The problem of a two-dimensional dynamical dislocation in an isotropic, homogeneous, unbounded, elastic medium has been discussed by the present authors in a recent paper (SINGH and SIKKA, 1988), assuming harmonic time-dependence. The aim of the present paper is to generalize the results to arbitrary time-dependence. Explicit theoretical expressions for the displacement and the stress components for a two-dimensional dislocation source have been obtained. These expressions can be used to calculate the theoretical seismograms due to a two-dimensional dislocation in an unbounded medium.

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The detailed rupture process of the 1975 central Oita earthquake is investi-gated by a non-linear waveform inversion technique using the near-field data of the strong-motion seismograms at JMA (Japan Meteorological Agency) stations. It is confirmed that the strike-slip mechanism is appropriate rather than the normal fault type, as was shown by Hatanaka and Shimazaki in 1988. The rupture process of this earthquake is characterized by two subevents: the first subevent at a deeper portion of the fault and the second subevent at shallower portion of the NW end of the fault. Total seismic moment of 3.8 x 10²⁵ dyn cm is obtained, which is larger than that of 2.9 x 10²⁵ dyn cm estimated by an analysis of far-field data. The seismic moment of 3.4 x 10²⁵ dyn cm is obtained by averaging the above two values. The synthetic far-field seismograms based on the parameters obtained in this study are consistent with observed ones. The result suggests that the faulting at a deeper portion triggered or guided the shallower rupture.

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We attempt to detect a high-velocity zone in the lower mantle beneath the Okhotsk Sea by analyzing travel time and amplitude data for teleseismic P-waves from intermediate and deep earthquakes in the Kurils. Travel time residuals with respect to the J-B tables are obtained by correcting ISC arrival time data for the Earth's ellipticity, station anomaly, and station elevation and are then inverted to obtain a P-velocity model for depths shallower than 1, 250 km in the Kuril subduction zone. The velocity model exhibits a dipping high-velocity zone down to a depth of 1, 200 km in the lower mantle immediately beneath the Kuril deepest seismicity. The velocity in the dipping zone is 2-3% higher than the ambient lower mantle. Resolution analysis supports the existence of a slablike high-velocity zone, but indicates that the depth extent of the zone cannot be well resolved by travel time data alone. We further constrain the velocity model by analyzing amplitude data, which are more sensitive to fine velocity structure than travel times. After correcting the amplitude data for the source radiation pattern, geometrical spreading, station anomaly, and instrument response, we obtain the amplitude anomaly due to laterally heterogeneous earth structure. The amplitude at a period of 1.5 s decreases by a factor of 2 to 4 at stations in the distance range between 50-65 and with azimuths perpendicular to the Kuril trench. To separate the effects of slab structure from those of large-scale mantle heterogeneity, we model the amplitude anomaly using a hybrid method combining a finite element (FEM) scheme with geometrical optics. The high-velocity zone obtained by the travel time inversion produces an amplitude reduction for the short-period P-waves which is comparable to the observed amplitude reduction. This suggests that the observed amplitude anomaly is primarily due to the lower mantle high-velocity zone. Comparison of the observed and theoretical amplitudes shows that a much larger velocity gradient is required near the upper boundary of the dipping high-velocity zone than near the lower boundary. A high-velocity zone with a maximum depth of 1, 000 km, thickness of 150 km, and velocity contrast of 3.5% gives the best fit to the amplitude data. Similarities in the velocity structure and the dip angle between the lower mantle high-velocity zone found by the present study and the upper mantle slab suggest that the former represents a lithospheric slab penetrating into the lower mantle.

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