Array measurements of P velocities in the upper mantle

Abstract

A velocity stucture for P waves in the upper mantle has been derived from dT/dΔ measurements with the extended array at the Tonto Forest Seismological Observatory in Arizona. Short-period P waves from earthquakes in the distance range 0 to 30 degrees have been used. Corrections for the crust underlying the array have been applied, using crustal models derived from seismic refraction and gravity data, By including later arrivals on the seismograms a fairly complete dT/dΔ curve has been constructed and inverted to obtain a velocity model for the upper 750 km of the mantle. The model includes a low-velocity zone with a high velocity gradient near its lower boundary. Two other regions with high velocity gradients are located near depths of 400 and 650 km and consist of 9–10% increases in velocity, spread out over 50 to 100 km. Travel times and amplitudes calculated for this model are consistent with observed data.