Crustal Thickness Variations in Southern California from Los Angeles Region Seismic Experiment Passive Phase Teleseismic Travel Times

Abstract

The goal of the 1993 Los Angeles Region Seismic Experiment (LARSE93) passive phase was to collect waveform data from local and distant earthquakes to study lower crust and upper mantle structural features in southern California, particularly under the San Gabriel Mountains and San Andreas fault. During LARSE93, approximately 88 stations were deployed in a 175-km-long, linear array across the Los Angeles basin, San Gabriel Mountains, and Mojave Desert northeast of Los Angeles. During the four weeks of continuous recording, teleseismic events recorded at each site provided a wide range of ray path backazimuths. The teleseismic events included a number of intermediate-magnitude earthquakes with epicenters in the Aleutian Island, Kamchatka, Kuril Island, mid-Atlantic Ridge, Solomon Island, Japan, Fiji Island, and Chile regions. This experiment was followed by LARSE94, which involved land refraction and deep-crustal seismic reflection profiles from offshore and onshore explosion sources. We have used the joint data sets in this study to distinguish upper crustal features from adjacent lower crustal and upper mantle structures. P-wave travel times were determined from 17 teleseisms, and upper crustal residual signatures were removed by incorporating LARSE94 upper crustal velocity model results. Within each backazimuth range, the resulting relative traveltime residuals increase from negative values (-0.5-sec average) recorded in the northern San Gabriel Valley-southern San Gabriel Mountain foothills to positive values (0.2-sec average) in the central and northern San Gabriel Mountains. The residual patterns display variations for different backazimuths and incidence angles but show almost no lateral spatial shift of maximum or minimum residual along the array, indicating that the dominant source of the residual pattern is shallow (<50 km). The patterns of residuals require a sharp lateral gradient in shallow velocities between the northern San Gabriel Valley (located in the northernmost Los Angeles basin) and the San Gabriel Mountains over a distance of less than 50 km. Most of the residual pattern can be explained by laterally varying crustal thickness and a high-velocity anomaly in the upper mantle. In our model of Moho depth variations, the northern San Gabriel Valley to the southwest of the array is underlain by thinned continental crust. Crustal thickness increases laterally by 12 km over a distance of less than 50 km into the San Gabriel Mountains. This conclusion is supported by widespread surface evidence for rift-style volcanism and crustal extension in southern California crustal rocks.

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