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Published July 2010 | Published
Journal Article Open

Spatial heterogeneity of tectonic stress and friction in the crust: new evidence from earthquake focal mechanisms in Taiwan

Abstract

We performed inversions of earthquake focal mechanisms in central Taiwan to investigate the heterogeneity of the stress field and fault strength, and temporal variations of stress parameters, friction and pore pressure associated with the 1999 Chi-Chi earthquake. We divided the focal mechanism data into two groups: before and after the Chi-Chi earthquake, and analysed them separately. With the assumption of a uniform stress field, the friction coefficient is mostly within a range of 0.2–0.4 in central Taiwan, which is lower than the commonly quoted laboratory result, 0.6–0.85. The low friction coefficient is also inferred by the rotation of principal stress axes after the Chi-Chi earthquake. By contrast, if we assume that the friction is constant and failures occur on optimally oriented planes, we find that the resulting stress orientations must be spatially variable. However, a large dispersion of stress orientations is not seen in borehole breakouts and fault slip data, implying a constant friction model might be ruled out. Our analysis suggests that either the distribution of the coefficient of friction or pore pressure changed during the 1999 Chi-Chi earthquake. We infer that the pore pressure probably rose in the Chi-Chi rupture area and northern Longitudinal Valley and dropped in the areas south of the coseismic rupture area after the main shock.

Additional Information

© 2010 The Authors. Journal compilation © 2010 RAS. Accepted 2010 March 26. Received 2010 March 26; in original form 2009 November 18. We thank the Editor, Dr. I. Grevemeyer, and the two reviewers, Dr. S. Ghimire and Dr. J. Suppe for their thoughtful reviews and valuable comments that helped to improve the manuscript. This study is supported by the Institute of Earth Sciences, Academia Sinica, the National Science Council of the Republic of China grant NSC 98-2119-M-001-033-MY3. This is a contribution of the Institute of Earth Sciences, Academia Sinica, IESASXXX.

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