A Unified Approach to Joint Regional/Teleseismic Calibration and Event Location with a 3D Earth Model
MASSACHUSETTS INST OF TECH CAMBRIDGE
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This project investigates the problem of locating seismic events from combined data sets of regional and teleseismic arrival times, based on the use of a unified 3D model of the Earths velocity structure to predict travel times for both types of data. Inherent to this problem is the joint tomographic calibration of the Earth model with both regional and teleseismic ground-truth data. The goal of the unified approach is to remove the inconsistencies that result when travel-time predictions are performed with a mixture of separately calibrated regional and global models and empirical corrections, which can lead to degradation of event location performance. Numerical experiments we have performed with joint regionalteleseismic travel-time tomography have demonstrated, for example, that joint calibration preempts a baseline shift between regional and teleseismic travel-time predictions. Our project has focused largely on the practical difficulties of implementing a unified locationcalibration capability. One such difficulty is the computational challenge of performing tomographic inversion with very large numbers of data and model parameters. Available data bases, for example, contain many millions of regional and teleseismic arrivals while 3D Earth models with appropriate resolution contain millions of grid nodes. Travel-time computation in 3D models presents another significant challenge. It can require days of central processing unit CPU time to forward model a travel-time data set when a fully 3D ray tracing methods is used. An efficient alternative to full 3D ray tracing is travel-time linearization, which approximates the travel time in a 3D model by integrating its slowness function along the ray path computed in a 1D reference model.