HR: 0800h
AN: T11D-1310    [Abstracts]
TI: Dynamic Deformational Characteristics of Co-seismic Shear Zone of the 1999 MW 7.6 Chi-Chi Earthquake,Taiwan
AU: * Lin, A
EM: slin@ipc.shizuoka.ac.jp
AB: The structures of fault rocks formed pass entirely throughout faulting periods, due to a long history there is a broad suite of deformational mechanisms involved in their deformational structures. Fault rocks formed at deeper levels by ancient faulting have been exhumed by uplifting, and if fault movement continues throughout that process there will be exposed in the fault zone, a variety of fault rocks formed under different conditions. It is therefore possible to gain insight into the process operating throughout the faulting history by studying the structures of fault rocks exposed. The drill cores through the Chelungpu fault zone related to the Chi-Chi (Taiwan) earthquake provide fresh and continuous core samples of fault rocks for studying the faulting process and tectonic history. In this paper, we report the analytic results on the meso- and micro-scopic structures of the co-seismic slip zone on the basis of the observations of the fault rocks within the Chelungpu fault zone from both the shallow drill cores and fault outcrops, and discuss their tectonic implications. The analytic results of the meso- and micro-scopic structures show that the 100-km-long surface rupture zone of the 1999 Mw 7.6 Chi-Chi earthquake was controlled primarily by the pre-existing Chelungpu fault zone, which is distributed in a wide zone of up to 60 m, and that the main co-seismic slip is localized in a narrow shear zone of <0.3 cm. Our results support the idea that the seismic slip is concentrated in a mm-scale narrow zone along pre-existing active faults. The Chelungpu fault zone is composed of cataclasite, fault breccia, and gouge zones that are well observed in both the fault outcrops and the drill cores taken throughout the fault zone. The foliations developed in the cataclasite and fault breccia zones are oriented parallel to that of the fault gouge zone where the main co-seismic slip occurred. The structural analyses of the shear zone and fault rocks show that the Chelungpu fault zone has slipped as a thrust fault with a significant left-lateral slip component since it formed from the Pleistocene to present. This oblique thrust motion is caused by the oblique convergence between the Philippine Sea plate and the Eurasian plate from the southeast.
DE: 7221 Paleoseismology
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8100 TECTONOPHYSICS
DE: 7200 SEISMOLOGY
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting