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[1] The dynamics of a spring-slider system with a single degree of freedom was investigated, focusing on two different rate-and state-dependent friction laws. While the inertia-controlled behavior and stick-slip cycles for a system that obeys the slip law have been extensively simulated, this study presents a comparative study of a system that obeys the slowness law. A key conclusion is that for both friction laws the overall stress drops are linearly related to the logarithm of the loading velocity (and the recurrence time) through the velocity-weakening parameter b À a and normal stress. Relatively higher peak stresses, larger quasi-static stress drop, and larger effective fracture energy are associated with a system that obeys the slowness law. Consequently, the partitioning of stress drop between quasi-static and dynamic slips, as well as dynamic overshoot and strength recovery, varies according to whether the slowness or slip law has been adopted. Analytic approximations were derived that elucidate the interplay of dynamics, energetics, and frictional constitutive behavior in controlling the scaling of stress drops with loading velocity and recurrence time. Seismological implications of the scaling behavior are also discussed.

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The presence of metamorphic epidote on faults has been implicated in the transition from stable to unstable slip and the nucleation of earthquakes. We present structured laboratory observations of mixed epidote and simulated Pohang granodiorite (analogous to the EGS‐enhanced geothermal system site) gouges to evaluate the impact of heterogeneity and contiguity of epidote‐patch structure on frictional instability. Experiments are at a confining pressure of 110 MPa, pore fluid pressures of 42–63 MPa, temperatures 100–250°C and epidote percentages of 0–100 vol.%. The simulated Pohang granodiorite gouge is frictionally strong (friction coefficient ∼0.71) but transits from velocity‐strengthening to velocity‐weakening at temperatures >150°C. This velocity‐weakening effect is amplified in approximate proportion to increasing epidote content. Modes of epidote precipitation likely control the size and contiguity of the epidote‐only patches and this in turn changes the response of 50:50 epidote‐granodiorite mixed gouges for different geometric configurations. However, 50:50 epidote‐granodiorite mixtures that are variously homogeneously mixed, encapsulated and checkerboarded in their structures are insensitive to their geometries – all reflect the high frictional strength and strong velocity‐weakening response of 100:0 pure epidote. This suggests that the epidote present as thin coatings on fractures/faults can enhance velocity‐weakening behavior, independent of individual patch size and can thereby support the potential seismic reactivation of faults. Considering the frictional and stability properties of epidote at conditions typical of shallow depths, the presence of low‐grade metamorphism exerts a potentially important control on fault stability in granitoids with relevance as a marker mineral for susceptibility to injection‐induced seismicity.