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Seismic velocity structure of the metamorphic belt of central California

ALBUQUERQUE SEISMOLOGICAL LABORATORY KIRTLAND AIR FORCE BASE-EAST, ALBUQUERQUE, NEW MEXICO 87115

Abstract

A seismic-refraction experiment was conducted in the Melones Reservoir region of the metamorphic belt of central California in May 1972. The experiment was designed to produce data for analyses by seismic-refraction-line and time-term methods. The belt is geologically complex. Three seismic-refraction lines of northwest bearing were made in and parallel to it. Useful data were recorded on them in the range 0 < 75 km. These data appear as if the P waves were propagated by a layered structure. Rocks at the surface range from 1.7 km/sec to 5.0 km/sec in P-wave velocity. For depths of a few hundred meters, the rocks range regionally from 5.6 to 6.0 km/sec in P-wave velocity. In the range 20 75 km, the travel times for all three lines appear as if the P waves were propagated by a refractor of elevation 1.4 km below sea level that has a P-wave velocity of 6.3 km/sec. There is no evidence of dip. Notable travel-time deviations from this simple model correspond to P-wave paths on one of the lines that was made in and almost parallel to fault zones. The zones contain ultramafic rocks. The anomalies may be related to paths in these rocks that are above the 6.3-km/sec refractor. The time-term method is appropriate for reducing the data of regional extent, and it was so employed. Assumptions for anisotropy and isotropy were tested. The model of anisotropy fits the data much better than the more elementary one, and the statistical parameters of the fit are almost as low as the repeatability of the measurements. The slowest P-wave velocity (6.00 km/sec) is for directions in the azimuth bands 5° to 95° and 185° to 275°, and the fastest P-wave velocity (6.32 km/sec) is for azimuths of 140° and 320°, bearings which are approximately the trend of the belt. The correspondence of the directions suggests that the anisotropy might relate to structural features of the belt.

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