Seismic Reflection Profile Across Los Angeles Basin

Abstract

A seismic reflection profile was run along the line of an earlier study in 1931, from the San Pedro hills south of Harbor City, northeastward between Signal hill and the Dominguez hills, across the basin axis, through West Coyote hills and the Puente hills, south of Puente. The profile consists of four parts, separated by faults. Southwest of Inglewood fault, reflections recorded from the deepest surface range from 3½ km. in the San Pedro hills immediately south of Harbor City, to 7 to 8 km. west of the Inglewood fault, crossed 1 km. (0.6 mile) west of Los Cerritos. This deepest surface is presumably the granitic basement because nowhere could deeper reflections be secured with increased charges of dynamite. Near the middle of the second block the deepest reflections occur at a maximum depth of 13-14 km. (45,000 feet), south of Bellflower, beneath the synclinal axis in the upper formations, and at several thousands of feet less depth near the Inglewood and Norwalk faults. On the third block the deepest reflections rise from a depth of about 6 km. near the Norwalk fault, southwest of La Mirada, to about 5 km. depth near the Whittier fault. North of the Whittier fault the few shots fired indicate that the deepest reflecting surface lies between 3½ and 2 km. in depth. The vertical off-set at the Inglewood and Norwalk faults, loci of many earthquakes, of roughly 5 km. in the deepest reflecting surface, suggests strongly that the synclinal Los Angeles basin is a graben at depth.

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