HR: 0800h
AN: T11D-1309    [Abstracts]
TI: Changes in Regional Stress Associated with the 1999 Chi-Chi, Taiwan, Earthquake Based on Focal Mechanisms
AU: * Jiang, J
EM: jim@scman.cwb.gov.tw
AU: Huang, B
EM: hwbs@earth.sinica.edu.tw
AU: Tsai, Y
EM: ybtsai@geps.gep.ncu.edu.tw
AU: Shin, T
EM: shin@taiwan.cwb.gov.tw
AB: The 1999 Mw7.6 Chi-Chi earthquake was the most recent prominent earthquake in Taiwan. The mainshock was followed by substantial number of strong aftershocks. Pre- and post-mainshock focal mechanisms of the 1999 Chi-Chi earthquake are analyzed to characterize spatial and temporal variation around the Chelungpu fault, which ruptured during the mainshock. The Frohlich­Ýs triangle diagram used to define the classification of earthquake focal mechanisms. We categorized the types of earthquake focal mechanisms as either normal , strike-slip, or thrust according to the corresponding maximum plunges is the pressure axis (P), null axis(B), or tension axis (T). The strain and stress orientations was estimated based on moment tensor summation and focal mechanism stress inversion respectively. The stress is caused by tectonic sources, but the strain results from the application of stress to particular structure of the crust. We show the difference between stress and strain directions and on what we can learn from this observation about tectonic processes in the volume studied. A comparison of the directions of strain and stress tensors showed close agreement for subvolumes with predominantly thrust and strike-slip faulting. This article describe and characterize the spatial and temporal distribution of earthquake focal mechanisms in the region of the 1999 Chi-Chi earthquake before(107 event) and after(401 event) the mainshock within the period from 1 January 1991 to 31 December 2001. Whereas the earthquake focal mechanism data suggest some spatial and temporal complexity in Central Taiwan, the overall pattern can be characterized by cluster analysis of grouped earthquake according to the seismicity trend and type of focal mechanisms. The aftershock focal mechanisms are dominant by strike-slip type even though the mainshock is thrust fault type. In contrast, for temporal distribution of different kind of aftershock focal mechanisms, which occurrence sequence is strike-slip, thrust and then normal types. In general, The most P axes of earthquakes which occurred before and after the 1999 Chi-Chi earthquake are consistent with the regional strain and stress orientations. These results indicate that the structural impact caused by 1999 Chi-Chi earthquake show a consistent trend of west-northwest, parallel to the direction of plate convergence.
DE: 7230 Seismicity and seismotectonics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting