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. ¿ American Geophysical Union 1986 |