Trans-California seismic profile-Pahute Mesa to San Francisco Bay

Abstract

Seismologists from the University of California at Berkeley and the ESSA Earthquake Mechanism Laboratory at San Francisco cooperated in an experiment to record seismic waves along a profile from the Nevada Test Site to San Francisco Bay, by utilizing four high-yield nuclear explosions detonated under the Pahute Mesa in Nevada as energy sources. The selected profile includes the Yosemite Valley and California's Central Valley a few miles south of Stockton, and instrumental coverage included 18 temporary and nine permanent seismographs in addition to local coverage in the source area and the Lamont-Doherty ocean bottom seismometer off Point Arena. Pn travel times along the profile may in general be represented by the equation t = ( 6.2 ± 0.1 ) + Δ / ( 7.88 ± 0.02 ) with deviations from this under the Central Valley probably because of deep sediments, and under Owens Valley and the White Mountains probably as a result of a thickened crust. Early travel times to stations located in the Sierra Nevada do not permit an appreciable root under the Sierra Nevada portion of the profile unless the material of that root, if it exists, has a relatively high velocity. Crustal layers under the Central Valley, the Pahute Mesa, and the Sierra Nevada have total thicknesses approximating 30 km and thicknesses of 35 to 40 km under Owens Valley and the White Mountains. Because of high source energies, temporary stations located in the Central Valley were able, for the first time, to record from artificial sources initial seismic waves above heavy local noise.