Robust inversion of IASP91 travel time residuals for mantle P and S velocity structure, earthquake mislocations, and station corrections
Abstract
Using both P and S arrival time information, 41,108 events in the International Seismological Centre (ISC) catalog for the years 1964 to 1987 are 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 are common in the ISC data, do not unduly influence the velocity estimates. Both the P and S models of lateral heterogeneity contain prominent circumPacific 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 200400 km. The continental cratons are underlain by highvelocity anomalies with maximum amplitudes of 2%. Iceland and the Azores are underlain by lowvelocity mantle material that extends down to at least 400 km. The Benioff zones are only intermittently imaged as 12% highvelocity regions in the uppermost 400 km. They are best resolved in the P velocity variations. Both the P and S velocity models contain a circumPacific ring, beneath the Benioff zones, of 12% positive velocity deviations in the depth range 660870 km. Coherent highvelocity features are seen below South America, Borneo, TongaFiji, the Marianas, and the northern Kuriles. The anomalies beneath South America and Borneo extend into the 8701070 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 coremantle 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.
 Publication:

Journal of Geophysical Research
 Pub Date:
 July 1994
 DOI:
 10.1029/93JB02023
 Bibcode:
 1994JGR....9913727V
 Keywords:

 Earth Mantle;
 Earthquakes;
 P Waves;
 Propagation Velocity;
 S Waves;
 Seismic Waves;
 Seismology;
 Velocity Distribution;
 Inversions;
 Iterative Solution;
 Mathematical Models;
 Robustness (Mathematics);
 Geophysics;
 Seismology: Structure of mantle and core;
 Seismology: Body waves;
 Seismology: Structure of the lithosphere and upper mantle;
 Tectonophysics: Composition and state of the Earth's interior