| Abstrakt: |
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. Plain Language Summary: Stress in the Earth's crust defines the forces acting on faults which drive earthquakes. Therefore, tracking how stress accumulates due to plate motion and is released due to fault slip have been a central topic in the study of earthquake mechanics and associated seismic hazard. Previous studies which analyzed data collected from Taiwan Chelungpu‐fault Drilling Project (TCDP), drilled after the 1999 Chi‐Chi earthquake, show that the post‐earthquake stress inferred from those studies is inconsistent with each other. This study reviewed previous measurements, conducted laboratory experiments, and analyzed borehole data to revisit the stress state after the Chi‐Chi earthquake. We show that stress state varies along the TCDP borehole due to lithology and fault‐related changes in mechanical properties, which explains why past studies indicated significant variability. We also estimated the pre‐earthquake stress before the Chi‐Chi earthquake to understand stress conditions leading to the rupture. We calculated the stress change caused by the Chi‐Chi earthquake and subtract it from the post‐earthquake stress to obtain the pre‐earthquake stress. Our pre‐earthquake stress estimate suggests that the stress before the Chi‐Chi earthquake was low in magnitude compared to the frictional strength of rocks. Key Points: We obtain a near‐continuous stress profile which captures the lithology‐dependent and fault‐related stress variation along the boreholesVariability in stress states reported from previous studies is explained by stress variations caused by lithological variationsThe estimated pre‐earthquake stress profile indicates subcritical stress state within the Chelungpu fault zone prior to the earthquake [ABSTRACT FROM AUTHOR] |
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