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Abundant heterogeneity has been documented on faults in nature across a wide range of length scales, including structural, mineralogical, and roughness variations. The role of complex heterogeneity on fault mechanics and frictional stability is not well established, and experiments investigating heterogeneity have typically incorporated a single source of heterogeneity. Here, we conduct rock friction experiments on rough, bimaterial faults that are creeping, or steadily sliding, to explore the role of lithological heterogeneity on fault mechanics and stability. When strong asperities juxtapose weak gouge, stable sliding occurs with a low friction coefficient, µ. Encounters of strong diabase asperities on talc gouge lined faults initiate dramatic increases in µ and transitions to unstable sliding characterized by frequent stick-slip events (StSE). Seismic moments and stress drops of StSE decrease with increasing asperity abundance. Stress is concentrated at asperities during encounters, increasing with decreasing asperity abundance and leading to extensive mechanical damage. Interactions between strong, velocity weakening asperities provide a model to explain the nucleation of seismic and aseismic slip events on nominally stable, creeping faults. 

Key Points Changes in hydraulic diffusivity and pressurization factor during thermal pressurization (TP) balance each other in low permeability and low porosity fault rocks Hydraulic diffusional length scales as time 0.7 when considering TP parameters that depend on temperature and pressure The constant case model should be considered with ambient initial conditions and not time‐averaged ones 

Nearly all frictional interfaces strengthen as the logarithm of time when sliding at ultra-low speeds. Observations of also logarithmic-in-time growth of interfacial contact area under such conditions have led to constitutive models that assume that this frictional strengthening results from purely time-dependent, and slip-insensitive, contact-area growth. The main laboratory support for such strengthening has traditionally been derived from increases in friction during “load-point hold” experiments, wherein a sliding interface is allowed to gradually self-relax down to subnanometric slip rates. In contrast, following step decreases in the shear loading rate, friction is widely reported to increase over a characteristic slip scale, independent of the magnitude of the slip-rate decrease—a signature of slip-dependent strengthening. To investigate this apparent contradiction, we subjected granite samples to a series of step decreases in shear rate of up to 3.5 orders of magnitude and load-point holds of up to 10,000 s, such that both protocols accessed the phenomenological regime traditionally inferred to demonstrate time-dependent frictional strengthening. When modeling the resultant data, which probe interfacial slip rates ranging from 3 . μ m · s − 1 . to less than 10 − 5 μ m · s − 1 , we found that constitutive models where low slip-rate friction evolution mimics log-time contact-area growth require parameters that differ by orders of magnitude across the different experiments. In contrast, an alternative constitutive model, in which friction evolves only with interfacial slip, fits most of the data well with nearly identical parameters. This leads to the surprising conclusion that frictional strengthening is dominantly slip-dependent, even at subnanometric slip rates. 

Abstract During rock friction experiments at large displacement, room temperature and humidity, and following a hold test, the fracture energy increases approximately as the square of the logarithm of hold duration. While it's been long known that failure strength increases with log hold time, here the slip weakening distance,d_h, also increases. The weakening distance increase is large, hundreds of percent change over a few thousand seconds. The initial bare surface and simulated fault gouge experiments were conducted in rotary shear at 25 MPa normal stress, 21 MPa confining stress and at displacements greater than 100 mm. In contrast, initially bare surface experiments at 5 MPa normal stress, unconfined at displacements less than 10 mm show effectively no change ind_h. We attribute the difference to the presence of an appreciable shear zone that develops due to wear over significant displacements, confined at elevated normal stress. Prior published studies of sheared simulated fault gouge at short displacement show both acknowledged and unacknowledged increases ind_hthat may relate to our observations. Since natural faults have well‐developed shear zones, the observations have more direct relevance to earthquake nucleation than prior laboratory studies that use short displacement data and focus on frictional strength recovery alone. However, the physics underlying this increase in weakening distance are not known; candidates are compaction (Nakatani, 1998) and delocalization (Sleep et al., 2000). Additional caveats are that these are room temperature and humidity experiments, at a single normal stress that have not yet been reproduced in other laboratories.