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Effects of three-dimensional velocity structure on the seismicity of the 1984 Morgan Hill, California, aftershock sequence

U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025

Abstract

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 corresponds well to the surface geology of the region, predominantly showing a low-velocity region associated with the sedimentary sequence to the southwest of the Madrone Springs fault. This low-velocity sequence extends to a depth of approximately 5 km. Previously constructed geologic cross-sections for this region had suggested that these sediments extended to a depth of less than 2 km. 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 Calaveras 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.

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