ROBUST INVERSION OF IASP91 TRAVEL-TIME RESIDUALS FOR MANTLE P AND S VELOCITY STRUCTURE, EARTHQUAKE MISLOCATIONS, AND STATION CORRECTIONS

Using both P and S arrival time information, 41,108 events in the International Seismological Centre (ISC) catalog for the years 1964 to 1987 axe relocated relative to the IASP91 velocity model. The mean absolute horizontal relocation is 7.7 km and the mean absolute depth relocation is 9.1 km. The mean absolute origin time shift is 1.2 s. The relocation procedure increased the P residual standard deviation slightly from 2.3 s to 2.5 s while decreasing the S residual standard deviation from 6.8 s to 6.1 s. When plotted as bottoming point averages, the resulting IASP91 P and S arrival time residuals show coherent patterns as a function of geographic location. An iterative l(p) residual norm minimization algorithm is used to estimate the set of P and S velocity variations as well as the earthquake relocation and seismographic station parameters which best explain the travel time residuals. The procedure is robust in that extremely large travel time residuals, which axe common in the ISC data, do not unduly influence the velocity estimates. Both the P and S models of lateral heterogeneity contain prominent circum-Pacific low velocities, 1% to 2% perturbations, underlying the back arc basins between 35 and 200 km depth. This ring of negative deviations continues into the depth interval 200-400 km. The continental cratons are underlain by high-velocity anomalies with maximum amplitudes of 2%. Iceland and the Azores axe underlain by low-velocity mantle material that extends down to at least 400 km. The Benioff zones axe only intermittently imaged as 1-2% high-velocity regions in the uppermost 400 km. They are best resolved in the P velocity variations. Both the P and S velocity models contain a circum-Pacific ring, beneath the Benioff zones, of 1-2% positive velocity deviations in the depth range 660-870 km. Coherent high-velocity features axe seen below South America, Borneo, Tonga-Fiji, the Maxianas, and the northern Kuriles. The anomalies beneath South America and Borneo extend into the 870-1070 km depth range. Below depths of 1270 km for P variations and 1070 km for S variations the amplitude of the heterogeneity has decreased significantly. It is only in the lowermost mantle, 2670 km to the core-mantle boundary, that the level of P heterogeneity rises significantly above the estimated noise level. In this depth range a partial ring around the Pacific basin is observed, although this pattern is somewhat obscured by poor resolution in the southern hemisphere. The hypocentral relocation parameters are sizable; they have a mean absolute horizontal relocation of 3.9 km and a mean absolute vertical relocation of 10.3 km. However, the event relocations do not alter the patterns of P and S velocity significantly. Deviations in P to S velocity ratios for the upper mantle are greatest (-3%) in the depth range 35-200 km beneath the oceanic trenches. Additional large variations of 2% are found below the continental cratons, particularly between the depths 200 and 400 km.