Source process of deep and intermediate earthquakes as inferred from long-period P and S waveforms, 1. Intermediate-depth earthquakes in the southwest Pacific region

Abstract

The source process of four intermediate earthquakes with magnitudes of 6.0-6.8 and focal depths between 100 and 200km has been re-investigated from the analysis of long-period P and S waveforms. The source process times recovered from the times of the first half cycle of recorded P waves or the group delay times derived from the slopes of equalized phase spectrum show an azimuthal dependence with respect to the orientation of a nodal plane and of the null vector. If we assume shear dislocation models for these earthquakes, the dependence yields a solution to the problem of which of two nodal planes corresponds to the slip plane. Various source parameters have also been estimated from the azimuthal dependence by least squares technique. The dimension of the slip plane or the fault length and width range in 25-40km and 8-18 km, respectively, and the rise time of dislocation is found to be about 1sec. The rupture velocity might be as low as 3.2km/sec, if two-dimensional propagation is assumed. The theoretical seismograms of both direct P and S waves appropriate to each recording station have been synthesized on the basis of the estimated source parameters, taking into account the combined effects of wave propagation in the mantle and the crust and of the seismograph response. A good agreement of general features between the observed and synthesized waveforms on the three component seismograms gives support to the above slip dislocation model. Comparison of the amplitudes on the both kinds of seismograms yields seismic moment of the order of 1.6-3.0×10[26] dyne·cm and the amount of dislocation of 80-140cm. The stress drop at these earthquakes ranges from 50 to 90 bars but might exceed 170 bars for one shock. The effective initial stress to produce shear dislocations is also estimated in relation to the frictional stress on the slip plane.