Travel-time residuals were calculated for Pn arrivals from four nuclear explosions and 10 earthquakes recorded by the U.S. Geological Survey's San Andreas seismic telemetry network. Arrivals at stations west of the fault are earlier than at stations east of the fault. Stations on basement rocks indicate a variation in crustal thickness of about 5 to 9 km across the fault at the center of the network. The average mantle velocity after correcting for the residuals is 8.0 km/sec. The Pn waves traveling from the east, when corrected for crustal delay, have a velocity of 7.7 to 7.8 km/sec. Waves traveling from the north or south, after the same corrections, have a Pn velocity of 8.0 to 8.2 km/sec. This difference may be caused by an anisotropic upper mantle, but additional data from seismic sources in the Pacific will be required to confirm this result.

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