Routine, Ensemble-Based Global Centroid Moment Tensor Inversions using the Spectral-Element-Method

The centroid moment tensors (CMT) provided by the Global CMT Project (https://www.globalcmt.org) have been a staple for global tomography for more than 40 years. With ever-advancing tomographic methodology and computational resources, more detail in Earth's interior structure is rapidly being revealed. Over the last few years, both tomography and moment-tensor inversions have shown that in a fully three-dimensional forward-modeling regime, the classic Global-CMT solutions are on average 3 km too deep, the centroid time about 1 s late, and the seismic moment approximately 5% too low. Therefore, to keep pace with the increased capacity and demands of global tomography, a new catalog of improved, global CMT solutions suggests itself as the next step. In an ensemble-based fashion, we are developing an automated workflow that takes Global-CMT solutions and routinely inverts for new, improved CMT solutions. The workflow includes four main steps: [1] launching of the forward, three-dimensional (3D) spectral-element-method (SEM) simulations to generate synthetics, [2] downloading of the observed seismic data, [3] processing of synthetic and observed data, and [4] inversion for the six moment-tensor parameters and the centroid latitude, longitude and depth, as well as a grid search for moment magnitude and centroid time. The computational cost of the 3D forward simulations is high; making our workflow ensemble-based ensures its simplicity and scalability, both critical factors for the recomputation of the thousands of Global-CMT solutions that already exist today. Finally, the CMT inversion results will be integrated into the existing workflow for routine computation of global earthquake synthetics, made available by the ShakeMovie website (http://global.shakemovie.princeton.edu) which provides seismologists with a simple interface to access the new CMT solutions as a complement to the original ones.