中文摘要：

2008年5月12日在四川西部发生的汶川地震是一次以逆冲运动为主，兼有右旋走滑运动的斜滑型地震，形成了有史以来最长、最复杂的地表破裂之一。其中，很多复杂现象到目前为止还没有得到很好的解释或一致的认识，如小鱼洞地区出现的NW走向的小鱼洞断裂，在小鱼洞以北出现的2条相距11km的平行断裂同时破裂的现象等。通过在小鱼洞地区的详细野外调查，获得了详细的地表破裂分布及同震位移分布，在此基础上对小鱼洞地区地表破裂的机制进行了分析。结果表明，造成上述复杂地表破裂的根本原因是汶川地震的主断层北川-映秀断裂的产状变化，即北川-秀断裂在小鱼洞以北向NW偏移约3．5km。其破裂机制是：1）北川-映秀断裂的右旋走滑运动在小鱼洞西侧的左阶挤压阶区引起的挤压隆升形成前冲断层，即小鱼洞断裂；2）由于北川-映秀断裂在小鱼洞以北向NW偏移3．5km，导致其断层面倾角变大，逆冲运动引起的断层上盘对下盘的挤压方向变化，结合右旋走滑引起的上盘对下盘的侧向推挤，两者共同作用突破了彭灌断裂，从而形成了2条相距11km的平行断裂同时错动的现象。另外，文中建议应该重视北川-映秀断裂右旋走滑运动分量、断层产状变化以及断层上、下盘的岩性差异对汶川地震地表破裂过程及地表破裂分布的影响。

英文摘要：

The Mw 7.9 Wenchuan, China, earthquake is a large oblique reverse slip shock, whose main fault is dominated by reverse slip with right-lateral strike-slip component. It generated one of the longest and most complicated surface ruptures, and to many of the phenomena, we haven＇t had an appropriate interpretation or a common understanding, e. g. on the 7km-long NW-trending Xiaoyudong Fault and the coinstantaneous fracture on the two parallel thrust faults which are llkm apart on the north of Xiaoyudong area. Field investigation in the Xiaoyudong area shows clear co-seismic rupture and displacement, and on these bases, we analyzed the mechanism of the surface rupture in the Xiaoyudong area. Our study indicates that the change of attitude of Beichuan-Yingxiu Fault （ BYF）, that is, the ca. 3.5km step-over in the west of the Xiaoyudong area, is the primary cause of the above complex phenomena. Specific mechanisms are as follows： 1 ）The dextral strike-slip of the BYF results in compressional uplift in the left-restraining step-over, creating a frontal reverse fault, known as the Xiaoyudong Fault. ; 2）The Pengguan Fault, which is parallel to and llkm apart from the BYF, is activated in the north of the step-over by a combination of the increased dip angle in the north of the step-over due to the ca. 3.5km left step of the BYF and the lateral push of the hanging wall to the footwall of the BYF caused by the dextral strike-slip of the BYF. These results are helpful in deepening our understanding of the dynamic processes that produced surface ruptures during the Wenchuan earthquake. We also suggest that more attention shall be paid to the impact of the dextral strike-slip component, the change of primary fault＇s attitude and the difference of the rocks of the fault＇s two walls on the process and distribution of surface rupture.