Two seismic refraction profiles were recorded in the Santa Clara Valley region of central California to determine the upper crustal seismic velocity structure. A reversed, 8-km-long profile was recorded across the valley near Gilroy and an unreversed 38-km-long profile was recorded in the southern Santa Cruz Mountains. The data indicate that the valley is wedge-shaped in cross section with the basement dipping 10° to the east. The alluvial fill of the valley has a maximum thickness of 1.5 km and has an average compressional-wave velocity of 2.3 km/sec. The top of the basement has a velocity of 4.3 km/sec; the velocity reaches 6.0 km/sec at a depth of 2 km. The Calaveras fault zone, located due east of the valley, was crossed by a short, unreversed profile. It appears to be characterized by a low velocity (Vp = 2.7 km/sec) to a depth of ∼2.5 km. The southern Santa Cruz Mountain profile indicates a 4.6-km/sec near-surface refractor and a layer with a velocity of 6.1 km/sec at 2-km depth. Thus, considered together, the similarity in velocity structure determined from the two profiles indicates that the Franciscan rocks of the Santa Cruz Mountains continue beneath the valley.

The presence of high compressional-wave velocities (≧6.0 km/sec) at 2-km depth in this Franciscan terrain can best be explained by an abundance of metabasalts (greenstones) at that depth. These seismic observations indicate that an area of at least 120 km2 is underlain by this high velocity, basaltic material. Therefore, the upper crustal structure in the study area differs dramatically from the Franciscan terrain of the Diablo Range where greywackies and metagreywackies (Vp ≦ 5.4 km/sec) dominate in the uppermost crust.

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