Crustal structure along the coast of California from seismic-refraction measurements

Abstract

Two reversed seismic-refraction profiles were recorded between Los Angeles and San Francisco in 1961. The three shot points were located in Santa Monica Bay near Los Angeles, offshore near San Francisco, and at Camp Roberts, about halfway between Los Angeles and San Francisco. The velocity of P along these profiles is 6.1 plus or minus 0.1 km/sec with possible exceptions near San Francisco and near Los Angeles, where the scatter in the arrival times indicates complex near-surface velocity variations. The velocity of P, between Los Angeles and Camp Roberts is 8.2 plus or minus 0.1 km/sec, and between Camp Roberts and San Francisco it is 8.0 plus or minus 0.2 km/sec. There is no indication of an intermediate crustal layer in the travel times of first arrivals. Computed depths to the M discontinuity, if the crust consists of a single layer, are 35 km at Jos Angeles, 23 km at Camp Roberts, and 23 km at San Francisco. Refractions from crustal layers of intermediate velocity need not appear as first arrivals, and, in the extreme, the depth to the M discontinuity may be one-third greater than the thickness of a one-layer crust. Amplitude measurements on seismograms from the drilled-hole shotmore » point at Camp Roberts give the attenuation with distance for P/sub g/as r/sup -2.13/. The combined data for P from the two shot points in the ocean give the attenuation with distance as r/sup -1.74/. The scatter in the measured amplitudes of P/sub n/ is too large to determine a rate of attenuation with distance, but the data indicate that attenuation in the 150 to 300-km range is less than r/sup -3/. (auth)« less