SOURCE PARAMETERS OF EARTHQUAKES IN THE ALEUTIAN ISLANDS SUBDUCTION ZONE

The source parameters of seven moderate-sized earthquakes representing different parts of the Aleutian Islands subduction zone are presented . Fault plane solutions are determined by the relative amplitude metho d. A simple 2-D kinematic source model is assumed and synthetic broad- band (BB) seismograms of displacement are generated. Comparisons with observed BB records of displacement allow the estimation of source dim ensions, focal depth, rupture velocity, mean stress drop and the stati c seismic moment for each event. Normal faulting associated with exten sional stresses generated within the subducting plate is determined fo r one event. However, the majority of solutions indicate compressive m echanisms consistent with the subduction environment. Focal depth is w ell resolved as depth phases are apparent on most of the seismograms d ue to the frequency range (0.05-5.0 Hz) of the broad-band records used . The circular or elliptical faults assumed for the source model vary from 3.5 to 8 km in radius depending on body-wave magnitude. The stres s drop values obtained are typical of interplate earthquakes (similar to 30 bar). However, the higher stress drop found for one event at a d epth of approximately 180 km in the subducting lithosphere can be attr ibuted to the intraplate character of this earthquake. Seismic moments obtained are similar to those obtained in centroid moment tenser (CMT ) solutions, even though the latter are calculated using lower frequen cy seismic waves. This implies a fairly flat spectral level below the corner frequency.