INTRODUCTION

Crustal and upper mantle structure under Southern California has been investigated by a variety of techniques including P-wave travel-time tomography, surface wave analysis, and seismic reflection lines (e.g., Hadley and Kanamori, 1977; Hearn and Clayton, 1986ab; Humphreys and Clayton, 1990; Li et al., 1992ab; Magistrale et al., 1992; Sung and Jackson, 1992; Zhao and Kanamori, 1992; Zhou, 1994; Zhao et al., 1996). The Southern California Seismic Network (SCSN) now comprises 300 stations with an average spacing of 20-30 km. In order to study in detail Moho depth, anisotropy, and fault structure at depth, much smaller station spacing is required. This was achieved in the 1993 phase of the Los Angeles Region Seismic Experiment (LARSE93). The travel times provided by LARSE93 data should improve the resolution of Los Angeles basin and Southern California tomographic models. Variations in Moho depth across the Los Angeles basin, San Gabriel Mountains, and Mojave Desert have important implications for velocity structure, depth of seismicity, as well as tectonic evolution and present tectonic setting of Southern California. Seismic anisotropy in Southern California also has a bearing on the present tectonic setting and upper mantle structure (Savage and Silver, 1993; Liu et al., 1995). Changes in anisotropy characteristics between basin and mountain stations, or across the San Andreas fault, provide an opportunity to study the shear strain field related to mantle flow and plate motion variations across Southern California. In addition, the structure of the San Andreas fault is not well understood at depth. Data collected during LARSE93 have the potential of elucidating fault structure in the lower crust and upper mantle.

The seismograms collected during the passive phase of LARSE are complemented by the acquisition of deep-crustal multichannel seismic-reflection and refraction profiles using onshore and offshore sensors, and airgun and explosion sources (LARSE94, active phase; Brocher et al., 1995; ten Brink, et al., 1996; Okaya et al., 1996ab; Murphy et al., 1996). The reflection and refraction data are especially useful in resolving upper crustal structure in order to better understand and constrain lower crustal and mantle structures.

LARSE93 was a joint effort involving scientists from the University of California at Los Angeles (UCLA), the U.S. Geological Survey (USGS), the California Institute of Technology (Caltech), and the University of Southern California (USC). It took place between November 11, 1993 and December 16, 1993 and involved the installation and maintenance of approximately 88 digital seismometers along a southwest-northeast array in Southern California. The stations were deployed in a 175-km-long, linear array across the Los Angeles basin, San Gabriel Mountains, and Mojave Desert. For this survey, energy sources were local, regional, and teleseismic earthquakes. The seismometers were placed an average of 1 or 2 km apart along the part of the line from Azusa, in the southern foothills of the San Gabriel Mountains, to the northeastern Mojave Desert (Fig. 1; Table 1). Four stations were placed in the Los Angeles basin (two near Seal Beach and two near Whittier); the signals recorded at these stations contain relatively high noise levels. The denser part of the array was located in the San Gabriel Mountains with 1 km spacing; the sparser part, in the Mojave Desert, had 2 km spacing. A few stations were added to the array during the course of the experiment and, except for the first few and last few days of the experiment, most stations recorded data continuously during the four weeks. One station (#331) was located in the San Andreas fault.

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