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U.S. Geological Survey Crustal Studies Technical Letter Number 5

In cooperation with the Defense Advanced Research Projects Agency

Crustal Structure along the Coast of California from Seismic-Refraction Measurements

By J.H. Healy


Thumbnail of and link to report PDF (49 MB)Abstract

Two reversed seismic-refraction profiles were recorded between Los Angeles and San Francisco in 1961. The three shotpoints were located in Santa Monica Bay, offshore near San Francisco, and at Camp Roberts, about halfway between Los Angeles and San Francisco.

The velocity of Pg 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 8.0 ± 0.2 km/sec. There is no indication of an intermediate crustal layer in the traveltimes of first arrivals.

Computed depths to the Mohorovicic 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 Mohorovicic discontinuity may be one-third greater than the thickness of a one-layer crust.

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For additional information:
Contact Information, Earthquake Science Center, Menlo Park Science Center
U.S. Geological Survey
345 Middlefield Road, MS 977
Menlo Park, California 94025

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Suggested citation:

Healy, J.H., 1962, Crustal structure along the coast of California from seismic-refraction measurements: U.S. Geological Survey Crustal Studies Technical Letter Number 5, 30 p. (Available at




Field Operations

Geologic Setting

Accuracy of the Data

Traveltime Data

Secondary Arrivals

Interpretation of Traveltimes

The effect of Intermediate Layers


Comparison with Earlier Results



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