Abstract: The East Kunlun fault zones are located in the northern margin of the Bayanhar block. Investigation of earthquake rupture behaviors of these fault zones is helpful for future seismic risk estimation for northwest Sichuan. We carried out a number of field geological investigation, typical micro-topography DGPS measurement and sample dating. Our results show that earthquake activities of the East Kunlun fault zones extend to the north bound of the Zoige basin. This part has been named Luocha segment, which belonged to the Tazang fault. Satellite images for Luocha segment are clear on the gray strips. The earthquake rupture zones are represented mainly by fault scarp, slope in the valley, gully and terraces dislocation, plant distribution in linear, plunge, fault springs, broken plug pond, landslide, collapsed and talus. These phenomena are distributed along the fault and formed an ~50 km long inverse “L-shaped” deformation strip. Fault activities produced gullies and left movement of terraces. Horizontal dislocations are in the range of 5.5~6 m, 18~23 m, 68~75 m, 200~220 m respectively. An earthquake happened recently between 340±30～500±30 BP, its macro-epicenter was located 5~7 kms northwest from the Benduo village, magnitude M_W7.3~7.4, with a maximum of 6 m coseismic displacement, horizontal dislocation 5.5~6 m, vertical dislocation 0.2~0.5 m. These phenomena show that the Tazang fault was the causative fault of the earthquake and raptured in Holocene. The fault has been controlled by compressive shear recently and shows horizontal movement with slight thrust components, similar to other segments of the East Kunlun fault zones. The results supports the “mainland escape” model.

Key words: Earthquake rupture, The East Kunlun fault zones, Luocha segment, The Tazang fault