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Global centroid moment tensor solutions in a heterogeneous earth: The CMT3D catalog


 For over forty years, the Global Centroid-Moment Tensor (GCMT) project has determined location and source parameters for globally recorded earthquakes larger than magnitude 5.0. The GCMT database remains a trusted staple for the geophysical community. Its point-source moment-tensor solutions are the result of inversions that model long-period observed seismic waveforms via normal-mode summation for a one-dimensional (1-D) reference earth model, augmented by path corrections to capture three-dimensional (3-D) variations in surface wave phase speeds, and to account for crustal structure. While this methodology remains essentially unchanged for the ongoing GCMT catalog, source inversions based on waveform modeling in low-resolution three-dimensional (3-D) earth models have revealed small but persistent biases in the standard modeling approach. Keeping pace with the increased capacity and demands of global tomography requires a revised catalog of centroid-moment tensors, automatically and reproducibly computed using Green functions from a state-of-the-art 3-D earth model. In this paper, we modify the current procedure for the full-waveform inversion of seismic traces for the six moment-tensor parameters, centroid latitude, longitude, depth, and centroid time of global earthquakes. We take the GCMT solutions as a point of departure but update them to account for the effects of a heterogeneous earth, using the global three-dimensional wavespeed model GLAD-M25. We generate synthetic seismograms from Green functions computed by the spectral-element method in the 3-D model, select observed seismic data and remove their instrument response, process synthetic and observed data, select segments of observed and synthetic data based on similarity, and invert for new model parameters of the earthquake’s centroid location, time, and moment tensor. The events in our new, preliminary database containing 9,382 global event solutions, called CMT3D for “3-D centroid-moment tensors”, are on average 4 km shallower, about 1 s earlier, about 5 percent larger in scalar moment, and more double-couple in nature than in the GCMT catalog. We discuss in detail the geographical and statistical distributions of the updated solutions, and place them in the context of earlier work. We plan to disseminate our CMT3D solutions via the online ShakeMovie platform.