Source parameters of earthquakes in the Aleutian Islands subduction zone

Abstract

SUMMARY 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 method. 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 dimensions, focal depth, rupture velocity, mean stress drop and the static seismic moment for each event. Normal faulting associated with extensional stresses generated within the subducting plate is determined for one event. However, the majority of solutions indicate compressive mechanisms consistent with the subduction environment. Focal depth is well resolved as depth phases are apparent on most of the seismograms due 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 stress drop values obtained are typical of interplate earthquakes (-30 bar). However, the higher stress drop found for one event at a depth of approximately 180 km in the subducting lithosphere can be attributed to the intraplate character of this earthquake. Seismic moments obtained are similar to those obtained in centroid moment tensor (CMT) solutions, even though the latter are calculated using lower frequency seismic waves. This implies a fairly flat spectral level below the corner frequency.