Alarm rent Age Dependence of the Fault Weakening Distance in ROCK Friction

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 in d(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 in d(h) that 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. Plain Language Summary The slip weakening distance in room temperature and humidity, confined, large displacement faulting experiments increases quasi-linearly with the logarithm of time since the last slip event. The increase is absent in experiments that lack an appreciable shear zone and we attribute the observed effect to changes in fault properties within the wear or gouge material. The observations suggest 'toughening' of natural faults zone during the interseismic period, where the energy dissipated in initiating failure increases more dramatically with age than suggested in prior studies.

Automated summary

In rock friction experiments at large displacement, it is observed that the fracture energy and slip weakening distance increase with the square of the logarithm of hold duration. The increase in slip weakening distance is attributed to the development of a shear zone due to wear over significant displacements. These observations have direct relevance to earthquake nucleation and suggest a 'toughening' of natural faults during the interseismic period.