Automatic simultaneous source localization and focal mechanism inversion based on strain Green’s function database for 3D background models

Abstract:

The simultaneous inversion of the source location and focal mechanism may be able to provide more consistent results on which the inference of seismicity pattern and tectonic stress typically relies. Hence, we present an inversion method to simultaneously determine the source location and focal mechanism by reducing the residuals in the travel time and waveform of phases based on wave simulations in 3D background models by the spectral-element methods (SEM). The method takes advantage of the source-receiver reciprocity and builds the strain Green’s tensor (SGT) database from sources placed at individual stations. As the travel times provide critical constraints on source location, we utilize an adaptive likelihood picker (ALP) to identify the arrival time of the phases and build a synthetic travel time table for the 3D background model, which accelerates the source localization inversion. With these inverted results as the initial source locations, we can further perform the grid search to update the source locations and invert for the focal mechanism by improving the waveform fits between data and synthetics.

We further apply the simultaneous inversion method to determine the source location and focal mechanism of the 2019 Ridgecrest, California, earthquake sequence. We use a radially anisotropic 3D model of southern California crust previously determined based on adjoint tomography (Wang et al 2020) as our background model and perform forward simulations based on the SPECFEM3D_Cartesian package. Our source location and mechanism results show consistency with other published catalogues, including those using the P polarity and S/P ratio and HASH method (Yang et al 2012, https://scedc.caltech.edu/research-tools/alt-2011-yang-hauksson-shearer.html), those based on the SGT database constructed using the finite-element method and the SCEC community model (CVM-S4.26) (Wang & Zhan 2020), and the Southern California Earthquake Data Center centroid moment tensor (SCEDC-CMT) catalogue (https://earthquake.usgs.gov/earthquakes/search/) determined using the long-period surface waveform. We are able to obtain the mechanisms for a significant number of earthquakes that are not in the SCEDC-CMT catalogue with the lowest magnitude of Mw3.18. The improved source location and mechanisms can be used to further interpret the fault orientations and tectonic stress.