The evolution of permeability on fractures and faults during the full earthquake cycle is shown to be sensitive to sealing during the repose phase. We explore the combined effect of static loading followed by fracture reactivation on permeability evolution via slide‐hold‐slide experiments. During the hold periods, permeability exhibits a slow but continuous reduction. The permeability decay is consistent with power law compaction of the aperture coupled with cubic law flow. With increasing hold periods, permeability evolves following reactivation from net reduction to net increase with the magnitude of the permeability change dependent on the hold period. This implies that the tight interlocking of asperities during interseismic repose primes the fault for permeability enhancement following reactivation. The inferred mechanism is via shear dilation with the probable involvement of unclogging. This result identifies that preslip sealing during repose is an essential component in the cyclic permeability evolution throughout the seismic cycle.

A growing body of observations worldwide has documented fault slip transients that radiate little or no seismic energy. The mechanisms that govern these slow slip events and their wide range of depths, slip rates, durations, stress drops, and recurrence intervals remain poorly known. Here we show that slow slip can be explained by a transition from rate-weakening frictional sliding at low slip rates toward rate-neutral or rate-strengthening behavior at higher slip rates, as has been observed experimentally. We use numerical simulations to illustrate that this rate-dependent transition quantitatively explains experimental data for natural fault rocks representative of materials in the source regions of slow slip events. With a standard constant-parameter rate-and-state friction law, slow slip events arise only near the threshold for slip instability. The inclusion of velocity dependent friction parameters significantly broadens the range of conditions for slow slip

Cyclic growth and decay of permeability in fractures is explored during repeated reactivation and repose of saw-cut fractures of Green River shale. These slide-hold-slide experiments are supported by measurements of fracture normal deformation and optical surface profilometry. Overall, we observe continuous permeability decay during repose (holds) and significant permeability enhancement during slow reactivation (slide). The permeability decay is accompanied by fault compaction. Both hydraulic aperture change (Δb h ) and measured compaction (Δb s ) are consistent with time-dependent power law closure with a power exponent of~0.2-0.4. These dual compaction magnitudes are positively correlated but Δb h > Δb s in late stage holds. Permeability enhancement during reactivation is typically also accompanied by fault dilation. However, we also observe some cases where hydraulic aperture change decouples from the measured deformation, conceivably driven by mobilization of wear products and influenced by the development of flow bottlenecks. Pretest and posttest surface profiles show that the surface topography of the fractures is planed down by shear removal. The shear removal is significant with initial laboratory prepared surface (~10 μm of aperture height) but less significant following consecutive reactivations (~2 μm). The flattened surfaces retain small-scale,~10-20 μm wavelength, roughness. Flow simulations, constrained by the surface topography and measured deformation, indicate that small-scale roughness may control permeability at flow bottlenecks within a dominant flow channel. These results suggest cycles of permeability creation and destruction are an intrinsic component of the natural hydraulic system present in faults and fractures and provide an improved mechanistic understanding of the evolution of permeability during fault repose and reactivation.