Observation of Temporal Variations in Seismic Anisotropy Within an Active Fault-Zone Revealed From the Taiwan Chelungpu-Fault Drilling Project Borehole Seismic Array
Observation of Temporal Variations in Seismic Anisotropy Within an Active Fault-Zone Revealed From the Taiwan Chelungpu-Fault Drilling Project Borehole Seismic Array
复制标题
台湾车笼埔断层钻探工程钻孔地震台揭示活动断层带内地震各向异性随时间变化的观测
DOI：
10.1029/2021jb023050
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发表时间：
2022
期刊：
影响因子：
3.4
通讯作者：
Hung R
中科院分区：
文献类型：
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作者：
Hung R
时间上的断裂带观测对于更好地了解断裂结构和应力结构的演化是很重要的。然而，在大地震发生后，在断裂带进行长期监测的情况很少。本文利用台湾车伦堡断裂钻探工程1公里深度断裂带的地震资料，研究了1999-Mw7.6集集地震后的长期各向异性。直接的S波分裂测量解决了地壳浅层的总体弱各向异性。为了解决断层破坏带的各向异性，我们对2007-2013年间的794次微震进行了尾波互相关技术。我们估计了断层破坏带背景横波速度、快横波偏振方向(FSP)和各向异性强度(AANI)的时间变化。结果表明，FSP的平均方向为N93°E，显著的AANI约为12%，这可能是由于地震后产生的普遍的垂直微裂缝所致。各向异性的时间变化表现出季节性变化，每隔9-12个月具有周期性，与降雨事件相关。此外，长期各向异性显示，在头4年的观测期间，FSP方向逐渐旋转约15°。同时，各向异性强度从17%下降到10%，剪切波速增加，表明地震后断层已经愈合。这项研究报告了两个关键观测的现场证据：(A)大地震后断裂带的长期愈合和(B)地表水文过程对1公里深断裂带性质的调节。这些观测结果可能对断层破坏带在地震间隔期的响应提供约束。
Temporal fault‐zone observations are important to better understand the evolution of fault structure and stress configuration. However, long‐term monitoring in the fault zone is rare after a large earthquake. Here, we use seismic data in the fault zone at 1‐km depth from the Taiwan Chelungpu‐fault Drilling Project to study long‐term anisotropy after the 1999 Mw7.6 Chi‐Chi earthquake. The direct S‐wave splitting measurements resolve the overall weak anisotropy in the shallow crust. In order to resolve fault damage zone anisotropy, we perform coda cross‐correlation technique for 794 microearthquakes between 2007 and 2013. We estimate the temporal change in background shear‐wave velocity, fast shear‐wave polarization direction (FSP), and strength of anisotropy (Aani) in the fault damage zone. We show the average FSP direction is N93°E with a significantAaniof about 12%, likely due to the pervasive vertical microcracks created after the earthquake. Temporal variations of anisotropy exhibit seasonal variation with periodicity every 9–12 months that correlates with rainfall events. Furthermore, long‐term anisotropy shows a gradual rotation of FSP direction of about 15° during the first 4 years of observation. At the same time, the strength of anisotropy reduced from 17% to 10% and shear‐wave velocity increased, suggesting the fault healed after the earthquake. This study reports in‐situ evidence for two key observations: (a) long‐term, fault‐zone healing after a major earthquake and (b) modulation of 1‐km deep fault‐zone properties by surficial hydrologic processes. These observations may provide constraints on the response of the fault damage zone in the interseismic period.