Previous experiments and modeling on the nucleation process of an earthquake reveal accelerating slip and development of a final patch of fixed or expanding length, but whether the nucleation phase is spatially large enough to be detected in real-Earth conditions with durations long enough to be helpful is unknown. This study performed a new round of simulation of the nucleation process based on the rate-and-state friction law. Our results reveal the development of a weakening-zone core where stress releases continuously, which expands first to a dimension less than the half fault length then shrinks to a final length before re-expanding, consistent with seismicity migration before large subduction earthquakes. Onset times of the weakening-zone core are weeks before large subduction earthquakes under high loading rate but years for continental faults with loading rate below 10mm/yr. The onset of weakening-zone core is marked by fault-length-dependent slip rates>3.6-4.8 times the loading rate.

To fully understand the constitutive parameters and the associated mechanism in the velocity-weakening behavior of pyroxene observed in a previous study (He et al., 2013), we employed pure augite (clinopyroxene) as simulated gouge sample to run velocity stepping sliding tests under hydrothermal conditions with temperatures of 101-607 °C, effective normal stress of 200 MPa with 30 MPa pore pressure and axial loading rates of 0.1-1.0 μm/s. From our experiments, we found that: (1) Velocity-strengthening behavior was observed at temperatures of 101-203 °C, the steady-state velocity dependence transitioned to velocity weakening at ~215 °C, and the velocity weakening persisted up to 607 °C, the highest temperature in our experiments. The absolute (b-a) values were revealed to range from 0.0009 to 0.0014, and the inferred average b/a values ranged from 1.15-1.18, both indicating quite weak velocity weakening at temperatures of 303-607 °C. (2) Inferred constitutive parameters through numerical fitting to rate and state friction laws show that the healing effect of friction (b value) has an increasing trend with temperature increase up to 403 °C, indicating an Arrhenius-type thermally-activated creep mechanism behind the healing effect. (3) In addition to microstructural observation of deformed samples that shows remarkable size reduction (crushed grains reduced to scales typically below 1-2 μm) in both intensely sheared regions and moderately sheared regions, ubiquitous precipitated particles (50-100 nm) with platy morphologies were observed to attach to the surfaces of crushed grains, which are typical signatures of intergranular pressure solution process, suggesting that pressure solution was commonly activated at intergranular contacts. (4) No recognizable crystalline plasticity was observed. Our microstructural observation together with the comparison of experimental data with model’s prediction, implies that intergranular pressure solution process at the frictional contacts may be the most likely mechanism operating at the frictional contacts and governing the healing effect for augite.