No abstract

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.

This paper extends an earlier study (Sengupta & Julian) of travel times of P waves of deep-focus earthquakes to include shear waves. Primary advantage of deep-focus earthquakes is the reduction of anomalies caused by complex structures near the source. The standard deviations of travel times and station anomalies of this study are about half as large as those determined from the data of shallow-focus earthquakes (e.g. Herrin et ul.;Hales & Roberts). Spherically-symmetric velocity models derived from the travel times by a linearized inverse technique have resolving lengths of about 70 km for standard errors in velocity of about 0.02 km/s. No pronounced reversal of either compressional or shear velocity was required at the base of the mantle to satisfy the data, though a small velocity decrease could not be entirely ruled out. Some anomalous rapid changes in compressional velocity gradient were, however, found centred around the depths of 2400 and 2600 km. The models derived in this study agree most closely with that of Herrin er ul. for compressional velocity and the model 1066B of Gilbert & Dziewonski for shear velocity.

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 anomahes near the upper mantle, but not with velocity anomalies below the upper mantle. A. WESTERN AUSTRALIA i i i i ß ... ß ß ß ß ß ß ß ß ß ß. ß i ß b) GABON ß ß ß •ß ß ß ß ß ß ß ß ß ß ß ß ß : ß ß I t ß ß ßß ß ß ß I ß d) HAWAIIAN ISLANDS ß i , I i , i i i i i i I ß ß ß t t ß ß ß ß ß ß ß ß . ß ß ß ßß •,ß ß ß ß ß ß ß ß d•ß ß ß & ß ß ß ß ß eee e•e I ß ß ß ß e " ß ß ß ß ß e e ß ee . ß ß . ß • ß ß ß ß ee ß ß • ß ß ß ß ' ee ß ß e•e ee • ß ß .. . • ß ß ß e ß ß ee ß ß ß ß •e e ß e) SOUTHWEST OF SUMATRA s/Sz, Bruce Julian, and Bob Sheppard for technical assistance during this study, Belle Lasher for drafting assistance, and Charlotte Scott and Dot Lyle for typing the manuscript. The authors also would like to thank the anonymous reviewers for their comments which have im-. proved the manuscript.