The surface rupture of the 1999 Mw 7.6 Chi-Chi (Taiwan) earthquake was controlled primarily by the pre-existing Chelungpu fault zone, which is distributed in a wide zone of up to 60 m, although the main coseismic slip is localized in a narrow shear zone of <0.3 cm. 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 coseismic 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 its formation in the Pleistocene. This oblique thrust motion is caused by the southeast–northwest oblique convergence between the Philippine Sea plate and the Eurasian plate.

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