Abstract
The purpose of this study is to obtain an accurate pattern of the geographical distribution of travel-time residuals of P waves from deep earthquakes in and around Japan. The residuals were calculated on the basis of relocated hypocenters. The large-scale lateral heterogeneity of the upper mantle beneath the island arc was taken into account in the hypocenter determination, but the degree of heterogeneity was not assumed. Two kinds of residuals (I and II) were considered. The residual I is the observed travel-time minus the calculated travel-time corrected for the lateral heterogeneity. The residual II is the observed travel-time minus the calculated travel-time without such correction. The residual I is mainly controlled by the local structure of the crust and uppermost mantle beneath the station. It correlates with the gravity anomaly. The residual II is related to the large-scale anomalous upper mantle structure represented by a high-velocity oceanic lithosphere underthrusted beneath the island arc. The average P-velocity contrast between the underthrusted lithosphere and the upper mantle on the continental side of it was estimated to be at least 3%.
