3-DIMENSIONAL VP VS VARIATIONS ALONG THE LOMA-PRIETA RUPTURE ZONE/

An inversion of S - P seismic wave arrival time data primarily from th e Program for Array Seismic Studies of the Continental Lithosphere (PA SSCAL) array deployed in the aftershock zone of the 1989 Loma Prieta, California, earthquake has yielded a 3-D model for V(p)/V(s) variation s along the Loma Prieta rupture zone. Significant effort was devoted t o identifying S-to-P converted waves, which were prevalent at a few of the stations used. The inversion method uses 3-D ray tracing in an ex isting laterally heterogeneous P-wave velocity model to determine the initial paths for S waves, assuming a constant V(p)/V(s). The model re presents spatial variations in V(p)/V(s) on a 3-D grid with node spaci ngs of 3 to 15 km horizontally and 3 to 5 km vertically. The reduction in data variance, computed exactly by applying the V(p)/V(s) perturba tions to the S-velocity structure, exceeds 65% for the single-step inv ersion. We interpret the V(p)/V(s) model in conjunction with other ava ilable geological and geophysical information. Our major findings are 1) high V(p)/V(s) along the fault zone in the upper crust, indicative of upthrust gabbroic rocks, 2) predominantly low V(p)/V(s) in the lowe r crust, indicative of an underlying quartz-rich basement, and 3) two areas of low V(p)/V(s) in the near-surface associated with known quart z-rich rock sequences, one a sandstone and the other granite and / or granodiorite. We also tentatively identity a boundary between areas of low and slightly high V(p)/V(s) at depth in the immediate vicinity of the Loma Prieta mainshock hypocenter. Our results indicate the diagno stic power of a 3-D V(p)/V(s) model when it is combined with other geo logical and geophysical information.