ELLIPTICITY OF TELESEISMIC S-PARTICLE MOTION

In the horizontal components of GRF records, particle motions of the teleseismic S waves in the period range between 5 and 20 s are usually elliptic. We correct the particle motions for the effect of azimuthal anisotropy at GRF and explain the residual ellipticity by assuming that it is produced at the source side of the wave path by interference of two waves with orthogonal polarizations and differing traveltimes. The estimates of polarization direction of the fast wave and time delay between the waves can be found by analysing the residual particle motion for groups of closely spaced seismic events. The technique was applied to about 40 GRF records of events from the northern and north-western Pacific. In the source regions the S-wave pulses propagate in subduction zones. It is found that the surface projections of polarization of the fast wave of the shallow events coincide approximately with the strikes of the corresponding island arcs. This regularity can be explained by anisotropy in the subduction zones provided that the a axis of olivine is parallel to the plane of subduction. The fast direction for the deep events in the sea of Japan and the sea of Okhotsk is close to the strike of the Kurile island arc. The time delay between the fast and the slow waves for the deep events is smaller than for the shallow Kurile events (0.9 s versus 1.4 s).