RUPTURES OF DEEP-FOCUS EARTHQUAKES IN THE NORTH-WESTERN PACIFIC AND THEIR IMPLICATIONS ON SEISMOGENESIS

The high resolution of broad-band seismic data is highly effective in studying ruptures associated with deep-focus earthquakes. In this study, we report results from an analysis of P and SH waveforms from eight of the largest deep-focus earthquakes that occurred between 1987 and 1992 along the north-western Pacific. In this region, broad-band data recorded at close-in distances, directly above the source zone, offer an unusual source-receiver geometry that complements observations at teleseismic distances. Among the eight earthquakes studied, six showed clear indications of directivity. The best constraint on the extent of rupture came from the steeply dipping Izu-Bonin Wadati-Benioff zone, where subhorizontal ruptures typically originate in the interior of subducted slab and propagate toward the top at high angles. In particular, rupture of the 1988 September 7 event consists of two en echelon line sources. At a depth of approximately 480 km. the overall rupture length of this earthquake constrains the minimum thickness of the seismogenic zone to be 27 +/- 12 km. At this depth, our estimated thickness of the seismogenic zone seems remarkably close to the maximum thickness (similar to 35 km) predicted by the metastable olivine wedge model of seismogenesis. This model interprets deep-focus earthquakes as a result of transformational faulting within a region bounded approximately by the 700 degrees C isotherm, The extent of rupture during the largest events in this region seems to be only slightly larger than the 20-25 km spacing between the two layers of a double seismic zone, found at depths between 300 and 400 km in the same general area. Beneath the Island of Sakhalin, a rupture speed greater than that of the shear wave is not required for the large, deep-focus earthquake (similar to 615 km) of 1990 May 12. Almost all complexity in observed waveforms for this event can be explained by a northward propagating, subhorizontal rupture composed of only two subevents.