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汶川M_s8.0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SW 85°～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SW 76°～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SW 32°～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。
The Wenchuan M_s8.0 earthquake caused serious surface rupture in the Longmen Shan fault zone, but the phenomenon of surface rupture caused by the earthquake was rare. There was a well-preserved fault scar marking profile in Bajiamiao Village, Hongkou Township, Dujiangyan City, Sichuan Province, It is the only outcrop that can reflect the rupture process of earthquakes. We can see two kinds of scratches with great difference between the two sides. The total and detailed measurements of the scratches were carried out by using total station and GPS. The results showed that a scratch was close to vertical and the lateral bending angle was between 76 ° ~ 87 ° between the other side of the scratches on the side of the relatively small angle, between 32 ° ~ 33 °. Using the lateral volts of fault scratches, the fault plane dip angle and the rock internal friction angle, the horizontal projection of the principal stress of the fault is solved. The results show that the mode of motion of the fault is thrust and right-lateral slip. By analyzing and comparing, we believe that the near-horizontal scratches on the fault plane cut the scratches near the vertical direction, and the earth surface rupture process is divided into two stages. The fault movement in stage I mainly thrust upwards, and stage II is dominated by dextral strike-slip, which is consistent with the seismic data inversion results. Stage I consists of two sub-events. The first sub-event thrusts upwards at a nearly vertical angle with a movement direction of 85 ° -87 °. The second sub-event has a tilt in the NE direction , The direction of movement is SW 76 ° ~ 80 °, indicating that the fault movement is a thrust and a small amount of dextral strike-slip. The movement direction of Phase Ⅱ fault is SW 32 ° ~ 33 °, indicating that the fault movement is dominated by the right-lateral strike slip at this stage, and the slip-strike component is greater than the thrust component.