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Three-dimensional velocity structure in northern California Coast Ranges from inversion of local earthquake arrival times

U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025
GEOPHYSICS DEPARTMENT STANFORD UNIVERSITY, STANFORD, CALIFORNIA 94305

Abstract

The three-dimensional P-wave velocity structure in the upper crust in the northern California Coast Ranges from Santa Rosa to Ukiah is obtained by simultaneous inversion of local earthquake and refraction travel-time data for velocity and hypocentral parameters. In this region, where there are lateral heterogeneities, the three-dimensional inversion provides important results not found in two-dimensional refraction studies. The medium is parameterized with velocities at the intersections (gridpoints) of a nonuniform three-dimensional grid. Travel times are calculated with approximate ray tracing along arcuate paths, which are reasonable for lateral distances less than 45 km. Inversions with 5 to 10 and 2 to 3 km gridspacing achieved 80 per cent decreases in data variance. The inversion with 2 to 3 km gridspacing reduced the rms residual level to approximately the level of timing error. Anomalously low velocities to at least 6 km depth are found along the Maacama and Healdsburg-Rodgers Creek (HRC) fault zones. The low-velocity region is widest at the en-echelon junction of the Maacama and HRC fault zones. Low velocities are associated with the Clear Lake basin where there is a thick sequence of volcanics interbedded with sedimentary deposits. There is a high-velocity body, located southeast of The Geysers between the Maacama and Collayomi fault zones, which may be associated with igneous intrusives or high-grade metamorphism related to an underlying inferred partial-melt body, modeled in previous teleseismic travel-time studies. There is some indication in the vapor-dominated geothermal reservoir of lower velocities relative to surrounding water-saturated rock, as has been shown in some laboratory experiments. Hypocenter locations obtained in the inversion show little variation from hypocenters obtained through inversion for one-dimensional velocity model and station corrections. For the inversion with 2 to 3 km gridspacing, however, some groups of events become more tightly clustered.

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