Effect of fault surface evolution on slip behaviors in large-scale biaxial experiments

Abstract:

To investigate the frictional slip behavior under more realistic condition, we conducted stick-slip experiments using large-scale biaxial friction apparatus at the NIED in Tsukuba, Japan. We used two rectangular metagabbro blocks as specimen, whose contacting area was 1.5 m long and 0.1 m wide. The experiments were repeatedly conducted with same pair of specimens, which means the fault surface evolved with the frictional slip. We successively conducted a set of three experiments under the condition of constant normal stress of 6.7 MPa and loading rate of 0.01 mm/s. All wear materials were collected after each experiment. To artificially accelerate fault evolution from one stage to the next, we applied fast loading with long slip displacement between a set of three experiments As a result, we obtained three sets of the experimental result in different evolutional stages I, II and III; one and two fast-loading processes mentioned above were performed before the experimental sets in Stage II and III, respectively. In all experiments, we observed many stick-slip events, the number of which tended to increase with the maturity of the fault. Local strain array also showed slow propagation of shear stress drop, which would be derived from slow slip, before the main rupture. We found that the time and location, in which slow slip started to propagate, depend on the stage of fault evolution. In Stage I, both the temporal and spatial distribution of the slow slip were mono-modal, whereas various occurrence times were observed in Stage II and III. The occurrence locations in Stage I and II look consistent with the initial local frictional strength, which was estimated from accumulation rate of the local shear stress divided by the local normal stress before the first slip event in each experiment; the slow slips started to propagate from the location where the initial frictional strength is local minimum. On the contrary, we cannot find such clear relationship in Stage III, though the occurrence locations are mostly limited in a central part of the fault. These results suggest that the fault surface evolution may increase complexity of the slow slip behavior.