Upper mantle structure of the western United States

Body wave observations from nuclear events at the Nevada Test Site and several earthquakes in the western United States have been used to construct models of the P wave structure for the upper mantle. The Cagniard-deHoop technique for computing synthetic seismograms for laterally homogeneous earth models was used to fit both the amplitudes and the travel times of the observations. Below a depth of 300 km one model fits all the observations remarkably well. This model has sharp discontinuities at 430 and 650 km and a discontinuity in slope at about 550 km. Above 350 km the observations can be divided into two groups according to the location of the point of deepest penetration of the rays. The difference between the areas is due primarily to the size of the low-velocity zone and the magnitude of the velocity immediately below the low-velocity zone.