汶川Ms8.0地震的地表破裂过程——以都江堰八角庙村断层擦痕剖面为例

汶川MS8.0地震在龙门山断裂带造成了严重的地表破裂，但能反映地震地表破裂过程的现象却很少，四川省都江堰市虹口乡八角庙村有一处保存基本完好的断层擦痕剖面，是惟一能反映地震地表破裂过程的露头。剖面上可见两种侧伏产状相差很大的擦痕，我们利用全站仪和GPS对擦痕进行总体和细部的测量，结果表明，一种擦痕接近垂直，侧伏角在 76°～87°之间，另一种擦痕侧伏角相对较小，介于 32°～33°之间。利用擦痕侧伏角、断层面倾角、岩石内摩擦角进行求解断层主应力方位的赤平投影，结果表明发震断层的运动方式为逆冲兼右行走滑。通过分析对比，我们认为断层面上近水平方向的擦痕切割了近垂直方向的擦痕，据此将地震的地表破裂过程分为两个阶段。阶段Ⅰ断层运动以逆冲向上为主，阶段Ⅱ以右旋走滑为主，这个结论与地震波数据反演结果是一致的。其中，阶段Ⅰ由两个子事件组成； 第一个子事件断层以接近垂直的角度逆冲向上运动，运动方向为SW85°～87°； 第二个子事件断层的运动方向发生朝向NE方向的倾斜，运动方向为SW76°～80°，表明断层运动性质为逆冲兼少量的右旋走滑。阶段Ⅱ断层的运动方向为SW32°～33°，表明这一阶段断层运动以右行走滑为主，走滑分量大于逆冲分量。

SURFACE RUPTURING OF WENCHUAN MS8.0 EARTHQUAKE BASED ON FAULT SCRATCH AT BAJIAOMIAO|DUJIANGYAN

The Wenchuan MS8.0 Earthquake of 12th May 2008 ruptured two imbricate reverse faults(dipping NW)along the Longmenshan fault zone in the eastern margin of the Tibetan Plateau. But the surface rupturing process during the earthquake is rarely reflected directly，except for typical outcrop of fault scratch located at Bajiaomiao,20km away from Dujiangyan City(31°8.719′N,103°41.513′E). The Yingxiu－Baichuan Fault in the study area(200m×500m)which stretches to NEE direction(NE34°～45°) created a fault scrap which is about 100m long and 4.7±0.5m high in the floodplain. Two outcrops of the bedrock fault have been found in the foot of the fault scrap and a waterfall which has a 6.0±2.0m offset horizontally has been created. The scratch outcrop is a part of the fault scrap.
Measured by EDM and GPS，the outcrop of the fault scratch is 7.5±0.5m wide and 4.3±0.2m high. There are 7～8 groups of scratch on the eastern side of the profile. According to the plunge angle，these scratch can be divided into two types. The first type is approximately vertical whereas the other is approximately horizontal. The former can be further divided into the upper part and the lower part. The upper part has the plunge angle of SW85°～87° ，the length of 55～81cm，the width of 29～46cm，and the depth of 1.5～8.4cm whereas the lower part has the plunge angle of SW76°～80°，the length of 133～182cm，the width of 23～31cm，and the depth of 1.4～5.6cm. The scratch which is approximately horizontal has the plunge angle of SW32°～33°，the length of 15～21cm，the width of 18～40cm，and the depth of 3.0～3.2cm. By means of stereographic projection we confirmed the principal stress orientation of the fault. The result suggests that the motion character of the causative fault is thrust and dextral strike­slip，which is in agreement with the results of seismic wave inversion(Chen et al.,2008; Wang et al., 2008).
As the horizontal type of scratch incise the vertical type，we suggest a two­stage the rupturing process during the earthquake: Stage I and Stage II. The moving direction of the fault was SW 85°～87° during the former sub­stage of Stage I，and the direction gradually decreased to SW 76°～80° during the later sub­stage of Stage I，which suggests that the motion character of the fault is thrust with slight dextral strike­slip. The moving direction of the fault was SW 85°～87° in Stage Ⅱ，which suggests that the motion character of the fault is dextral strike­slip with slight thrust.