Three-dimensional Vp/Vs variations along the Loma Prieta rupture zone

Abstract

An inversion of S - P seismic wave arrival time data primarily from the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) array deployed in the aftershock zone of the 1989 Loma Prieta, California, earthquake has yielded a 3-D model for Vp/Vs variations along the Loma Prieta rupture zone. Significant effort was devoted to 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 existing laterally heterogeneous P -wave velocity model to determine the initial paths for S waves, assuming a constant Vp/Vs . The model represents spatial variations in Vp/Vs on a 3-D grid with node spacings of 3 to 15 km horizontally and 3 to 5 km vertically. The reduction in data variance, computed exactly by applying the Vp/Vs perturbations to the S -velocity structure, exceeds 65% for the single-step inversion. We interpret the Vp/Vs model in conjunction with other available geological and geophysical information. Our major findings are 1) high Vp/Vs along the fault zone in the upper crust, indicative of upthrust gabbroic rocks, 2) predominantly low Vp/Vs in the lower crust, indicative of an underlying quartz-rich basement, and 3) two areas of low Vp/Vs in the near-surface associated with known quartz-rich rock sequences, one a sandstone and the other granite and/or granodiorite. We also tentatively identify a boundary between areas of low and slightly high Vp/Vs at depth in the immediate vicinity of the Loma Prieta mainshock hypocenter. Our results indicate the diagnostic power of a 3-D Vp/Vs model when it is combined with other geological and geophysical information.