[S-CG45] Science of slow-to-fast earthquakes

[SCG45-50] Development of gouge triaxial shear experiments in Taiwan

*Ting-Yen Tai¹、Zhan-You Kuo¹、Szu-Ting Kuo¹、Li-Wei Kuo^1,2,3、Maciej Trzeciak⁴、Hiroki Sone⁴ (1.Department of Earth Sciences, National Central University, Taoyuan320, Taiwan、2.Earthquake-Disaster & Risk Evaluation and Management Center, National Central University, Taoyuan 320, Taiwan、3.Institute of Earth Sciences, Academia Sinica, Taipei 115, Taiwan、4.Department of Civil and Environmental Engineering, University of Wisconsin-Madison, Madison, Wisconsin, USA)

キーワード：rate-and-state friction law (RSF), triaxial friction experiments, confined direct shear assembly, fault-slip behaviors

In response to the 2050 net-zero emission in Taiwan, identifying potential sites for exploration of Enhanced Geothermal System (EGS) and Carbon Capture Utilization and Storage (CCUS) has become critical. Both technologies involve fluid injections into the subsurface, which can affect pore fluid pressure and fault stability, potentially leading to unstable slip and consequently increasing the risk of inducing seismicity. Recent studies have highlighted the linkage between induced seismicity and the rate-and-state friction law (RSF) to evaluate fault-slip instability during the operations of EGS and CCUS. Our team aims to establish the ability to run laboratory triaxial rock friction experiments in Taiwan. In this study, we used the direct-shear sample assembly for triaxial friction experiments on the GCTS servo-controlled Triaxial Rock Testing System (GCTS RTX-1000) at National Central University, Taiwan. We used kaolinite powder as synthetic gouge samples that were sandwiched between two 1-inch L-shaped stainless-steel platens. Normal stress was imposed through confining pressure (Pc) and maintained at 20 MPa, while the pore fluid pressure (Pp) was set at 10 MPa, resulting in an effective normal pressure of 10 MPa. After 8–12 hours of compaction to achieve equilibrium in Pp within the fault gouge, we applied a load-point displacement rate controlled by the load frame displacement transducer (DCDT). The initial velocity applied in all tests was set to 1 μm/s. Once the deviator stress (Sd) reached a steady state of 4 MPa, which represented the start of fault gouge shearing, we decreased velocity to 0.1 μm/s until the load-point displacement reached 1.5 mm. The velocity was then adjusted in a 10-fold step sequence (e.g., 0.2, 0.02, 0.2 μm/s), with each test featuring different velocity values, applied at displacement intervals of 0.3 mm per step. All experiments showed a coefficient of friction from 0.16 to 0.22 after achieving a steady state and exhibited a velocity-strengthening behavior. The RSF constitutive parameters, direct effect coefficient (a), evolution effect coefficient (b), and critical slip distance (Dc) were obtained using RSFit3000 (Skarbek & Savage, 2019), with the assumed elastic stiffness k = 0.15 mm^-1. Our results reveal that a-b, ranging from 0.001 to 0.002, is slightly lower than those documented by the previous studies (0.002-0.006 for saturated sample). The Dc was ranging from 10-100 μm . The results of fitting show an R-square of 0.6, indicating data dispersion that may be attributed to oscillations of DCDT and Sd. To improve the fitting quality of the a-b values, we can reduce noise by appropriately adjusting the sampling frequency or averaging data points during processing. We will conduct a stiffness test with dry gouge samples using a slide-hold-slide (SHS) stepping to obtain the elastic stiffness (k, units of mm^-1) for our confined direct shear assembly.