Abstract

Tectonically regionalized tau estimates are used to obtain seismic P-wave travel-time corrections for lateral variations in the Earth's crust and shallow-mantle velocity structure. The corrections, in the form of estimates of tau perturbations, are functions of slowness and are assessed for both the source and receiver regions. The functional form is derived analytically and allows interpretations of causative velocity anomalies. Corrections for both ray path endpoints constrain effects typically assigned solely to the receiver, thereby helping assess systematic errors in hypocentral parameters. Over 1.25 million ISC Bulletin P-wave ray paths are used to estimate tau perturbation functions for seven types of tectonic regions. Particular attention is given to the problem of uniformly sampling all tectonic regions at all propagation depths in the mantle.

Estimates of source and receiver tau perturbations are consistently less than 1 sec and show a definite and systematic difference between the travel-time correction functions for oceans and continents. Source tau perturbations are indicative of negative velocity anomalies in the shallow mantle beneath oceanic regions and positive anomalies beneath continental regions and oceanic trenches. Heterogeneity confined to the upper 250 km of the mantle suggests velocity variations within ±5 per cent of the lateral mean over a depth interval of approximately 100 km. Differences between the receiver and source perturbations for a common tectonic region are attributed to variations in the characteristic crustal structure of the region, constrained by the receiver perturbation, and errors in origin time and/or focal depth, determined from the source perturbation. The estimates of perturbation functions therefore suggest systematic regional biases in crustal structure and hypocentral parameters. A less than average crustal thickness in oceanic regions and greater thickness in stable continental regions is indicated. Furthermore, it appears that the origin times of sources in oceanic regions are systematically determined too late and/or focal depths located too shallow, whereas origin times of sources in continental regions are systematically determined too early and/or focal depths located too deep.

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