RELATIVE ARRAY ANALYSIS OF THE SOUTHERN CALIFORNIA LITHOSPHERE

Lateral velocity structure in the lithosphere beneath southern Califor nia is investigated using relative array analysis. In this form of ana lysis, P wave polarization values measured by a subset of receivers (s ubarray) are compared to values measured by the entire set of receiver s (array). The effects of lateral velocity structure beneath the subar ray can be distinguished from the effects of average velocity structur e beneath the entire array. Lateral velocity gradients can be mapped a s the subarray window is moved across the network. The method is insen sitive to effects of near-source and lower mantle heterogeneity, does not lose resolution neaT the periphery of the array, and utilizes a lo cally derived ray geometry. Lateral velocity variations beneath the Ca lifornia Institute of Technology-United States Geological Survey netwo rk detected by relative array analysis can be compared directly to vel ocity anomalies found by inversion of travel time residuals. Two regio ns near the edge of the tomographic image developed by Humphreys and C layton are investigated; both regions are experiencing active tectonic s. Relative array analysis suggests that relatively high velocity may be present beneath the westernmost Transverse Ranges. This feature ext ends to a depth of roughly 80 km and may be related to the high veloci ty upper mantle anomaly imaged beneath the central and western Transve rse Ranges. Relatively low velocity is associated with the crust and u ppermost mantle south of the Salton Sea. This feature is probably rela ted to hot, ascending mantle but is located east of the surface expres sion of the spreading axis in the Imperial Valley.