Abstract: Under the background of thrusting stress regime, a large number of strike-slip earthquakes occurred on the Miyaluo Fault during the Wenchuan earthquake sequence process, which is in the southern part of the Longmenshan Fault. In order to find the cause of their occurrence, stress tensors in subregions near the Miyaluo Fault are estimated. The result shows that in both north and south side of the Miyaluo Fault, the direction of principal compressive stress is nearly perpendicular to the Longmenshan Fault, and its dip is nearly horizontal, and the direction of tensile stress is nearly vertical. While in the Miyaluo fault zone, the direction of principal compressive stress is SWW-NEE, and its dip is nearly horizontal, the direction of principal tensile stress is NNW-SSE, also its dip is nearly horizontal. It is consistent with sinistral shear stress state in the Miyaluo fault zone. It was referred that the behavior of Miyaluo Fault during the Wenchuan earthquake sequence process was caused by tearing effect generated from unbalanced forces of two sides of the fault. To understand the rupture mode of the aftershocks in subregions as described above, the total seismic moment tensors are estimated by adding the corresponding component separately of the seismic moment tensor of aftershocks in each region. The result shows the similar trend of total seismic moment tensor components in the north and south side of the Miyaluo Fault(indicating the consistency of rupture mode in the north and south side of the Miyaluo Fault), and most seismic moment tensor components in the south side is higher than that in the north side, especially the compression component perpendicular to Longmenshan Fault and expansion component in the vertical direction. It indicates that thrusting component in the southeast direction in the south side is greater than that in the north side, and the thrusting difference causes the sinistral tearing effect of the Miyaluo Fault. We also find that the sinistral tearing component of the Miyaluo Fault is the same order of magnitude with the thrusting difference of its two sides, which indicates that the tearing effect of Miyaluo Fault can be completely explained by thrusting difference of its two sides. According to the analysis, we put forward the dynamic model of the Miyaluo Fault, which can explain the above phenomenon.

Key words: the Longmenshan Fault, the Miyaluo Fault, tearing effect, total seismic moment tensor

摘要： 在挤压应力的背景下，龙门山断裂带南段的米亚罗断裂在汶川地震序列过程中却发生了大量的走滑型地震。为研究其原因，分区反演了米亚罗断裂附近的应力张量。结果显示：米亚罗断裂南北两侧主压应力轴走向近乎垂直于龙门山断裂带走向，倾角近水平；主张应力轴走向近竖直。而米亚罗断裂主压应力轴走向为SWW-NEE，倾角近水平；主张应力轴走向为NNW-SSE，倾角近水平，与该断裂受到左旋剪切力的应力状态相符。据此推测米亚罗断裂在汶川地震序列过程中的活动是由于该断层两侧受力不均衡导致撕裂引起的。为理解以上分区内余震的破裂模式，将各区余震的矩张量进行叠加得到各区的综合地震矩张量，发现米亚罗断裂南侧与北侧综合地震矩张量各分量分布趋势相似（表明其破裂模式具有一致性），且南侧大部分分量大于北侧，尤其是垂直于龙门山断裂带走向的水平压缩分量和竖直方向的膨胀分量，说明南侧向SE逆冲的分量大于北侧的对应分量，两者的差异导致了米亚罗断裂的撕裂，并且这种撕裂的分量与南北两侧逆冲分量的差异大致在同一量级，故完全可以由南北两侧的逆冲差异解释其活动原因。根据以上研究提出了能对上述现象进行解释的米亚罗断裂活动的动力学模型。

关键词: 龙门山断裂带, 米亚罗断裂, 撕裂, 综合地震矩张量