SOURCE MECHANISMS OF TWENTY-SIX LARGE, SHALLOW EARTHQUAKES (Ms~ 6.5) DURING 1980 FROM P-WAVE FIRST MOTION AND LONG-PERIOD RAYLEIGH WAVE DATA

Source mechanisms of 26 large shallow earthquakes are determined in terms of a double-couple point source with a correction for the nondirectional part of source finiteness by using P-wave first motions and long-period Rayleigh wave spectra recorded on WWSSN, IDA, and GDSN networks. The combined use of both data sets allows us to determine the double-couple mechanism uniquely in most cases. Constrained linear moment tensor inversion (Mxz =My.= 0) correctly determines the strike of the fault, but fails to estimate the dip, and underestimates the scalar moment. All thrust events along the deep-sea trenches analyzed in this study show nodal planes which dip perpendicular to the trench axis at an angle shallower than 45°. The fit to data of the double-couple inversion is comparable to that of the constrained moment tensor inversion. Using the phase spectra of surface waves we can detect a slow source process with an accuracy of about 10 to 20 sec.