Earthquake Focal Mechanisms and Stress Environment in Southern Taiwan
Date Issued
2012
Date
2012
Author(s)
Lee, Chiao-Ying
Abstract
The largest on-land earthquake in Taiwan since the 1999 Chi-Chi earthquake (Mw7.6) occurred on March 4, 2010, near the town of Jiashian in southern Taiwan. The earthquake (Mw5.7) has a hypocentral depth of 18 km and is located in an area of relatively low seismicity. To examine the tectonic environment surrounding the Jiashian earthquake, we conduct a systematic determination of the focal mechanisms of earthquakes in its source region. Waveform records at stations of Broadband Array in Taiwan for Seismology (BATS) are obtained for earthquakes of magnitudes 3.5 ≤ ML ≤ 6.4 from 1997 to 2010 within a 50-km radius from the Jiashian epicenter. A total of 694 earthquakes are collected, and the source parameters of 318 earthquakes, including focal mechanisms, focal depths and moment magnitudes, are determined with good confidence by fitting three-component waveforms in the frequency bands of 0.05-0.3 Hz for P wave and 0.02-0.1 Hz for S and surface waves using the Cut And Paste (CAP) method of Zhao and Helmberger (1994) and Zhu and Helmberger (1996).
A first look at these focal mechanism solutions suggests that they can be grouped into events of similar fault types in four subareas in our study region: thrust faults in the west; strike-slip faults in the northeast; normal faults in the southern Central Range (CR); and mixed thrust and strike-slip faults in the southern Longitudinal Valley (LV). Horizontal projections of the P-axes of these focal mechanisms show patterns consistent with that of the surface deformation in southern Taiwan: an overall SE-NW compression resulted from the convergence of the Philippine Sea Plate and the Eurasian Plate. To further investigate the spatial pattern of the principal stress axes as well as their relationship with the focal mechanisms of earthquakes, we conduct linear stress tensor inversions (Hardebeck and Michael, 2006) using our focal mechanism solutions for the earthquakes around the Jiashian source area. The stress inversion results show distinct spatial patterns: a compressive regime in southern CR with the maximum compression axis in WNW direction around the Chukou Fault and in EW near the northern tip of the Chaochou Fault; alignments of the horizontal maximum extension and maximum compression axes in NE-SW and NW-SE directions, respectively, near the southern tip of the Hsueshan Range (HR) around the triple junction of the Western Foothills, HR and CR; an extensional zone in the CR west of southern LV with a NE-SW maximum extension axis; and a compressive region in southern LV with a SE-NW maximum compression axis. These results from focal mechanisms and variations in principal stress axes have important implications for understanding the characteristics of regional tectonics and their relationship with regional earthquakes and deformation.
Subjects
Focal mechanism
waveform fitting
stress field
linear stress inversion
Taiwan
Type
thesis
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