Mounting evidence suggests that fault zone heterogeneity may play a crucial role in the localization of rupture in earthquakes [Aki, 1995]. The heterogeneity can take several forms: spatial variations in physical properties (elastic properties, pore fluid pressure, etc.) or complexity in the fault surface (bends, offsets, etc.). High-resolution, three-dimensional models of the P and S wave velocity (Vp and Vs) structure and accurate hypocenters provide tools for studying the effects of fault zone heterogeneity on rupture localization.
A dense, passive array of 48 seismic instruments was deployed south of Hollister, California, from mid-November 1994 to late May 1995. The array obtained local-earthquake seismograms for high-resolution, three-dimensional imaging of the (Vp and Vs) structure in the San Andreas fault zone. In mid-May, anactive seismic experiment was also carried out. Figure 1 shows a 7-km wide, 3- to 4-km deep zone of very low Vp imaged during the experiment. The zone is bounded on the southwest by the San Andreas fault and the adjacent Northern Gabilan Range. The San Andreas fault appears to be approximately vertical to at least 5 km depth (the limit of good model resolution), contrary to an existing model based on gravity data.
