P-wave station anomalies and the structure of the upper mantle

Regional differences in seismic travel times at teleseismic distances have been estimated by various authors, and the agreement between them is sufficient to establish that the effects are real. Many data exist about these teleseismic anomalies and the corresponding Pn anomalies in the United States. Comparison of these values with model studies of the variation of P station anomalies with distance indicates that the major source of the anomalies is in the upper mantle. Furthermore, the variation in the anomalies between the western and eastern United States can be explained satisfactorily by a change in the thickness of the upper mantle low-velocity layer. The possibility that the temperature changes associated with the development of a thicker low-velocity layer were responsible for vertical movements of the crust is discussed.