Moho orientation beneath central California from regional earthquake travel times

Abstract

Table Al is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B84‐011; $2.50. Payment must accompany order.Analysis of relative Pn travel times recorded by 238 stations of the U.S. Geological Survey central California seismic network (CALNET) from 77 regional earthquakes and explosions shows that arrivals are progressively delayed to the northeast in both the Coast Ranges and Sierra Nevada foothills. By dividing the CALNET into 10 subarrays and assuming laterally uniform velocities for a crust over an upper mantle half space, the orientation of a planar Moho beneath each subarray is calculated by inversion of relative Pn arrival times. The results show that the Moho dips to the northeast between 3° and 6° beneath the Coast Ranges and 8°–17° beneath the Great Valley and is horizontal beneath the foothills. Pn residuals west of the San Andreas fault are relatively early by 0.35 s but are interpreted as resulting from a lateral velocity contrast across the fault rather than variations in crustal thickness. Unexplained Pn travel time variance improves by approximately 60% after correction for the travel time due to dip and mean Pn residual at each station. The Pn data corrected for the travel time due to a dipping Moho indicate an isotropic upper mantle velocity of 7.97±0.05 km/s beneath the Coast Ranges, Great Valley, and foothills. Evidence of upper mantle velocity anisotropy in the azimuthal variation in Pn apparent velocity corrected for dip is only marginally significant at the 5% level. However, large azimuthal variations in corrected apparent velocity do exist and may be attributed to velocity heterogeneities in the crust, unmodeled higher‐order mantle structure, and gradients in upper mantle velocity. Arrival times of Cape Mendocino earthquakes recorded in the Coast Ranges south of San Francisco Bay indicate lateral variations in upper mantle velocity. The 7.4 km/s apparent velocity estimate from travel times of the Oroville earthquake recorded in the Coast Ranges out to distances greater than 400 km is anomalous in that Pn is normally observed at distances greater than 200 km. These arrivals may originate from updip propagation along a 6.9 km/s layer in the lower crust of the Coast Ranges subparallel to the Moho.