【摘要】 汶川地震在地表形成了北东向与北西向两个方向的地表破裂带,余震分布也清晰地显示沿着小鱼洞断裂存在一条北西向小震密集条带。为了研究北西向小鱼洞断裂在汶川地震破裂过程中的作用,在已有的地表破裂数据和认识的基础上,结合汶川地震前小震资料和余震资料,完善了汶川地震震源构造模型。分别计算了以北川—映秀断裂西南的虹口段(BY1)与小鱼洞断裂作为初始破裂段所产生的库仑静应力变化量(ΔCFS)分布图像。结果显示,以北川—映秀断裂虹口段(BY1)做为起始破裂段,虽然小鱼洞断裂西北段(XYD2)的局部地段处于应力触发区,但在出现地表破裂的小鱼洞断裂东南段(XYD1)却处于1.5 bar的应力抑制区,同时随着北川—映秀断裂向北东方向的进一步破裂,小鱼洞断裂仍处于应力抑制区,并且范围有所扩大。如果这样,在汶川地震过程中,小鱼洞断裂应该是稳定的,不可能产生地表破裂带及小震密集条带;以小鱼洞断裂作为起始破裂段,北川—映秀断裂BY1段的大部分区段处于1.0～1.5 bar的应力触发区,不但如此,小鱼洞断裂对彭灌断裂也有触发作用。基于以上结果,认为汶川地震破裂过程是以北西向小鱼洞断裂为起始破裂段,该断裂的破裂触发了北川—映秀断裂和彭灌断裂,并导致北川—映秀断裂向北东方向发生级联破裂。更多还原

【Abstract】 The 2008 Wenchuan Ms 8.0 earthquake has produced two long NE trending rupture zones as well as a short NW-strike rupture zone on the surface.The latter one is consistent with the Xiaoyudong fault and the distribution of aftershocks,and has been paid less attention.We speculate that this NW-directed fault may play a certain role during the main shock process.To test this idea,we established a seismic source model comprising three faults,the NE trending Beichuan-Yingxiu fault,the Pengguan fault,and the NW trending Xiaoyudong fault,and calculated Coulomb failure stress change during the source rupture.We considered two cases for the model.One is that the southwestern section of the NE directed Beichuan-Yingxiu fault served as the initial rupture.And the other is that the NW trending Xiaoyudong fault ruptured first during the main shock process.Under the first condition,most of the Xiaoyudong fault was inhibited by 1.5 bar Coulomb stress from failure(the stress shadows),though its northwestern section was in the stress trigger region.With the rupture of the Beichuan-Yingxiu fault spreaded toward NE,the stress shadowed on the Xiaoyudong fault became larger.If such a process was true,then the Xiaoyudong fault should had been stable during the main shock of the Wenchuan Ms 8.0 event and no surface ruptures and small aftershocks would have occurred along this NW trending fault.Evidently this is in contrary to the fact.Under the second condition,the Xiaoyudong fault ruptured first,and then other faults failed successively.In response to the initial rupture on the Xiaoyudong fault,most of the southwestern section of the Beichuan-Yingxiu fault was in the stress trigger region of 1.01.5 bar,and the parallel Pengguan fault was also brought closer to failure,implying the whole source fault system started to rupture toward northeast,in agreement with the observations.Thus,this study suggests that the NW trending Xiaoyudong fault should have been the initial rupturing member that triggered the NE directed Beichuan-Yingxiu fault and Pengguan fault to fail leading to the main shock.更多还原