从两个地表破裂点的几何学与运动学特征分析汶川地震的破裂方式

汶川MS8.0地震的地表破裂出现在中央和前山2条断裂上,分别形成了235km和72km长的地震地表破裂带,而且地表破裂表现出复杂的几何学和运动学特征。文中选取了中央断裂上虹口乡桐麻坎和擂鼓镇石岩村2个地表破裂点,通过几何学与运动学特征分析其地表破裂方式。桐麻坎的白沙河河床上4条右阶斜列的主破裂西南侧发育了一条反冲断层坎,精细地形测量反映出了破裂带内的断块特征与破裂过程中的地表掀斜;擂鼓镇石岩村逆冲破裂前缘的2条次级断层形成叠瓦式结构,地貌上表现为挠曲和其上部的地震鼓包。该两点地表破裂的几何学、运动学特征分析表明,本次地震在龙门山中央断裂上的破裂方式以逆冲为主,兼有右旋走滑。这一结果与美国地质调查局、哈佛大学和中国地震台网中心提供的震源机制解基本一致。此外,桐麻坎和石岩村横切破裂带的地形剖面显示出不同的地表掀斜方向,这种不一致主要由于前者位于断裂挠曲后缘,后者位于断裂挠曲前缘

RUPTURE ANALYSIS OF THE WENCHUAN EARTHQUAKE AT TWO OBSERVATION STATIONS BY GEOMETRY AND KINEMATICS

Surface ruptures of the MS 8.0 Wenchuan earthquake are distributed mainly on the Longmenshan central fault and front-range fault,extending 235km and 72km,respectively.The ruptures exhibit complicated characters in geometry and kinematics.On the riverbed of Baisha river,a backthrust scarp was formed in the southwest of the major fault that comprises four right-step sub-faults,and precise topographical measurement shows a kinematic character of fault block and surface tilting in the rupture process;In Shiyan village,the master fault and the secondary fault formed an imbricate structure,and show as flexures and earthquake swells on ground surface.We analyzed the ruptures at Tongmakan village and Shiyan village,both locating on the central fault,by geometry and kinematics,and the results indicate that the earthquake rupturing along the central fault is mainly characterized by thrusting associated with right-lateral strike-slipping.This result is consistent with the focal-mechanism solutions which are promulgated by U.S.Geological Survey,Harvard University and China Earthquake Networks Center.In addition,the profiles of Tongmakan and Shiyan display different tilting directions,and the reason is that the former locates at the trailing edge of the fault,while the latter in the leading edge of fault.