龙门山彭县－灌县断裂的活动构造于地表破裂

2008年5月12日在龙门山发生了8.0级特大地震，彭县－灌县断裂亦发生了同震地表破裂。在前期对龙门山活动构造研究的基础上，汶川特大地震发生后，在灾区进行了多次的野外调查和国际合作考察，重点对汶川地震的地表破裂和地质灾害开展了详细的详细野外地质填图，利用全站仪和GPS对地表破裂进行了精确的测量，研究了的地表破裂地貌错位、构造组合和运动学，已实地测得地表破裂数据70余组(其中彭县－灌县断裂地表破裂数据20余组)。文章以彭县－灌县断裂地表破裂为切入点，在彭县－灌县断裂的关键部位开展了详细的野外地貌测量，主要测量了彭州磁峰、白鹿、绵竹金花和汉旺等地的地表破裂，标定了彭县－灌县断裂破裂带的垂向断距和水平断距，结果表明该地表破裂南西起于彭州磁峰，向北东延伸经白鹿、绵竹金花至绵竹汉旺，全长约 40～50km。地表破裂带沿彭县－灌县断裂带的走向断续分布，单个破裂长度在几米到500余米不等，破裂带切割了多种类型的地貌单元，包括山脉基岩、河流阶地、冲洪积扇、公路、桥梁等，同时也使道路发生拱曲、破坏和桥梁垮塌或移位。其以脆性破裂为特征，以逆冲－右旋走滑为特点，断面倾角较陡，北西盘为上升盘，南东盘为下降盘，垂直位错介于 0.39～2.70m之间，水平位错介于 0.20～0.70m，平均垂直位错为1.6m，平均水平位错为0.6m; 地表最大错动量的地点位于彭州白鹿镇，其中最大垂直断错为 2.7±0.2m，最大水平断错为 0.7±0.2m。垂直位错与水平位错量之间的比值为2 ∶1，表明该地震地表破裂带不仅存在逆冲运动分量和右旋走滑运动分量，而且逆冲运动分量大于右旋走滑运动分量，显示了彭县－灌县断裂破裂带具有以逆冲和缩短作用为主、右旋走滑作用为辅的破裂性质。其与映秀－北川断裂带的地表破裂相比较，该断裂的地表破裂程度远小于映秀－北川断裂带的地表破裂程度，主要表现在地表破裂的长度较短，垂直位错和水平位错也相对较小，而且为以逆冲作用为主。初步研究结果表明，彭县－灌县断裂与映秀－北川断裂地表破裂的平面组合样式显示为两条在平面上近于平行的北东向地表破裂带，其间由一条南北向的次级地表破裂带(小鱼洞断裂)将它们相连结，地下破裂面的剖面组合样式显示为叠瓦状，并在汶川地震震源附近或震源的上方相连的，是同“根”的。

ACTIVE TECTONICS AND SURFACE RUPTURE OF THE PENGXIAN－GUANXIAN FAULT|LONGMEN MOUNTAIN|SICHUAN|CHINA

The MS 8.0 Wenchuan Earthquake of May 12th,2008 occurring in Longmen Mountain caused surface rupture in the Pengxian－Guanxian Fault. After the earthquake we carried out several field surveys in Cifeng Town and Bailu Town of Pengzhou City and Jinhua Town and Hanwang Town of Mianzhu County，focusing on detailed geological mapping in typical surface rupture and active fault belts and precise measurements by GPS and EDM.
The Pengxian－Guanxian Fault after the Wenchuan Earthquake is characterized by fragility，showing surface rupture cutting a wide range of terrain units. We measured the vertical and horizontal offsets of the Pengxian－Guanxian Fault in some key positions. The surface rupture started from Xiang′e Town of Dujiangyan City，extended through Cifeng，Bailu，Jinhua，Hanwang，and to Sangzao Town of Anxian County，with a total distance of about 40～50km. It is characterized by dextral strike­slips and steep dipping. The upthrown block was in the northwest while the downthrown in the southeast. The largest measured rupture is located in Bailu Town，of 2.7±0.2m vertical offset and 0.7±0.2m horizontal offset respectively. The vertical offset was 0.39～2.70m and the horizontal offset was 0.20～0.70m with the average of 1.6m and 0.6m respectively，indicating the existence of thrust displacement and dextral strike­slip displacement. The ratio of thrusting to strike­slipping is 2 ∶1 in the Pengxian－Guanxian Fault，which indicates that the thrust displacements are twice more than the dextral strike­slip displacements. Compared with the surface rupture of the Yingxiu－Beichuan Fault，the the Pengxian－Guanxian Fault rupture is far less remarkable: it is shorter，with smaller less vertical and the horizontal offsets and mainy of thrust displacement. It suggests that the surface rupture of Wenchuan Earthquake consists of two paralleled thrust faults linked by the Xiaoyudong Fault，while the subsurface rupture consists of two imbricate thrust faults.