Abstract
SUMMARY A central problem of seismology is the inversion of regional waveform data for models of earthquake sources. In regions such as Southern California, preliminary 3-D earth structure models are already available, and efficient numerical methods have been developed for 3-D anelastic wave-propagation simulations. We describe an automated procedure that utilizes these capabilities to derive centroid moment tensors (CMTs). The procedure relies on the use of receiver-side Green’s tensors (RGTs), which comprise the spatial-temporal displacements produced by the three orthogonal unit impulsive point forces acting at the receivers. We have constructed a RGT database for 219 broad-band stations in Southern California using a tomographically improved version of the 3-D SCEC Community Velocity Model Version 4.0 (CVM4) and a staggered-grid finite-difference code. Finite-difference synthetic seismograms for any earthquake in our modelling volume can be simply calculated by extracting a small, source-centred volume from the RGT database and applying the reciprocity principle. The partial derivatives needed for the CMT inversion can be generated in the same way. We have developed an automated algorithm that combines a grid-search for suitable focal mechanisms and hypocentre locations with a Gauss–Newton optimization that further refines the grid-search results. Using this algorithm, we have determined CMT solutions for 165 small to medium-sized earthquakes in Southern California. Preliminary comparison with the CMT solutions provided by the Southern California Seismic Network (SCSN) shows that our solutions generally provide better fit to the observed waveforms. When applied to a large number of earthquakes, our algorithm may provide a more robust CMT catalogue for earthquakes in Southern California.
Highlights
Accurate and rapid seismic source parameter inversions are important for seismic hazard analysis in earthquake-prone areas such as Southern California
Synthetic seismograms as well as their partial derivatives with respect to centroid moment tensors (CMTs) parameters are often computed in simple 1-D earth structure models using semi-analytic methods (e.g. Zhao & Helmberger 1994; Dreger 2003)
We extend the receiver-side Green’s tensors (RGTs)-based CMT inversion algorithm in Zhao et al (2006) to include a grid search in the vicinity of the reference locations for suitable focal mechanisms and source locations that minimize different measures of waveform misfit
Summary
With the advancement of digital broad-band instrumentation, complete Centroid Moment Tensor (CMT; Dziewonski et al 1981) solutions can be automatically recovered from regional broad-band waveform data for earthquakes with local magnitude larger than 3.0 (Dreger & Helmberger 1991; Zhao & Helmberger 1994; Thio & Kanamori 1995; Pasyanos et al 1996; Zhu & Helmberger 1996; Liu et al 2004; Clinton et al 2006). In the waveform inversion approach, an optimal CMT solution is found by minimizing a certain measure of the waveform misfit between observed and model-predicted (synthetic) seismograms either in time domain
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