Seismic potential of the Anninghe Fault zone, southeastern Tibetan Plateau: Constrains from friction experiments on natural granite gouge

The eastern boundary of the Sichuan-Yunnan tectonic block and Tibetan Plateau is marked by a highly active fault zone featuring four left-lateral strike-slip faults, the Xianshuihe, Anninghe, Zemuhe, and Xiaojiang faults. These collectively show the highest seismicity in southwestern China. Since 1977, a portion of the Anninghe faults (AHF) has experienced seismic quiescence for M_L≥4.0 earthquakes. The spatial extent of this quiescent portion has gradually dwindled with time resulting in the formation of the current 130-km-long “Anninghe seismic gap”. To evaluate the seismic potential and model seismogenesis on this part of the AHF, data are needed on the frictional properties of relevant fault zone materials under mid-crustal hydrothermal conditions.

In this study, we report both saw-cut and rotary shear friction experiments performed on sieved granite gouge collected from the AHF and believed to represent the fault rock composition at seismogenic depth. Experiments were conducted on 1 mm thick gouge layers at 100-600℃, effective normal stress of 100-200MPa, pore water pressures of 30MPa and 100 MPa, and sliding velocities of 0.01-100μm/s . The saw-cut tests reached shear displacements up to 4 mm versus 30 mm in the ring shear experiments. Friction coefficient lays in the range 0.6-0.8 in most samples, except that it drops to 0.4 at higher temperatures and low velocity. In the saw-cut experiments performed at 30MPa pore water pressure, velocity-strengthening behaviour occurred below 200℃ (Regime 1), whereas velocity-weakening occurred at 200-600℃ (Regime 2). By contrast, dry saw-cut experiments showed velocity-strengthening at all temperatures investigated (25-600℃). In the rotary shear experiments performed at 100MPa pore water pressure, three temperature-dependent regimes of behaviour were identified, showing potentially unstable, velocity-weakening behaviour at 100-400℃ (Regime 2) and velocity-strengthening at lower and higher temperatures (Regimes 1 and 3). These regimes moved towards higher temperatures with an increase in sliding velocity. Combining all the data, the importance of Regime 2, i.e. the temperature range characterized by velocity-weakening, potentially seismogenic behaviour, decreased with increasing pore water pressure, shear displacement and effective normal stress. Combined with our microstructural observation and previous studies, we explain our results qualitatively in terms of a microphysical model in which changes in friction coefficient and (a-b) are caused by competition between dilatant granular flow and grain-scale creep processes.

Since the geothermal gradient around AHF is approximately 30 ℃/km, direct application of our results suggests velocity-weakening (Regime 2) on the AHF at depths of 2.5-12.5 km, and velocity-strengthening at shallower and deeper levels. By comparison, the depth range of the AHF seismic gap (locking region) is 0 to 15 km, additionally, the relocated small earthquake distribution in southwestern China shows that the depth of hypocenters are mostly less than 15km, which is consistent with our experimental results. However, our experiments show that the velocity-weakening regime for AHF gouge is controlled by many factors besides temperature, such as effective normal stress, pore fluid pressure, shear displacement and velocity. Further progress towards understanding the seismic gap, and allowing rupture nucleation modelling, for example, therefore requires a more quantitative microphysical modelling approach in future.