Seismic-Refraction Measurements of Crustal Structure Between Santa Monica Bay and Lake Mead

Abstract

A reversed seismic-refraction profile was recorded between Santa Monica Bay, California, and Lake Mead, Nevada, during November 1961. Depth to the Mohorovičić discontinuity was determined to be approximately 29 km at Santa Monica Bay, 36 km under the Transverse Ranges, 26 km under the Mojave Desert, and 30 km at Lake Mead.

Prominent events on the seismograms in the distance range 30 to 150 km are interpreted as reflections from the Mohorovičić discontinuity and from a crustal layer of intermediate velocity. These reflected events are used to make a detailed interpretation of crustal structure. The velocity of compressional waves in the mantle immediately below the Mohorovičić discontinuity was determined to be 7.8 km/sec. The velocity of compressional waves in the intermediate layer is near 7.0 km/sec. The apparent velocity of the direct arrival in the crustal rocks near the surface is 6.l km/sec north-east of Santa Monica Bay, and 6.1 km/sec southwest of Lake Mead. The higher apparent velocity for the direct arrival from Santa Monica Bay seems to be the result of thinning toward the east of low-velocity rocks near the surface. These low-velocity near-surface rocks are Cenozoic sedimentary rocks and fractured and weathered granitic and metamorphic rocks. The velocity of S_g was determined to be 3.4. km/sec near Lake Mead.

A prominent phase with apparent velocity of 6.3 to 6.4 km/sec was recorded at distances beyond 200 km. This phase is identified as P and is interpreted as a reflection from the intermediate layer. Amplitude measurements support the conclusion that the P phase is a reflected arrival.

First posted July 27, 2012