abstract

Analysis of travel times from a reversed active seismic refraction experiment in the western foothills of the Sierra Nevada reveals a localized layer of high-velocity material within 10 km of the surface. This layer, with a velocity of 7.0 km/sec, has a five-fold increase in dip approximately 25 km south of the epicenter of the 1 August 1975 magnitude 5.7 earthquake near Oroville, California. We interpret this material to be a remnant of buried ophiolite. These data also suggest a 5 per cent anisotropy in the near-surface P-wave velocity; the compressional velocity is 6.3 km/sec parallel to the strike of the Sierra range and 6.0 km/sec normal to it.

A series of aftershocks to the magnitude-5.7 Oroville earthquake recorded on a linear array of portable seismographs extending 300 km southward along the west edge of the Sierra Nevada foothills show an apparent Pn velocity of 7.8 km/sec. The Mohorovičić discontinuity is located between 30- and 39-km depth. This wide range of depth reflects the facts that the profile is unreversed, the receivers are widely spaced, and the origin times and depths of the sources are not exactly known.

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