Three dimensional model of seismic velocity variation in the Earth's mantle

Three dimensional velocity models for the earth's mantle were obtained satisfying P, S and some PcP and ScS travel time anomalies from deep focus earthquakes. A method of successive approximation was used to compute the uniform relative velocity perturbation over three dimensional blocks of size 10°×10° in longitude and latitude, and 500 km in thickness. Features of these velocity perturbations indicated that lateral heterogeneity is the most pronounced in the upper mantle and near the core-mantle boundary. The upper mantle anomalies are correlated with surface tectonic features, but these are not correlated with deep mantle anomalies.