Relative travel-time residuals of teleseismic P waves recorded by the Saint Louis University network vary over a range of about 1.5 sec. The residuals depend on both position within the network and azimuthal direction to the source, the largest positive anomalies occurring at stations situated in the most seismically active region.
An inversion of these data using the method of Aki, Christoffersson, and Husebye (1976a) yields a three-dimensional model of velocity variation beneath the network. The most conspicuous feature of the derived model is a roughly cylindrical zone of low velocities at least 5 per cent lower than surrounding regions, which extends from near the surface to depths of 150 km or greater. Although the resolution for this model is poor for the crust, it is quite good for the upper mantle, where the zone of low velocities appears to angle toward the west with increasing depth.