利用原地应力实测资料分析芦山地震震前浅部地壳应力积累

Stress build-up in the shallow crust before the Lushan Earthquake based on the in-situ stress measurements

龙门山断裂带南段的芦山Ms7.0级地震给雅安地区人民群众和当地社会带来巨大的损失,如何从地壳应力积累的角度来研究该次地震对于深入认识孕震发震机理将有很大的帮助.项目组在过去21年中在该区域积累了22个钻孔的应力实测资料,这些应力资料显示该区域的应力状态为逆冲断层应力状态,最大水平主压应力方向为N44＆#176;-64＆#176;W,而且在过去20多年里,该区域的应力状态是稳定的.文中提出利用平均差应力和平均有效应力之比μm来表征地壳应力积累的能力和水平.通过理论分析可知μm的物理意义与μ是近似的,完全可以用μm代替μ来开展分析.利用实测应力数据计算得到的μm范围为0.39～0.56,接近于Byerlee定律所定义的μ=0.6的下限值,该范围与龙门山断裂带震后断层泥稳态摩擦系数的范围完全吻合,芦山地震的发生说明研究区的地壳应处于摩擦极限平衡或者亚平衡状态.芦山地震的震源机制解反应的应力状态为逆冲断层应力状态,主压应力轴方向为122°,与该区域的应力测试数据所反应的应力状态一致.通过与他人的研究成果对比分析可知,芦山地震可看作汶川地震对龙门山断裂带南段区域的断层加载而诱发产生的一次地震.

The Ms7.0 Lushan Earthquake in the southern segment of Longmenshan Fault has brought great loss to the local natives and societies. To analyze the earthquake from the viewpoint of stress buildup in the upper crust is helpful and meaningful to understand more about the seismogenic mechanism. In the past 21 years, the research group accumulated and collected lots of in-situ stress data from 22 boreholes. These in-situ stress data show that the stress regime is of reverse faulting, and that the orientation of the maximum horizontal principal stress is N44-64°W. At the same time, the stress measurements indicate that the stress state in theresearch area is relatively stable. In this paper, the authors suggest adopting the ratio of half maximum differential stress to the mean effective stress, μm, to reflect the capability and stress level of the upper crust. According to the theoretical analysis, Ezra has similar physical meaning with μ, so we can replace μ with/ μm to do rock friction analysis. The μm based on all the stress measurements ranges from 0.39 to 0.56, approximately approaching the lower limit defined by the Byerlee law, μ= 0. 6, which also agrees well with the static friction coefficients of gouge specimen from the Longmenshan fault after Wenchuan Earthquake tested in the laboratory. Therefore, the occurrence of the Lushan Earthquake proves that the upper crust of the research area is in the critical frictional equilibrium state or sub-equilibrium state. The focal mechanism solution of Lushan Earthquake is reverse faulting, with the main P axis of 122°, which is consistent with the stress state defined by the in-situ stress measurements. Compared with the research findings made by other geoscientists, the Lushan Earthquake can be regarded as one induced earthquake due to the stress increase caused by the Wenchuan Earthquake on the southern segment of Longmenshan fault.