Anisotropic upper crust above the aftershock zone of the 2013 Ms 7.0 Lushan earthquake from the shear wave splitting analysis
Anisotropic upper crust above the aftershock zone of the 2013 Ms 7.0 Lushan earthquake from the shear wave splitting analysis
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2013年芦山7.0级地震余震区上方地壳的各向异性剪切波分裂分析
DOI：
10.1002/2015gc005972
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发表时间：
2015-10
影响因子：
1.3
通讯作者：
Dong Shuwen
中科院分区：
文献类型：
--
作者：
Liu Ying;Zhang Haijiang;Zhang Xin;Pei Shunping;An Meijian;Dong Shuwen
我们使用2013年芦山7.0级地震中沿着中国西南龙门山断裂系的1000个M2级余震进行了系统的剪切波分裂分析。快剪切波的偏振方向呈双峰分布，一个主导方向近似平行于断层走向，另一个接近区域最大水平压应力方向。这表明该区地壳地震各向异性的形成机制既有应力诱发的，也有断裂带构造控制的。对于较深的地震，快、慢横波之间的延迟时间没有明显的增加趋势，说明各向异性带主要位于余震上方，余震一般位于8 km以下。我们进一步应用剪切波分裂层析成像方法测量延迟时间来表征地震各向异性的空间分布。三维各向异性百分比模型显示8 km以上各向异性较强，8 km以下各向异性较弱，主震滑动带及其余震的各向异性很小或可忽略，且速度较高，表明8 km以上各向异性高、速度低的区域力学性质较弱，应力难以在那里积累。主逆断层带和后逆断层带在108km以上具有高的各向异性异常，可能是由平行于断层带的剪切组构或微裂缝引起的。
We have conducted a systematic shear wave splitting analysis using 1000 selected aftershocks with M 2 from the 2013 Ms 7.0 Lushan earthquake along the Longmenshan fault system in southwest China. Polarization directions of fast shear waves show a bimodal distribution with one dominant direction approximately parallel to the fault strike and the other close to the regional maximum horizontal compressive stress direction. This indicates that in this area mechanisms causing crustal seismic anisotropy are both stress induced and fault zone structure controlled. Delay times between fast and slow shear waves do not show a clear trend of increase for deeper events, suggesting the anisotropic zone is mostly above the aftershocks, which are generally located below 8 km. We further applied a shear wave splitting tomography method to measured delay times to characterize the spatial distribution of seismic anisotropy. The three‐dimensional anisotropic percentage model shows strong anisotropy above 8 km but low anisotropy below it. The mainshock slip zone and its aftershocks are associated with very low or negligible anisotropy and high velocity, indicating that the zones with high anisotropy and low velocity above 8 km are mechanically weak and it is difficult for stress to accumulate there. The main and back reverse fault zones are associated with high anisotropic anomalies above ∼8 km, likely caused by shear fabric or microfractures aligned parallel to the fault zone.
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