abstract

An experiment to investigate the major earth structure along a profile from the Nevada Test Site (NTS) to the Pacific Ocean across two sections of the Sierra Nevada, one in the Kings Canyon area and the other which includes Huntington Lake, was undertaken by the Earthquake Mechanism Laboratory of the National Oceanic and Atmospheric Administration (NOAA). Instrumental coverage included 35 temporary and seven permanent seismographs. Energy sources included four high-yield and 20 intermediate- to low-yield nuclear explosions under the NTS, one high-yield explosion under Amchitka Island of Alaska, two earthquakes near Santa Rosa, and one earthquake in Monterey Bay. An upper-mantle speed of 7.9 km/sec satisfied most of the observed data except under the Sierra where the velocity was somewhat lower than this. An earthquake in Monterey Bay helped to close the west end of the profile. The Sierra Nevada “root” in large part can be attributed to relatively low upper-mantle speed under the Sierras (estimated at 7.64 km/sec) which extends to an indefinite depth and which possibly may serve as lens to refract late arriving high-energy waves to coastal areas. Crustal thickness from the Sierra foothills eastward varies from 25 to 35 km, the thinner portion under the Sierra crest and Owens Valley, near Independence, and the thicker sections under the western Sierra and the basin ranges east of Owens Valley. West of Fresno, the crust thins more or less gradually to about 20 km in thickness under the western coast ranges. This profile contrasts with that from an earlier study, across the Central Valley south of Stockton, where no such thinning under the Central Valley was observed. The sub-Sierra crust east of Fresno is somewhat complicated. Low-angle thrusting toward the east is indicated.

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