Crustal structure from San Francisco, California, to Eureka, Nevada, from seismic-refraction measurements

Abstract

Seismic-refraction measurements from chemical explosions near San Francisco, California, and Fallon and Eureka, Nevada, were made along a line extending nearly 700 km inland from San Francisco across the Coast Ranges, Great Valley, Sierra Nevada, and Basin and Range province. The velocity of Pg in the Basin and Range province was found to be 6.0 km/sec. Between Fallon and Eureka the velocity of Pn is 7.8 km/sec, and just east of the Sierra Nevada it is about 7.9 km/sec. Two prominent phases closely following the first arrival between 50 and 250 km from the source in the Basin and Range province were interpreted as reflections from an intermediate layer and from the M discontinuity. The velocity of P in the possible intermediate layer, deduced from the reflected phases because the refracted wave expected from this layer is nowhere a first arrival, seems to be 6.6 km/sec at the top of the layer and probably increases with depth. In the Coast Range northeast of San Francisco, the velocity of P in the crust, 5.4 to 5.6 km/sec, is substantially smaller than in the Basin and Range province. When corrected for abnormal delays in the sediments, first arrivals from San Francisco across the Great Valley suggest a Pn velocity of 7.9 km/sec. The low apparent velocity of first arrivals on this profile beyond the valley indicates that the M discontinuity dips downward beneath the Sierra Nevada. From a depth of about 20 km beneath the Coast Ranges and Great Valley, the M discontinuity descends to at least 40 km beneath the high Sierra Nevada. East of the Sierra Nevada it rises rapidly to about 22 km beneath Carson Sink and then dips downward toward Eureka, where it reaches a depth of 32 km. If the intermediate layer is real, the foregoing depths are increased by 2 to 5 km.