Source process of deep and intermediate earthquakes as inferred from long-period P and S waveforms, 2. Deep-focus and intermediate-depth earthquakes around Japan

Abstract

The dynamical processes at the source of eleven deep-focus and intermediate-depth earthquakes that occurred around Japan have been investigated from the analysis of long-period P and S waveforms. The recorded P waveforms have been equalized at some distance around the focal region to get the source function, eliminating the combined effects of wave propagation in the earth and of the seismograph characteristics. It is found that the source process times derived from the source function, as well as from the recorded first half-periods, indicate some azimuthal dependence with respect to the orientation of one nodal plane and of the null vector, in most of the earthquakes. This dependence is interpreted as a result of shear faulting over a finite fault area, and used to determine the slip plane, slip direction, fault length and width. The seismic moment and the average dislocation over the plane are evaluated from P wave amplitudes together with the estimated fault dimension. A close agreement in general features between the recorded and synthesized waveforms including the absolute amplitudes of both P and S waves supports the above shear dislocation model. The overall distribution of the orientations of the slip planes and slip vectors of these earthquakes does not seem to be definitely related to the local dip or strike of seismic zones in this region. The calculated stress drops during these earthquakes, together with some available data, appear to show a gradual increase with focal depths down to 400 km. A tentative interpretation of this increase is that the stress drop might be related, at least qualitatively, to partial loss of the intrinsic (cohesive) shear strength of the material under increasing hydrostatic pressures, if these earthquakes are caused by brittle fractures in the lithosphere.