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 Inte rnational Seismological Centre (ISC) catalog for the years 1964 to 198 7 axe relocated relative to the IASP91 velocity model. The mean absolu te horizontal relocation is 7.7 km and the mean absolute depth relocat ion is 9.1 km. The mean absolute origin time shift is 1.2 s. The reloc ation procedure increased the P residual standard deviation slightly f rom 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 res ulting IASP91 P and S arrival time residuals show coherent patterns as a function of geographic location. An iterative l(p) residual norm mi nimization algorithm is used to estimate the set of P and S velocity v ariations as well as the earthquake relocation and seismographic stati on parameters which best explain the travel time residuals. The proced ure is robust in that extremely large travel time residuals, which axe common in the ISC data, do not unduly influence the velocity estimate s. 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 dev iations continues into the depth interval 200-400 km. The continental cratons are underlain by high-velocity anomalies with maximum amplitud es 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 o nly intermittently imaged as 1-2% high-velocity regions in the uppermo st 400 km. They are best resolved in the P velocity variations. Both t he 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 Amer ica, Borneo, Tonga-Fiji, the Maxianas, and the northern Kuriles. The a nomalies 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 signific antly. 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 t he Pacific basin is observed, although this pattern is somewhat obscur ed by poor resolution in the southern hemisphere. The hypocentral relo cation parameters are sizable; they have a mean absolute horizontal re location 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 v elocity significantly. Deviations in P to S velocity ratios for the up per mantle are greatest (-3%) in the depth range 35-200 km beneath the oceanic trenches. Additional large variations of 2% are found below t he continental cratons, particularly between the depths 200 and 400 km .