Inferring the pre-earthquake stress state before the 1999 Chi-Chi earthquake at the TCDP site from borehole data and co-seismic stress modeling

Understanding the state of stress before and after an earthquake is essential to study the stress release due to rupture. Detailed studies of the 1999 Chi-Chi earthquake along with quality logging and core data make this dataset ideal to study pre and post-earthquake stress. This study integrates laboratory experiments, wellbore failure analysis, and boundary element modeling to study the stress state before and after the Chi-Chi earthquake.

The post-earthquake in-situ state of stress observed along boreholes of Taiwan Chelungpu-Fault Drilling Project (TCDP) is heterogeneous due to lithological contrasts. Along the borehole where the lithology varies from sand- to silt-rich layers, we observe that drilling induced tensile fractures are present only in sandstones. On the contrary, breakouts are ubiquitous features in the borehole but absent in sandstones. Laboratory experiments on TCDP cores also show that tensile and compressive strength are lower in sandstones than in silt-rich rocks. These observations indicate that both maximum and minimum horizontal principal stresses are higher in silt-rich intervals than in sand-rich intervals. Extended leak-off tests in the TCDP borehole also show lower minimum horizontal stress in sand-rich intervals, consistent with the above.

We take two approaches of calculating the co-seismic stress change: 1) analytical and 2) discrete boundary element method. We calculate the pre-earthquake stress by subtracting co-seismic stress change from the post-earthquake in-situ stress discussed above.

We observe from slip tendency calculation, that the state of stress was well below the frictional limit before the rupture. Low-velocity friction experiments using wet fault gouge material recovered from the Chelungpu fault in TCDP show that the coefficient of friction is about 0.63. However, the estimated pre-earthquake slip tendency in the vicinity of the Chelungpu fault is at most 0.33. This indicates that the Chelungpu fault at where the TCDP borehole penetrate was not critically-stressed before the Chi-Chi earthquake. This could be because the fault was only critically stressed at the hypocenter, which was further south of the TCDP boreholes, or it could be attributed to stress release due to afterslip or inter-seismic damage zone creep which our calculations do not incorporate.