干旱区生态环境知识资源中心

The increased concern over seismic hazards in the Los Angeles region has created interest in the crustal structure and mechanical properties associated with faults, like the Sierra Madre and San Andreas. This study presents the seismic properties of major lithologies found in the vicinity of the San Andreas fault zone in order to correlate with the Los Angeles Region Seismic Experiment (LARSE) results. Scientists from the LARSE group have recently acquired high-quality seismic data along three transects. Thirty-nine rock samples were collected along the LARSE Line 1, which trends NE-SW across the central Mojave Desert and the San Gabriel Mountains. These rock samples represent the following lithologies: Precambrian gneisses, San Gabriel anorthosite, Mesozoic granitic intrusives, Pelona schist, Mendenhall granulite gneisses and San Andreas fault zone cataclasites, The seismic properties were determined by measuring compressional and shear wave velocities as a function of confining pressure, Average compressional and shear wave velocities at 150 MPa show a wide range of values. The Mendenhall gneiss and the San Gabriel anorthosite complex have high velocities, 6.5 km/s and 6.4 km/s, respectively, The Pelona schist, the Mojave granitic intrusives, and the San Andreas cataclasitic rocks have average velocities of 5.8 km/s, 5.2 km/s, and 5.1 km/s, respectively. Average densities range from 2320 kg/m(3) for fault zone cataclasites to 3100 kg/m(3) for a Mendenhall granulite gneiss. Gneisses and schists show significant seismic anisotropy (5-20%) due to preferred mineral orientation, while most other rocks are nearly isotropic. Poisson’s ratios range from a high of 0.29 in the San Gabriel anorthosite to a low of 0.22 in the San Gabriel gneiss. Comparison of the measured seismic properties with the LARSE velocity model allows interpretation of crustal structure in terms of rock compositions. The upper crust southwest of the San Andreas fault contains granitic intrusives, gneisses and schists, while under the Mojave Desert, the upper crust is dominated by gneisses. High velocities under the San Gabriel Mountains represent either lower crustal granulites or anorthosite. The velocity decrease beneath the San Andreas fault zone originates from minor amounts of cataclasite or the presence of low-velocity lithologies such as Pelona schist. (C) 1998 Elsevier Science B.V. All rights reserved.