Azimuthal variations of teleseismic P wave residuals for northern California

Abstract

Teleseismic P wave residuals from 70 events (1971–1979) for seven stations in central and northern California are investigated as a function of azimuth to model upper mantle velocity structure. Relative residuals are determined for each station using Berkeley as the reference station. Maximum relative residual variation is +0.9 s in the northern Coast Ranges to −1.7 s in the Klamath Mountains. Interpretations of the residual variations represent lateral variations of the P wave velocity in the upper mantle. The modeling technique utilizes a simple ray‐tracing technique to calculate time‐residual equivalent path lengths in the anomalous velocity regions. A model of the paleosubduction zone, Farallon plate, beneath northern California represents a region (40‐ to 150‐km depth) of high‐velocity (8.3–8.5 km/s), which explains the negative residual pattern for the Klamath Mountains, Cascades, and Modoc Plateau region. A low‐velocity zone (7.2–7.4 km/s) beneath the Gorda plate (25‐ to 155‐km depth) produces the observed positive residual pattern of the northern Coast Ranges. The low‐velocity region is related to proposed models of the Mendocino triple junction migration and associated upwelling of asthenospheric material.