Fluid Pressure Heterogeneity in Rock Fractures and Its Influence on Injection-induced Earthquake Prediction = Fluid Pressure Heterogeneity in Rock Fractures and Its Influence on Injection-induced Earthquake Prediction

다국어 초록 (Multilingual Abstract)

Fluid injection into a highly heterogeneous fault causes the build-up of localized fluid pressure and initiates the rupture of pressurized fault segment. The rupture front may propagate along the fault subjected to tectonic stress and finally trigger a large earthquake. We carry out a suite of triaxial shear-flow experiments on sawcut fractures in granite to reproduce the injection-induced fracture instability due to heterogeneous fluid pressure distribution. When distilled water is injected at one side of a sawcut fracture in a granite sample subjected to triaxial compression, the pore pressure measured at the other side of the fracture rises at different increasing rate depending on the permeability of the fracture. We find that when the pore pressure is homogeneously distributed in a high-permeability fracture, the fracture instability is mainly due to the near-uniform reduction of effective normal stress. If the fracture permeability is low, the pore pressure amplifies near the injection borehole, and induces fracture rupture propagating in the unpressurized area. The pore pressure can only perturb limited area around the borehole in an extremely low-permeability fracture, and the entire fracture remains stable. The relationship between the seismic moment release and fluid injection volume assumes the uniform distribution of injected fluid in fault, and may not reasonably predict the seismic moment release due to the rupture of highly heterogeneous fault. This study provides a possible explanation of ~800 times unexpected seismic energy released during Pohang earthquake.