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

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 Pn along these profiles is 6.1±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 Pn between Los Angeles and Camp Roberts is 8.2±0.1 km/sec, and between Camp Roberts and San Francisco it is 8.0 it 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 Los 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 shot point at Camp Roberts give the attenuation with distance for Pg as r−2.13. The combined data for Pg from the two shot points in the ocean give the attenuation with distance as r−1.74. The scatter in the measured amplitudes of Pn is too large to allow us 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−3.