Tectonic implication of local seismic tomography and focal mechanism study in Northeast Taiwan

Abstract:

Evaluation of the stability of host rock and nearby seismic activities are critical for safety issue. Since 2011, a broad-band seismometer array with 15 seismometers has been gradually deployed to date in the study area to collect continuous waveform data for host rock characterization study. In this study, we presented a series of study results, including location/relocation of regional earthquakes, seismic focal mechanisms, and inversion of three dimensional Vp and Vp/Vs models. A double-difference tomography algorithm was adopted in both earthquake relocation and three-dimensional velocity inversion to improve location accuracy and model reliability. Spatial distribution of earthquakes could be separated into two clusters. The eastern group was distributed nearly along the eastern boundary of granite outcrop in the surface; the other group was mainly located beneath the western part of imbricated high-magnetic-susceptibility zones, which were identified by three-dimensional inversion of aeromagnetic data. In-between there exists a clear zone with relatively quiet seismicity. Moreover, our Vp and Vp/Vs inversion results also revealed patterns compatible with previous-mentioned imbricate structures and shown good correlation with the high magnetic susceptibility zones as well. In addition, more than one hundred earthquake events with clear P-wave first motion in waveform were identified for demonstration. Their focal mechanisms were also determined and shown dramatic variation with respect to the depth of hypocenters. Mechanisms of earthquakes in shallow crust with depth less than 10 km are mostly normal faults. In contrast, most of deeper events are reverse faults. It implies that tectonic stress regime in shallow crust is extensional and becomes more compressional in deeper part.