Three-dimensional velocity structure, seismicity, and fault structure in the Parkfield region

A three-dimensional velocity model for the area surrounding the 24 April 1984 Morgan Hill earthquake has been developed by simultaneously inverting local earthquake and refraction arrival-time data. This velocity model corre-sponds well to the surface geology of the region, predominantly showing a low-velocity region associated with the sedimentary sequence to the south-west of the Madrone Springs fault. This low-velocity sequence extends to a depth of approximately 5 kin. Previously constructed geologic cross-sections for this region had suggested that these sediments extended to a depth of less than 2 kin. Relocation of 2,179 hypocenters with the three-dimensional velocity model did not show great differences when compared to locations done with a one-dimensional velocity model with station corrections. In both cases, the Calaveras fault is steep but has a noticeable dip to the northeast and the seismicity does not intersect the surface along the trace of the Ca-laveras fault in the southern half of the rupture zone. The greatest differences between the two sets of locations was that the locations using the three-dimensional velocity model were deeper, and fewer earthquakes were located within the low-velocity region. The focal mechanisms were also determined for 946 earthquakes using both the one-dimensional and three-dimensional earth models. Both earth models yield similar focal mechanisms for these earthquakes