Centroid Fault-Plane Inversion in Three-Dimensional Velocity Structure Using Strong-Motion Records

The Chia-yi area in southwestern Taiwan is a region of relatively high seismicity. Frequent earthquakes of various magnitudes in the shallow crust provide valuable information for understanding the regional seismotectonic environment. In this study, we use strong-motion records from the 22 October 1999, ML 6.4 Chia-yi earthquake sequence to determine the focal mechanisms based on a criterion combin- ing the P-wave polarity and the cross correlations between recorded and synthetic waveforms. A recent 3D structural model for the study region is used, and to ensure the computational efficiency, a database is established for the Green's functions obtained by the finite-difference method. Optimal focal depths and fault-plane solu- tions are obtained through a grid-search scheme driven by the genetic algorithm to further improve the efficiency. We apply this approach to determine the focal mecha- nisms and centroid depths of the small and moderate events in the Chia-yi earthquake sequence. Results show dominant strike-slip and thrust mechanisms that are in good agreement with previous results based upon P-wave first-motion polarities and moment-tensor inversions. This highly automatic and efficient approach will enable the determination of focal depths and fault-plane solutions immediately following the occurrence of small- to moderate-sized earthquakes.

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