Rupture processes inferred from laboratory earthquakes

Abstract:

Since the proposal by Brace and Byerlee that the mechanism of stick-slip is similar to earthquake mechanics, many experimental studies have been conducted in order to improve our understanding of earthquakes. Here we report macroscopic stick-slip events in saw-cut Westerly granite samples deformed under controlled upper crustal stress conditions in the laboratory. Experiments were conducted under triaxial loading (σ₁> σ₂ =σ₃) at confining pressures σ₃ ranging from 10 to 100 MPa. A high frequency acoustic monitoring array recorded particle acceleration during macroscopic stick-slip events allowing us to estimate rupture speed. In addition, we were able to record the stress drop dynamically, and we show that the dynamic stress drop, measured locally close to the fault plane, is almost total in the breakdown zone, while the strength recovers to values of 0.4 within a few hundred of microseconds only. Enhanced dynamic weakening is observed to be linked to the melting of asperities, which can be well explained by flash heating theory, in agreement with our post-mortem microstructural analysis. Relationships between initial state of stress, rupture velocities, stress drop and energy budget suggest that at high normal stress (i.e. at supershear velocities), the rupture processes are more dissipative. Our experimental results were compared to natural observations, theory and existing experimental results. Our observations question about the current dichotomy between the fracture energy and the frictional energy in terms of rupture processes. A power law scaling of the fracture energy with final slip is observed over eigth orders of magnitude in slip, from the microns to tens of meters.