| Summary: | 170 p. Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1993. Precise source parameters, including focal depths and mechanisms, of 74 large to moderate-sized $(m\sb{b} \ge 5.5)$ earthquakes that occurred along the Kuril-Kamchatka arc between 1963 and 1991 were determined by inverting teleseismic P and SH waveforms. Based on these results, I delineated seismogenic structures at depths shallower than 200 km and proposed interpretations for systematic variation of such structures with depth and along the strike of the arc. Along the down-dip direction in central Kuril, a transitional thrust zone occurs between depths of $\sim$30-50 km, connecting a classic double seismic zone with the interplate thrust zone. The transitional zone is characterized by earthquakes of thrust faulting with a westerly dipping nodal plane of $\sim$40$\sp\circ$, $\sim$10-15$\sp\circ$ steeper than that of typical low-angle thrust earthquakes along the plate interface. Waveform data suggest that the source region of transitional thrust faulting is within a crust-like low velocity zone of $\sim$10 km in thickness. I interpreted that the transitional thrust earthquakes occurred beneath the plate interface, within subducted crust of the Pacific plate, and that these events may indicate a locked plate interface at depths of 30-50 km. Northeast of 53$\sp\circ$N, the double seismic zone turns into a single zone under compression. The changeover is caused by disappearing of the lower (extensional) layer of the double zone. Similarly, down-dip compressional earthquakes were not found southwest of 46$\sp\circ$C. It is uncertain, however, whether the single extensional seismic zone is a southwestern extension of the lower layer of the double seismic zone. Lateral variation of this Wadati-Benioff zone can be explained by a superposition of two factors: A conventional model for double seismic zones and an additional component of stress transmitted from deep slab. Toward the northern end of the arc, compression transmitted from below overshadows extension in the lower layer of the double seismic zone caused by effects such as unbending. A similar reasoning applies to the southwestern termination of the double seismic zone.
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