Moment tensor inversion of small earthquakes in southwestern Germany for the fault plane solution

Abstract

SUMMARY The amplitudes of direct P- and S-waves as well as P,, and other deep crustal body-wave phases from events in southwestern Germany are studied for the effects of the double-couple earthquake radiation pattern. The direct P- and S-wave amplitudes for six earthquakes in the southern Black Forest of West Germany with magnitudes between 2.2 and 0.5 were inverted for the source moment tensor, and the double-couple components were extracted from the solutions and refined by a further inversion. The inversion focal mechanisms using both P- and S-wave amplitudes were quite consistent with those found from a large set of first motion readings, even when very different crustal models were used. For the events at or above magnitude 1.4, inversions using only direct P-wave amplitudes also yielded good focal mechanisms. Furthermore, the vertical component record at a station at P,, distance from the largest of the events was successfully synthesized using local mechanisms representative of those found from the inversions. The phases modelled included P,,, PmP and several free-surface reflections. The effect of the earthquake radiation pattern was also noted in the amplitudes of the P,, arrivals of a set of azimuthally distributed stations for two other earthquakes. This study confirms that direct P and S amplitudes can be inverted for the source focal mechanism and suggests that deeper crustal phases, like P,,, PmP, etc., are also affected by the earthquake radiation pattern.