Lithology and Fault‐Related Stress Variations Along the TCDP Boreholes: The Stress State Before and After the 1999 Chi‐Chi Earthquake
Lithology and Fault‐Related Stress Variations Along the TCDP Boreholes: The Stress State Before and After the 1999 Chi‐Chi Earthquake
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DOI：
10.1029/2021jb023290
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发表时间：
2022-01
期刊：
影响因子：
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通讯作者：
M. Talukdar;H. Sone;L. Kuo
中科院分区：
文献类型：
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作者：
M. Talukdar;H. Sone;L. Kuo
了解地震前后的应力状态对于研究地震周期中断层上的应力如何演变至关重要。本研究整合了井眼破坏分析、实验室实验和刃位错模型，以研究集集地震前后的应力状态。台湾车笼埔断层钻探工程（TCDP）的地震后现场应力状态由于岩性的变化而在沿着观测到不均匀性。沿着钻孔，我们观察到钻井引起的拉伸裂缝仅存在于砂岩中，而崩落大多存在于富含淤泥的岩石中。TCDP岩心的实验室实验还表明，砂岩的抗拉和抗压强度比富含淤泥的岩石弱。这些观察结果表明，最大和最小水平主应力在富含淤泥的层段中较高。TCDP钻孔中的扩展漏失试验也显示，富砂层段的最小水平应力较低，与上述观察结果一致。我们将联合收割机这些观察估计剖面的应力大小沿着以及解释的变化，在以前的研究中发现的应力状态。我们观察到的应力异质性强调了承认应力数据所代表的空间尺度的重要性。然后，我们使用集集地震期间获得的GPS地表位移约束的边缘位错模型来计算同震应力变化。通过从震后应力中减去同震应力变化得到的我们推断的震前应力大小表明，尽管在集集地震期间在TCDP遇到断层的地区观察到了大位移，但地震前的应力状态仍处于亚临界状态。
Understanding the stress state before and after an earthquake is essential to study how stress on faults evolves during the seismic cycle. This study integrates wellbore failure analysis, laboratory experiments, and edge dislocation model to study the stress state before and after the Chi‐Chi earthquake. The post‐earthquake in‐situ stress state observed along boreholes of the Taiwan Chelungpu‐fault Drilling Project (TCDP) is heterogeneous due to lithological variations. Along the borehole, we observe that drilling‐induced tensile fractures are only present in sandstones, whereas breakouts are mostly present in silt‐rich rocks. Laboratory experiments on TCDP cores also show that tensile and compressive strength are weaker in sandstones than in silt‐rich rocks. These observations imply that both maximum and minimum horizontal principal stresses are higher in silt‐rich intervals. Extended leak‐off tests in the TCDP borehole also show lower minimum horizontal stress in sand‐rich intervals, consistent with the above observations. We combine these observations to estimate a profile of stress magnitudes along the well which explains the variability of stress states found in previous studies. The stress heterogeneity we observed underlines the importance of acknowledging the spatial scale that the stress data represent. We then use an edge dislocation model constrained by GPS surface displacements obtained during Chi‐Chi earthquake to calculate the coseismic stress changes. Our inferred pre‐earthquake stress magnitudes, obtained by subtracting the coseismic stress change from the post‐earthquake stress, suggest subcritical stress state before the earthquake despite the large displacements observed during the Chi‐Chi earthquake in the region where TCDP encountered the fault.