A Progressive Inversion Scheme for Three-dimensional Imaging of Seismic Velocities in Earth's Mantle

We investigate the effects of mislocated earthquakes on the velocity model obtained in tomographic inversions for mantle structure and the ability of simultaneous and progressive inversion techniques to correct earthquake mislocations and produce an accurate velocity model. We solve a system of tomographic travel time equations in three ways: (a) directly, neglecting source terms, (b) simultaneously for both velocity model terms and corrections to the source locations, and (c) progressively, for each set of terms in succession. The algorithms all perform least-squares inversions; they differ primarily in their treatnent of source mislocation terms. Simulations demonstrate that ignoring the effects of source mislocation results in underestimating velocity anomalies by up to 50%, creates smeared anomalies in adjacent voxels with values up to 50% of the retrieved velocity of its neighbor, and creates anomalies elsewhere in the mantle with values greater than those estimated for input anomalies. Clearly, careful treatnent of the source location problem is critical to the accurate estimation of three-dimensional velocity variations. The progressive inversion developed here generally produces more accurate source corrections and velocity anomaly estimates than does an inversion scheme in which both source corrections and velocity terms are found simultaneously. These results are superior particularly with respect to the suppression of artificial anomalies. Electronic mail: [email protected], [email protected]