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2008年汶川M_S8.0地震后,沿龙门山断裂带系列水压致裂地应力测量结果表明,断裂带东北段(茂县-绵竹连线东北侧)北川、平武及广元等地浅表层最大主应力方向与区域应力场相比发生了25°~69°不等的偏转,亦称地应力解耦。初步分析认为,龙门山断裂带东北段的右旋走滑活动直接导致了区域内最大主应力方向的解耦,且最大主应力方位向偏转角度与断裂右旋走滑活动量正相关。为合理解释这一现象,探讨走滑断裂活动导致地应力解耦的机理,本文采用二维有限元数值模拟方法,选取汶川M_S8.0地震断裂(龙门山中央断裂)上的同震右旋走滑位移作为边界条件,首先模拟了龙门山断裂带东北段右旋走滑活动产生的扰动应力特征,之后结合断裂带附近典型钻孔地应力测量结果,叠加该扰动应力作用,从力学机理上深入剖析断裂右旋走滑活动产生的扰动应力对区域应力场(或初始应力场)的影响,并对其附近不同构造部位最大主应力方向偏转现象进行定量解释。初步结果表明:1)走滑断裂活动导致地应力解耦主要是通过断裂滑动产生的扰动应力对区域应力场的附加影响实现的;2)在固定的断裂活动长度和摩擦强度情况下,走滑断裂活动导致区域最大主应力方向偏转角度基本与活动断裂走滑位移量呈正比;3)受汶川M_S8.0地震影响,龙门山断裂带大致以茂县-绵竹为界,西南侧的宝兴-映秀-汶川一带最大主应力方向偏转角度很小,基本与区域应力场一致,而东北侧的北川-平武-广元一带则发生角度不等的偏转,3个地区最大主应力方向平均偏转角度分别为65°、56°和29°,该角度与龙门山断裂带东北段同震右旋走滑位移量呈正相关。
After Wenchuan M_S8.0 earthquake in 2008, the series of hydrofracturing in-situ stress measurements along the Longmenshan fault zone show that the maximum principal stress of the superficial layers in the northeastern part of the fault zone (Beichuan, Pingwu and Guangyuan, northeast of the Muxian-Mianzhu connecting line) Direction and the regional stress field compared to 25 ° ~ 69 ° ranging from deflection, also known as stress decoupling. The preliminary analysis shows that the dextral strike-slip event in the northeastern segment of the Longmenshan fault zone directly leads to the decoupling of the direction of maximum principal stress in the region, and the azimuthal deflection angle of the maximum principal stress is positively correlated with the amount of right-lateral strike-slip activity. In order to explain this phenomenon reasonably and to explore the mechanism of the decoupling of stress caused by the strike-slip faulting, a two-dimensional finite element numerical simulation method was used to select the right-earth coseismic strike on the Wenchuan M_S8.0 earthquake fault (central Longmenshan fault) Slip displacement as the boundary condition, the disturbing stress characteristics generated by the dextral strike-slip event in the northeastern segment of the Longmenshan fault zone are modeled first, and then the perturbation stress effect is superimposed based on the results of typical borehole stress measurements near the fault zone. The influence of disturbing stress caused by right-lateral strike-slip fault on the regional stress field (or initial stress field) is analyzed, and the direction deflection of the maximum principal stress at different tectonic sites in the vicinity of the fault is quantitatively explained. The preliminary results show that: 1) the decoupling of the geostress caused by the strike-slip faulting is mainly caused by the additional influence of the disturbing stress caused by fault slip on the regional stress field; 2) under the condition of fixed fault activity length and friction strength, The angle of deflection of the direction of maximum principal stress in the area is basically proportional to the strike-slip displacement of the active faults; 3) Affected by the M_S8.0 Wenchuan earthquake, the Longmen Shan fault zone is roughly bounded by Mao County-Mianzhu and Baoxing- - The direction of maximum principal stress deflection in Wenchuan area is small, basically consistent with the regional stress field, while the Beichuan-Pingwu-Guangyuan area in the northeast side is angularly deflected. The average deflection angles of the maximum principal stress in the three areas are 65 °, 56 ° and 29 °, which is positively correlated with the right-lateral strike-slip displacement of the northeastern segment of the Longmenshan fault zone.