龙门山小鱼洞断裂在汶川地震中的地表破裂及地质意义

在龙门山中段小鱼洞地区映秀－北川断裂和彭县－灌县断裂发生了同震位移，同时在该地区新出现了一条北西向的同震破裂带——小鱼洞断裂的破裂带，并分割了两条近于平行的北东向逆冲－走滑型的主断裂。汶川特大地震发生后重点对小鱼洞断裂的地表破裂开展了详细的野外地质填图，利用全站仪和GPS对地表破裂进行了精确的测量。初步调查的结果表明，小鱼洞断裂位于映秀－北川断裂与彭县－灌县断裂之间，走向近于北西向，延伸约15km，以脆性破裂为特征，穿过小鱼洞大桥，并切割了多种类型的地貌单元，使道路发生拱曲、破坏和桥梁垮塌或移位。单个破裂长度在几米到300m不等，其南西盘为上升盘，北东盘为下降盘，平均垂直位错为1.0m，平均水平位错为2.3m，垂直与水平位错量之比1 ∶1～1 ∶2，显示为逆冲分量小于或等于左旋走滑分量，并以左旋走滑作用为主。同时，研究表明小鱼洞断裂属于映秀－北川断裂与彭县－灌县断裂之间的捩断层，其主要特征包括： 1)是在汶川地震中由于龙门山逆冲体之间的差异逆冲运动过程中而形成的断裂； 2)其走向近于北西向，垂直于龙门山北东向的主干断裂，而平行于逆冲体的逆冲运动方向； 3)具有高角度断面的断层，以左旋走滑作用为主。

SURFACE RUPTURE OF XIAOYUDONG FAULT CAUSED BY WENCHUAN EARTHQUAKE AND ITS GEOLOGICAL SIGNIFICANCE

The MS8.0 earthquake occurred on May 12th,2008 in Longmenshan(Longmen Mountain)caused surface ruptures in the Yingxiu－Beichuan Fault，the Pengxian－Guanxian Fault，and a new fault，the Xiaoyudong Fault in the Xiaoyudong area in the middle segment of the Longmenshan(Pengzhou City，Sichuan Province). Detailed geological mapping and surveying on typical surface ruptures and fault belts were taken by GPS and EDM after the earthquake in this area, e.g.，including the measurement of vertical and horizontal offsets of all three faults. The results suggest that the plane fabric pattern of the surface rupture caused by the Wenchuan Earthquake in this area is consisting of two paralleled thrust faults，which are cut by the Xiaoyudong Fault. The Xiaoyudong Fault is located between the Yingxiu－Beichuan Fault and the Pengxian－Guanxian Fault with NW­SE striking，and extends about 15km through the Xiaoyudong Bridge，and cuts a wide range of terrain units. Its southwest block is a hanging wall and the northeast block is a foot wall with an average vertical offset of 1.0 meter and an average horizontal offset of 2.3 meters. The ratio of the thrust to strike­slip componenet is 1∶1～1∶2，showing the vertical offset is smaller than the horizontal offset，and from south to north the thrust displacement is increasing.
The surface rupture pattern in the Xiaoyudong area implies that the Xiaoyudong Fault is a tear fault which was formed by differential movements of thrust belt in Longmenshan during the Wenchuan Earthquake. It striked in the direction of tectonic thrusting from NW to SE and dipped at high angles with left slipping.