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NATIONAL CENTER FOR EARTHQUAKE RESEARCH U.S. GEOLOGICAL SURVEY, MENLO PARK, CALIFORNIA 94025
Abstract
Time terms resulting from an analysis of Pg seismic travel-time data from the Coast Ranges of central California yield a pattern that is remarkably consistent with other geological and geophysical estimates of depth to basement. A set of high-quality data for more than 330 paths was selected from all available travel-time observations from quarries and explosions. To assure a statistical estimate of the uncertainty of each time term, time terms were calculated only for sites recording arrivals from more than one source.
Time terms ranged from 0.0 to 1.5 sec, indicating depths to the Pg refractor of near-surface to more than 6 km, depending on assumed near-surface velocities. The Pg refractor is presumed to be granitic and metamorphic rocks southwest of the San Andreas Fault and volcanic or ultrabasic rocks within or beneath the Franciscan Formation northeast of the San Andreas Fault. Small time terms (< 0.4 sec) are associated with exposed granitic basement in the Santa Cruz and Gabilan ranges and with exposures of the Franciscan Formation in the central Diablo Range and in the Santa Teresa Hills (south of San Jose). Large time terms are associated with large thicknesses of upper Cretaceous and younger sedimentary rocks in the northern Santa Cruz Mountains, in the northern part of the Diablo Range, in structural troughs between the Zayante and San Andreas faults, and between the Sargent and San Andreas faults and in the Hollister Trough. Time terms for sites athwart structural discontinuities, particularly the San Andreas Fault, vary with the direction of the approaching ray. Time terms for sites on the relatively homogeneous granitic mass of the Gabilan Range increase with elevation.
The large differences in magnitude of these time terms and the nature of the distribution clearly demonstrate that the laterally inhomogeneous Earth's crust in this area must be taken into account in the location of local earthquakes where high accuracy is required. These time terms provide a firm basis for the necessary station corrections.
Footnotes
Publication authorized by the Director, U.S. Geological Survey.
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