【摘要】 汶川地震(M_s8.0)形成了具逆冲兼右旋走滑性质的映秀-北川破裂带、纯逆冲性质的灌县-安县破裂带以及它们之间具左旋走滑-逆冲性质的小鱼洞破裂带,成为世界上迄今为止空间上分布最为复杂、长度最大的逆冲型地震地表破裂带。其地表破裂过程对理解发震机理具有重要的科学意义。文章根据地表破裂带的不同运动特性以及分段性,并结合不同地段同震断层擦痕行迹,探讨汶川地震地表破裂过程。通过详细分析,认为沿地表破裂带出现南北两个大的滑移量峰值,以及南北两段破裂带的几何分布和运动特性均不同,表明汶川地震是由南北两个次级破裂事件组成,这个结论与地震波数据反演结果一致,并根据八角庙和北川地区同震断层擦痕的运动学对比研究,推测汶川地震初期破裂以逆冲运动为主,沿映秀断裂和灌县-安县断裂,同时形成约100～80km长近纯逆冲性质的映秀-清平段地表破裂带和灌县-安县地表破裂带,并且在这两条破裂带向NE方向扩展过程中形成了小鱼洞地表破裂带。由于南部初期破裂事件触发了北川断裂的活动,造成后期破裂事件的发生。后期破裂事件以右旋走滑(或右旋斜冲)运动为主,沿映秀-北川断裂形成龙池-深溪沟-八角庙-清平-擂鼓-北川-南坝-石坎等一线破裂带,该破裂带在映秀断裂带上叠加了第一次破裂事件形成的部分破裂带,整体地表破裂带南段(映秀-清平段)叠加了两次不同性质的破裂过程,北段(擂鼓-北川-南坝-石坎段)只反映了第二次破裂事件的过程。所以,汶川地震地表破裂带中约270km长的映秀-北川破裂带具有逆冲兼右旋走滑性质,而约80km长的灌县-安县破裂带为纯逆冲性质。地震破裂过程是一动态复杂的物理过程,上述地表破裂模型可以解释两条不同性质破裂带的形成。但是否符合实际的地震破裂过程,还需要通过近台网地震波数据精细反演地震震源破裂过程的验证。更多还原

【Abstract】 On 12 May 2008,the Wenchuan earthquake(M_S8.0)produced the most complicated yet longest thrust-type co-seismic surface rupture zone,which comprising the dextral-slip thrusting Yingxiu-Beichuan fault,the approximately pure thrusting Guanxian-Anxian fault and the sinistral-slip thrusting Xiaoyudong rupture zone between them.Aiming at understanding the focal mechanism,here we discuss the surface rupture process by examining the segmentation and kinematic feature of the surface rupture zones,together with the co-seismic fault striations occur in various sites.Basing on the two displacement peaks,the different geometric and kinematic patterns for the southern and northern segment of the surface rupture zone,it is implied that the Wenchuan earthquake might contain two rupture events,which is in agreement with the seismic wave inversion results.By comparing the kinematic feature of fault striations occur in the Bajiaomiao and Beichuan area,it is suggested that at the first stage,thrusting along both the Yingxiu-Beichuan fault and Guanxian-Anxian fault produced the about 80～100km long Yingxiu-Qingping surface rupture segment and Guanxian-Anxian surface rupture zone,and just during their northeastward extension, the Xiaoyudong rupture zone has also developed;then,Beichuan fault was triggered by the first rupture process in the south,and ruptured along the Longchi-Shenxigou-Bajiaomiao-Qingping-Leigu-Beiehuan-Nanba-Shikan segment and was characterized by dextral strike-slip(or dextral oblique thrusting).Due to the overlap of the two rupture stages,the southern segment(Yingxiu-Qingping)shows two different processes while the northern segment(Leigu-Beichuan -Nanba-Shikan)only suggests the second one.That is the reason why the Yingxiu-Beichuan surface rupture zone is about 270km long and indicates both the thrusting and the dextral strike-slip,while the Guanxian-Anxian rupture zone is only about 80km long and is approximately pure thrusting.Considering that the earthquake is a dynamic and complicated physical process,though it can explain the development of the two different rupture zones, whether the model we discussed here is agree with the actual rupture process still need to be verified by the precise local seismic wave inversion.更多还原