Abstract: Field investigation on the surface ruptures of the M_S8.0 Wenchuan earthquake along the segment between Beichuan and Qingchuan shows that there is one surface rupture zone developed on this segment,which extends generally along the Beichuan-Qingchuan Fault zone.Observation at Huangjiaba Chenjiaba,Guixi,Pingtong,Nanba,and Shikan suggests that the surface ruptures on this segment spread continuously along the trend of the fault,with single structure and a length of 60～90km.The surface rupture has not reached Guanzhuang of Qingchuan county.The observable rupture zone is about 62km,between Beichuan and Shikan,trending 20°～55° in general,dipping NW with an angle of 70°,showing mainly thrusting with dextral strike-slipping.The most distinct feature of the surface ruptures of this earthquake is the vertical surface bending,which indicates the thrusting of the deep fault.Its horizontal motion on this segment displays as dextral strike slipping,without sinistral slipping component.The value of the vertical coseismic displacements decreases gradually from 3m at Huangjiaba to about 1.5m at Nanba and Shikan;The amount of the dextral displacements does not change evidently,generally between 1.5m and 2.0m.Features of the surface rupture show that the causative tectonics of this M_S8.0 Wenchuan earthquake is the Yingxiu-Beichuan-Qingchuan Fault,whose movement is characterized mainly by thrusting,with a dextral slipping component,and the thrusting direction is from west to east.

Key words: M_S8.0 Wenchuan earthquake, surface rupture, coseismic displacement, thrusting, dextral strike slip

摘要： 对北川-青川间汶川M_S8.0地震地表破裂的野外地质调查表明,在这一段内主要存在一条地震地表破裂带,总体沿北川-青川断裂带分布。沿黄家坝、陈家坝、桂溪、平通、南坝、石坎等地的观察显示,该段地表破裂沿走向连续分布,结构单一,破裂长度为60～90km,地表破裂没有到达青川县关庄镇。可观察到的破裂长度在北川北至石坎之间,长62km,走向总体为20°～55°,运动学性质主要为右旋走滑逆冲。地震形成的地表破裂主要表现为垂向上的地表拱曲,指示了深部断层的逆冲性质;在水平运动方向上则主要表现为右旋走滑,不存在左旋走滑分量。地震地表破裂显示的同震垂直位移从西南段黄家坝的3m左右,向东北逐渐降低至南坝、石坎的1.5m左右;右旋水平位移没有明显变化或者略有增加,一般在1.5～2.0m之间。地表破裂特征表明,引起本次汶川M_S8.0地震的发震构造是映秀-北川-青川断裂带,该断裂以逆冲运动为主,兼具右旋走滑分量,逆冲方向由NW向SE。

关键词: 汶川M_S8.0地震, 地表破裂, 同震位移, 逆冲, 右旋走滑