Source parameters of moderate size earthquakes and the importance of receiver crustal structure in interpreting observations of local earthquakes

Abstract

Broadband observations of three central California earthquakes as recorded on opposite sides of the San Andreas fault zone are studied. The earthquake mechanisms are of the strike-slip type occurring along the fault at epicentral distances between 15 and 30 km. The seismograms obtained at the two sites are distinctly dissimilar in both amplitude and wave shape even though they are at roughly the same azimuth. We suppose that the earthquake excitation is identical for the two sites and that the differences in seismograms are caused by the receiver structure. The problem is idealized by assuming that the first 10 sec of each record can be modeled synthetically with a point shear dislocation embedded in a half-space with a two-layer upper-crustal model appropriate for each site. The results determined by matching the observations indicate that the durations for these events with M_L = 4 to 5 are about 0.3 to 0.6 sec. Furthermore, the results demonstrate that accurate estimate of source parameters can only be accomplished after a detailed appreciation of crustal structure.