Development of a Regional 3-D Velocity Model of the Pakistan Region for Improved Seismic Event Location
WESTON GEOPHYSICAL LEXINGTON MA
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In this report, we detail the development of a new regional 3-D tomographic P-wave velocity model WINPAK3D of a region in Southern Asia centered on Pakistan. Our primary goal in developing a 3-0 model of the crust and upper mantle in this region is to improve regional seismic event location. A detailed initial Pn model was iteratively refined using a nonlinear, conjugate-gradients technique that adjusts the velocity model of model to minimize the misfit between calculated and observed travel times from multiple stations and events, subject to smoothness constraints. We compute the travel times and their sensitivities to the velocity structure with an extension of the 3-D Podvin-Lecomte 1991 method. Because the event locations are not fixed, we also relocate all events using a 3-D grid search location method after each update of the 3-D velocity model. The data set consisted of a suite of 535 events containing over 6,600 arrivals from the Engdahl, el al. 1998 database. Our results show that the new model better fits the data compared to both the initial model and the global l-D IASP91 model. The root mean square RMS error for the updated 3-0 model is 1.81, compared to 2.01 for the initial 3.0 model and 2.42 for the IASP91 model.
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