Upper mantle structure of western North America from apparent velocities of P waves

In a study of variation of the compressional velocity with depth in the C region of the upper mantle in western North America, we determined apparent velocities of first arrivals across the Tonto Forest array in Arizona for about 70 shallow-focus earthquakes. The epicenters ranged from 10° to 30° in distance and were mostly south of the array. The method gives the slope of the travel-time curve directly, the parameter required for a velocity-depth determination. For this distance range we found that although the slope of the P wave travel-time curve decreased substantially with distance, i.e., increasing apparent velocity, the variation was not uniform. Two relatively abrupt changes were observed at distances of about 17° and 24°. These are most readily interpreted as two second-order discontinuities in the region of the mantle above 1000 km. Assuming an average crustal structure as derived from seismic refraction measurements and taking the uppermost 200 km of the mantle given by Gutenberg or Jeffreys, we investigated various possible forms of variation of the P wave velocity with depth. The discontinuities in the rate of change of velocity with depth which cause the observed breaks in the apparent velocity curve are found to be at depths of about 320 and 640 km. Although no absolute times are required in the method, the travel times for the various derived models are computed and compared with standard tables