To understand mechanism for unstable velocity‐weakening slip behavior of plagioclase at hydrothermal conditions in the lower crust, we employed nominally dry experiments on plagioclase gouge to isolate fluid‐assisted processes that may be activated at hydrothermal conditions and obtained constitutive parameters fitted to rate and state friction laws with temperatures of 99–617°C and confining pressure of 150 MPa, allowing comparison to the hydrothermal counterpart. While the direct effect (a value) has relatively high values of 0.007–0.01 in the high temperature range above 300–400°C, the evolution effect (b value) shows a diminishing trend from peak values of 0.0075 and 0.0057, respectively, at 305°C and 400°C down to ~0 at 617°C, in contrast to the increasing trend in both effects at hydrothermal conditions. The evolution effect in the dry experiments may be controlled by structural changes related to degrees of shear localization and evolution of porosity change rate in response to a change in loading rate. The evolution effect in dry friction compared to the thermally enhanced, much stronger one at hydrothermal conditions suggests activation of fluid‐assisted time‐dependent mechanism operating in the wet experiments.

The deformation model of antigorite and its relationship to intermediate‐depth earthquakes are still controversial. Here, we conducted deformation experiments on foliated iron‐poor antigorite at confining pressures of 1,100–1,500 MPa, temperatures of 400–600°C, and axial displacement rates of 3.75 × 10⁻⁶–7.24 × 10⁻⁴ mm/s using a solid‐medium Griggs‐type apparatus. As the temperature and/or pressure increases, the stress exponent decreases, and all samples show stable fault slip. Microstructural observations show ductile behavior indicated by kinking and bending of the originally straight foliation, and brittle characteristics, such as Riedel shear, comminution, and microcracking. Therefore, antigorite deformation is controlled by a semibrittle regime. Antigorite decomposed to forsterite and enstatite in samples sheared at temperatures ≥550°C but not in samples triaxially compressed at 600°C. This observation, combined with phase boundary information, implies that the deformation geometry (e.g., triaxial compression or shear) may change the phase stability of antigorite. Following dehydration, samples still show stable fault slip, indicating that dehydration does not directly trigger earthquakes. In cold subduction zones, fine‐grained antigorite in the core of the shear zone likely induces seismic activity by producing water and talc (or a talc‐like phase) in the process of shear friction because such phases within the fault zone may reduce the rate dependence, thereby inducing unstable slip. The comparison of the semibrittle strengths of antigorite and dunite and the weak strain rate dependence of stress suggest that a thin antigorite layer immediately above the subducting slab is not sufficient to cause significant decoupling between the slab and the mantle wedge.

The bottom of the lithosphere is characterized by a thermally controlled transition from brittle to ductile deformation. While the mechanical behavior of rocks firmly within the brittle and ductile regimes is relatively well understood, how the transition operates remains elusive. Here, we study the mechanical properties of pure olivine gouge from 100 to 500°C under 100 MPa pore‐fluid pressure in a triaxial deformation apparatus as a proxy for the mechanical properties of the upper mantle across the brittle‐ductile transition. We describe the mechanical data with a rate‐, state‐, and temperature‐dependent constitutive law with multiple thermally activated deformation mechanisms. The stress power exponents decrease from 70 ± 10 in the brittle regime to 17 ± 3 and 4 ± 2 in the semi‐brittle and ductile regimes, respectively. The mechanical model consistently explains the mechanical behavior of olivine gouge across the brittle‐ductile transition, capturing the gradual evolution from cataclasis to crystal plasticity.

Laumontite is a common and potentially frictionally unstable hydrothermal alteration product present in deep faults of the Gonghe EGS reservoir. We characterize the friction‐stability characteristics of synthetic laumontite gouge under in situ reservoir conditions. The pure laumontite gouge is frictionally strong (μ = 0.73–0.98) and the quartz/laumontite mixture (1:1) is generally less strong (μ = 0.73–0.78) under experimental conditions (P_c = 95 MPa, T = 90–250°C, P_f = 0–90 MPa). The shear velocity was stepped between 6.1, 0.61, then 0.061 μm/s for our experiments. For both gouges, the friction coefficient is independent of temperature and increases with elevated pore pressures. The pure gouge and mixture are strongly velocity‐weakening over a broad range in temperatures (∼90–220°C) and excess pore pressures (0–90 MPa) relevant to the Gonghe stimulation. Microearthquakes (MEQs) observed during stimulation are confined to within the broad depth range of inferred frictional instability—although fluid overpressures are also limited to this region. The observation that laumontite mixtures are frictionally unstable over a broad range of pressures and especially temperatures representative of EGS reservoirs and insensitive to the presence of the coexisting mineral phase (quartz) suggests its presence is a strong indicator of potential seismic hazard.