Advancement in Source Estimation Techniques Using Broadband Regional Seismograms

One important constraint on source retrieval from regional seismograms comes from the amplitude difference between various phases (such as Pnl/surface wave, SV/SH). Because the misfit errors used in some waveform inversions are normalized by the data and synthetics, the amplitude information in the data has not been fully utilized. In this article, we modify the "cut and paste" source estimation technique (Zhao and Helmberger, 1994) by removing this type of normalization. It is shown that the modified method increases the stability and resolution of inversion. When multiple stations at different distance ranges are used, a distance scaling factor is introduced to compensate for the amplitude decay with distance. By applying the technique to the TERRAscope data, we have determined source mechanisms and depths of 335 southern Californian events with M_L ≧ 3.5. The amplitude decays with distance are r^(1.13) for Pnl, r^(0.55) for Love waves, and r^(0.74) for Rayleigh waves. In contrast to generally shallow source depths reported by the southern California short period network, the depth distribution from waveform inversion shows a strong peak around 12 km with few earthquakes occurring above 5 km and below 20 km.