汶川8级大地震的地表破裂特征及分段

Surface Rupture Features and Segmentation of the M_S8.0 Wenchuan Earthquake

2008年5月12日14时28分,四川省汶川县境内发生MS8.0地震。地表破裂多以跌水、陡坎形态发育在河流沟谷或晚新生代沉积层内,位移明显。山地受崩塌、滑坡影响,位移量较难获得。发震断裂主要有三条,即北川-映秀断裂、彭县-灌县断裂和小鱼洞断裂。北川-映秀断裂地表破裂由南向北活动性质从逆冲为主逐渐转变为走滑为主,长约220 km,平均垂直位移量约3 m,按位移量沿断裂走向的变化可以分为虹口段、北川段和南坝段;彭县-灌县断裂地表破裂以逆冲活动为主,长约82 km,平均垂直位移量约1.5 m,可以划分为白鹿段和汉旺段,断裂断距分布的几何特征与北川-映秀断裂的中南段相近;小鱼洞断裂是一条新生北西走向的次级破裂,长约5.6 km,平均垂直位移量约1.5 m,调节两侧构造单元变形差异,具有捩断层特征,活动以逆(左行)走滑为主,可划分出小鱼洞段和中坝段。姚都镇地表破裂可能说明南坝以北的地震具有不同的活动特征。活动断裂的运动方式反映区域应力场有北西西向挤压特征。

Detailed field investigation showed that three seismogenic faults ruptured in the Wenchuan Ms8.0 earthquake, which are Beichuan-Yinxiu fault, Penxian-Guanxian fault, and Xiaoyudong fault. The Beichuan-Yinxiu fault is characterized by reverse faulting in the south and right-lateral faulting in the north, which lasted for 220 km long and vertically displaced 3 meters on average, and can be divided into three segments. The Penxian-Guanxian fault rupture is characterized by reverse faulting which is 5.6 km long and vertically displaced 1.5 meters on average, and can be divided into two segments. Xiaoyudong secondary fault is characterized by tear faulting and was formed in this earthquake, leftlaterally faulted 5.6 km long with 1.5 m vertical displacement on average, which accommodated the different deformations of tectonic units on both side. The surface rupture on Yaodu town may show the different characteristics between Nanba segment and Beichuan segment. The geometry and motion stress in this region is in the direction mode indicate that the maximum horizontal main of NWW.