Lithology and Fault‐Related Stress Variations Along the TCDP Boreholes: The Stress State Before and After the 1999 Chi‐Chi Earthquake

Abstract Understanding the stress state before and after an earthquake is essential to study how on faults evolves during seismic cycle. This integrates wellbore failure analysis, laboratory experiments, edge dislocation model Chi‐Chi earthquake. The post‐earthquake in‐situ observed along boreholes of Taiwan Chelungpu‐fault Drilling Project (TCDP) heterogeneous due lithological variations. Along borehole, we observe that drilling‐induced tensile fractures are only present in sandstones, whereas breakouts mostly silt‐rich rocks. Laboratory experiments TCDP cores also show compressive strength weaker sandstones than These observations imply both maximum minimum horizontal principal stresses higher intervals. Extended leak‐off tests borehole lower sand‐rich intervals, consistent with above observations. We combine these estimate a profile magnitudes well which explains variability states found previous studies. heterogeneity underlines importance acknowledging spatial scale data represent. then use constrained by GPS surface displacements obtained calculate coseismic changes. Our inferred pre‐earthquake magnitudes, subtracting change from stress, suggest subcritical despite large region where encountered fault.