Structural analysis of the coseismic shear zone of the 2008 Mw 7.9 Wenchuan earthquake, China

Field investigations reveal that the surface rupture of the 2008 Mw 7.9 Wenchuan earthquake, China, occurred along a pre-existing shear zone in the Longmen Shan Thrust Belt. Structural analyses of the coseismic fault zone and fault rocks show that i) the main coseismic shear zone consists of a fault core that includes a narrow fault gouge zone of <15 cm in width (generally 1–2 cm) and a fault breccia zone of <∼3 m in width, and a wide damage zone of >5 m in width that is composed of cataclastic rocks including fractures and subsidiary faults; ii) the foliations developed in the fault core and damage zones indicate a dominantly thrust slip sense, consistent with that indicated by the coseismic surface rupture; and iii) coseismic slip was largely localized to within a narrow fault gouge zone of <2–3 mm in width. The structural characteristics of the coseismic shear zone and cataclastic rocks indicate that the location of coseismic slip zone associated with the 2008 Wenchuan earthquake was controlled by a pre-existing shear zone and that the main active fault of the Longmen Shan Thrust Belt has moved as a thrust since the formation of cataclastic rocks along the fault during the late Miocene or early Pliocene.