Frictional stability of metamorphic epidote in granitoid faults under hydrothermal conditions and implications for injection-induced seismicity

The effect of low-grade metamorphic epidote on fault stability in granitoids at shallow depth is not well understood. We present structured laboratory observations of epidote and simulated Pohang granodiorite (an EGS site) gouges and mixtures to evaluate their frictional responses. The experiments were performed on powdered rock gouges at a constant confining pressure of 110 MPa, pore fluid pressures of 42 and 63 MPa, temperatures of 100-250℃ and epidote percentages in the range of 0-100 vol.%. Results show that the simulated Pohang granodiorite gouge is frictionally strong (friction coefficient of ~0.71) but exhibits a transition from velocity-strengthening to velocity-weakening behavior at in-situ temperatures >150℃. Epidote gouge displays similar frictional strength to the simulated granodiorite gouge but a stronger velocity-weakening response at 150℃.  Increasing the epidote content in the homogeneously mixed gouges maintains the high frictional strength but increases velocity-we...