Preparatory Process in Advance of Runaway Fault Rupture through Fluid Injection Observed in Laboratory Experiments Using a Large Specimen of Sub-meter scale

Abstract Fault slip is initiated by locally applied fluid pressure, and it can expand unstably over a wide area causing elastic waves having magnitudes that induce felt or destructive earthquakes. Thus, it is important to examine the unstable expansion of initial slips. However, it is hard to reproduce the process by general setup of laboratory experiment such as triaxial loading tests on cylindrical specimens with inclined faults. In this study, we prepared a cubic specimen of sub-meter scale, which was separated into two triangular prisms by a model fault. The specimen was subjected to bi-axial compressions with different magnitudes. A 2D array of strain gauges was embedded beneath the fault plane to measure the changes in shear strain with the fault slip driven by fluid injection. Based on the experimental results, we discussed the features of fault slips that lead to injection-induced earthquake. The strain accumulated around the edge of the fault slipping area. The accumulation increased locally the strain by ~ 10 µε, which was equivalent to ~ 0.1 MPa in shear stress. The fault slipping area expanded gradually first, and it expanded unstably beyond the fluid invasion area ~ 3 s later after the slip initiated. The unstable expansion of initial slips was suppressed due to reducing the initial shear stress on the fault by 0.3 MPa. In this case, the initial shear stress was too small for the additional stress accumulated at the edge of the fault slipping area to overcome the static friction on the fault.