Earthquake travel times extracted from the International Seismological Centre data set have been used to map the laterally varying Pn velocity structure of the uppermost mantle and to estimate the crustal thickness at each recording station in the western United States. The mapping (see cover) was done by Thomas Hearn, New Mexico State University, Las Cruces, and Noureddine Beghoul and Muawia Barazangi, both at the Institute for the Study of the Continents, Cornell University, Ithaca, N.Y. First arrival times from regional distances (200–1200 km) are shown in Figure 1.
A tomographic backprojection method was used [Hearn and Clayton, 1985] that images the uppermost mantle velocity in the western U.S. The method also solves for the station and event static, or time delays. Results correlate well with major tectonic features, as seen on the cover image. Slower uppermost mantle velocities (V < 7.9 km/s) were found centered in the Basin and Range Province surrounded by upper mantle of higher, more normal Pn velocities (V > 7.9 km/s). These low Pn velocities are directly due to the effects of isostasy, hotspot volcanism and crustal extension.