Three‐dimensional seismic velocity structure of the Earth's mantle using body wave travel times from intra‐plate and deep‐focus earthquakes

Abstract

This paper combines the recent measurements of P and S wave travel times from intra‐plate earthquakes with travel times from deep‐focus earthquakes reported earlier to infer the velocity structure of the mantle. It is reasonable that the source bias for intra‐plate earthquakes is less pronounced than that for other shallow‐focus earthquakes such as those occurring in island arcs. The inclusion of intra‐plate earthquakes compensates for the lack of uniform areal distribution exhibited by the deep‐focus earthquake data and augments the number of high quality travel time observations. The features observed in the variation of teleseismic travel times from intra‐plate earthquakes are consistent with the travel time variation determined from deep‐focus earthquakes (especially the P wave travel time variation). The combined intra‐plate and deep‐focus earthquake travel time data were converted to determine an average (spherically symmetric) compressional and shear velocity model and, subsequently, a three‐dimensional compressional velocity model for the earth's mantle. Lateral variation in the mantle accounted for most of the scatter in the observed P wave travel times. The average compressional velocity model suggests two rapid velocity increases at 500 and 670 km depths and a small velocity gradient in the depth interval of 100–450 km. The 3‐D model determined from the augmented travel time data set corroborated the earlier findings (1) that the lateral heterogeneities are more pronounced near the upper‐mantle and near the core‐mantle boundary and (2) that the surface tectonic features are correlated with compressional velocity anomalies near the upper mantle, but not with velocity anomalies below the upper mantle.