We have examined local earthquake shear wave travel times from a four-station, intermediate-period, three-component seismograph array located in the Coast Ranges north of San Francisco Bay. Our preferred interpretation of the data indicates an upper crustal layer with a shear velocity of 3.05 ± 0.10 km/sec over a lower crustal layer having a shear velocity of 3.87 ± 0.15 km/sec. We estimate the thickness of the upper crustal layer to be 13.0 ± 3.0 km. The shear velocity of the lower crustal layer, 3.87 km/sec, is higher than crustal shear velocities reported elsewhere in the Coast Ranges and corresponds to the shear velocity of layer three of the oceanic crust.

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