汶川M_S8.0地震地表破裂带白沙河段破裂及其位移特征

RUPTURE OF THE M_S 8.0 WENCHUAN EARTHQUAKE ALONG BAISHAHE RIVER

2008年5月12日14时28分，四川省汶川县发生Ms8．0特大地震。这次地震在地表形成2条地表破裂带，主破裂沿龙门山中央断裂发育，长约240km；次级破裂沿龙门山前山断裂发育，长约72km.白沙河破裂带位于中央主破裂带的南端，沿都江堰市北约11km的白沙河河谷展布，长14km。该破裂带几何结构复杂，多条短小破裂或斜列或平行或斜交组成破裂带，总体走向N50°E，但几乎每条次级小破裂走向都不与平均走向一致，在0°～90°之间变化。同震位移沿该破裂带也同样表现出复杂性和多样性，总体以NW盘逆冲为主，最大垂直位移为6．5m，局部存在反冲断层挠曲坎；走向滑动以右旋为主，最大水平位移为4．8m，局部存在左旋位移，而且同震位移分布特征与白沙河破裂带的几何展布特征密切相关。该破裂带北段出露的地表断层产状为40°／NW∠76°，其上的擦痕有2组，侧伏角分别为75°SW和80°SW，不仅反映了该次地震在此段以逆冲运动为主，兼有少量的右旋走滑分量，还反映了该次特大地震包括了2次破裂事件。分析破裂几何和同震位移分布特征得出如下认识：1）破裂面从震源深度的低角度逐渐向地表转化为高角度，迫使垂直断层走向的水平缩短转化为隆起的垂直位移，直接导致了走滑分量与水平缩短量的比例接近1，还导致了主破裂后侧反冲断层挠曲坎的形成；2）地震地表破裂带走滑分量变化受地表破裂走向的变化、标志线方位角变化产生的视走滑位移和重力作用等三大因素影响；3）上覆松散堆积物的厚度影响破裂带出露的规模和方式。

At 14：28, May 12,2008, a great earthquake of Ms 8.0 occurred in Wenchuan County, Sichuan Province. It produced two ruptures along the Longmenshan Thrust Belt, with the major one of ca. 240km along the central fault zone and minor one of ca. 70km along the mountain front fault zone, respectively. Baishahe rupture section,located at the southern end of the rupture of Wenchuan earthquake ,extends 1dkm along Baishahe River, 11 km north of Dujiangyan City. This rupture section presents complicated geometric features, and is composed of several segments. Almost each segment has different strike from others（from 0 to 90°） ,but the section has an average strike of 50°. Co-seismic slip along the rupture section also shows complexity and muhiformity. Up-thrust of the northwest wall is the main vertical co-seismic slip,and the maximum height of fault flexure scarp is 6.5m, but local backward flexure scarps exist there; the right-slip is the main co-seismic strike slip, and the maximum right-slip offset is 4. 8m, but local left-slip offset also appears. A rupture plane of 40°/NW∠76° crops out at the northern end of the rupture section. Two groups of striations present on the plane,75° SW and 80°SW. This indicates not only that reverse faulting is major, and strike-slip faulting is minor, but also two rupture events in the earthquake. Based on the analysis of the rupture geometric features and co-seismic slip distribution, we have found the following knowledge ： 1 ） The gradual change of the rupture plane from low angle at source depth to high angle at surface compels the horizontal shortening to transform into uplifting, causing directly the ratio of strike-slip to horizontal shortening approaching to 1, and forming backward fault flexure scarp behind the main rupture zone; 2）The direction and amount of strike slip are determined by three factors of strike of rupture, azimuth of reference and gravity; 3）The thickness of the overlying unconsolidated deposits affects the scope and style of rupture at the surface.