The Anninghe Fault (ANHF) is a major left‐lateral strike‐slip fault in southwestern China and one of the main seismogenic fault zones with a history of strong earthquakes. To understand the frictional properties of natural granitic gouges from the principal slip zone, we conducted hydrothermal friction experiments using both saw‐cut and ring shear methods. These experiments were performed at temperatures (T) of 25–600°C, pore pressures (P_f) of zero (dry), 30 and 100 MPa, sliding velocities (V) of 0.01–100 μm/s and effective normal stresses () of 68, 100, and 200 MPa. The (apparent) friction coefficient is low (μ < 0.5) at high T (600°C), high P_f (100 MPa) and low V ( 0.6) under all other T, P_f and V conditions. Under high P_f, the velocity dependence of friction, (a‐b), displays three regimes with increasing temperature, from positive below ∼100°C to negative at 100–300°C (at V = 1–3 μm/s) or else 100–450°C (at V = 30–100 μm/s), becoming positive again above 300–450°C. At low P_f, the negative (a‐b) expands to the range ∼300–600°C. Microstructural observations and microphysical interpretation imply that the frictional weakening and transitions in (a‐b) are related to competition between dilatant granular flow and deformation of the fine‐grained gouge by intergranular pressure solution accompanied by healing phenomena (leading to cavitation‐creep‐like behavior). Our results provide a possible explanation for the distribution of earthquakes at different depths in the continental crust, in particular for the depth range of the seismogenic zone between 4 and 24 km along the ANHF.

As a particularly common mineral in granites, the presence of feldspar and feldspar‐chlorite gouges at hydrothermal conditions has important implications in fault strength and reactivation. We present laboratory observations of frictional strength and stability of feldspar (K‐feldspar and albite) and feldspar‐chlorite gouges under conditions representative of deep geothermal reservoirs to evaluate the impact on fault stability. Velocity‐stepping experiments are performed at a confining stress of 95 MPa, pore pressures of 35–90 MPa, and temperatures of 120–400°C representative of in situ conditions for such reservoirs. Our experiment results indicate that the feldspar gouge exhibits strong friction (μ ∼ 0.71) at all experimental temperatures (∼120–400°C) but when T > 120°C, the frictional response transitions from velocity‐strengthening to slightly velocity‐weakening. At constant confining pressure and temperature, increasing the pore pressure increases the friction coefficient (∼0.70–0.85) and the gouge remains slightly velocity weakening. The presence of alteration‐sourced chlorite leads to a transition from velocity weakening to velocity strengthening in the mixed gouge at experimental temperatures and pore pressures. As a ubiquitous mineral in reservoir rocks, feldspar is shown to potentially contribute to unstable sliding over ranges in temperature and pressure typical in deep hydrothermal reservoirs. These findings emphasize that feldspar minerals may increase the potential for injection‐induced seismicity on pre‐existing faults if devoid of chloritization.

Fine‐grained gouge found in fault cores controls earthquake generation through its frictional behavior and microstructural evolution. Here we present friction experiments on natural granitic gouges of various initial grain sizes at hydrothermal conditions. To obtain the velocity( V )‐dependence of friction of both fine‐ and coarse‐grained samples, velocity‐stepping tests were conducted between 0.01 and 100 μm/s at 25–600°C, 100 MPa pore pressure and 100 MPa effective normal stress. In the rate‐and‐state friction framework, we observed that fine‐grained gouge is strongly velocity‐weakening over a substantially broader range in temperature than coarse‐grained gouge at V of 1–100 μm/s. Velocity‐weakening may result from a combination of mechanical comminution (grain size reduction) and dilatant granular (i.e., cataclastic) flow, operating in competition with compaction and creep involving grain scale mass transfer by pressure solution. We also infer that temperature‐dependent grain growth of ultrafine particles plays a role in determining the upper temperature limit of velocity‐weakening.

The Anninghe Fault (ANHF) is a large left-lateral strike-slip fault in southwestern China, and it is one of the major seismogenic fault zones where several strong earthquakes happened in history with a high level of seismic potential. To understand the frictional properties of natural granitic gouges in the principal slip zone, we performed hydrothermal friction experiments on both saw-cut and ring shear tests. Velocity-stepping and constant velocity experiments were performed at temperatures (T) of 25-600 ℃, pore pressure (Pf) of dry, 30 and 100 MPa, sliding velocities (V) of 0.01-100 μm/s and effective normal stress (σneff ) of 68, 100 and 200 MPa. The results showed the friction coefficient (μ) is high (μ=0.64-0.80), when this field remains more or less constant with T, Pf and V; Conversely, friction was substantially lower (μ=0.37-0.48), but μ in this field is sensitive to T, Pf and V. The velocity dependence of friction, (a-b), varies with T, Pf, V, and fine grain size. At high Pf, velocity-weakening at 100 to 350 ℃, but as V increases and Pf decreases, the unstable slip at T will increase to 600 ℃. Microstructural observations suggest that the values of μ and (a-b) are controlled by competition between dilatation by granular flow and compaction by intergranular pressure solution of fine grains. Our microphysical-based results provide a possible explanation for the distribution of earthquakes at different depths in the continental crust, and earthquake nucleation is up to ~4-24 km along the ANHF.

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 ML≥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.

As a particularly common minerals in granites, the presence of feldspar and altered feldspar-chlorite gouges at hydrothermal conditions have important implications in fault strength and reactivation. We present laboratory observations of frictional strength and stability of feldspar (K-feldspar and albite) and altered feldspar-chlorite gouges under conditions representative of deep geothermal reservoirs to evaluate the impact on fault stability. Velocity‐ stepping experiments are performed at a confining stress of 95 MPa, pore pressures of 35-90 MPa and temperatures of 120-400°C representative of in situ conditions for such reservoirs. Our experiment results show that the feldspar gouge is frictionally strong (μ~0.71) at all experimental temperatures (~120-400℃) but transits from velocity-strengthening to velocity-weakening at T>120°C. Increasing the pore pressure increases the friction coefficient (~0.70-0.87) and the gouge remains velocity weakening, but this weakening decreases as pore pressures increase. The presence of alteration-sourced chlorite leads to a transition from velocity weakening to velocity strengthening in the mixed gouge at experimental temperatures and pore pressures. As a ubiquitous mineral in reservoir rocks, feldspar is shown to potentially contribute to unstable sliding over ranges in temperature and pressure typical in deep hydrothermal reservoirs. These findings emphasize that feldspar minerals may increase the potential for injection-induced seismicity on pre-existing faults if devoid of chlorite alteration.