Rate-strengthening Friction Research Articles

Seasonal slow slip in landslides as a window into the frictional rheology of creeping shear zones.

Whether Earth materials exhibit frictional creep or catastrophic failure is a crucial but unresolved problem in predicting landslide and earthquake hazards. Here, we show that field-scale observations of sliding velocity and pore water pressure at two creeping landslides are explained by velocity-strengthening friction, in close agreement with laboratory measurements on similar materials. This suggests that the rate-strengthening friction commonly measured in clay-rich materials may govern episodic slow slip in landslides, in addition to tectonic faults. Further, our results show more generally that transient slow slip can arise in velocity-strengthening materials from modulation of effective normal stress through pore pressure fluctuations. This challenges the idea that episodic slow slip requires a narrow range of transitional frictional properties near the stability threshold, or pore pressure feedbacks operating on initially unstable frictional slip.

Poroelastic effects destabilize mildly rate-strengthening friction to generate stable slow slip pulses

Slow slip events on tectonic faults, sliding instabilities that never accelerate to inertially limited ruptures or earthquakes, are one of the most enigmatic phenomena in frictional sliding. While observations of slow slip events continue to mount, a plausible mechanism that permits instability while simultaneously limiting slip speed remains elusive. Rate-and-state friction has been successful in describing most aspects of rock friction, faulting, and earthquakes; current explanations of slow slip events appeal to rate-weakening friction to induce instabilities, which are then stalled by additional stabilizing processes like dilatancy or a transition to rate-strengthening friction at high slip rates. However, the temperatures and/or clay-rich compositions at slow slip locations are almost ubiquitously associated with rate-strengthening friction. In this study, we propose a fundamentally different instability mechanism that may reconcile this contradiction, demonstrating how slow slip events can nucleate with mildly rate-strengthening friction. We identify two destabilizing mechanisms, both reducing frictional shear strength through reductions in effective normal stress, that counteract the stabilizing effects of rate-strengthening friction. The instability develops into slow slip pulses. We quantify parameter controls on pulse length, propagation speed, and other characteristics, and demonstrate broad consistency with observations of tectonic slow slip events as well as laboratory tribology experiments.

Glacier sliding, seismicity and sediment entrainment

ABSTRACTThe evolution of glaciers and ice sheets depends on processes in the subglacial environment. Shear seismicity along the ice–bed interface provides a window into these processes. Such seismicity requires a rapid loss of strength that is typically ascribed to rate-weakening friction, i.e., decreasing friction with sliding or sliding rate. Many friction experiments have investigated glacial materials at the temperate conditions typical of fast flowing glacier beds. To our knowledge, however, these studies have all found rate-strengthening friction. Here, we investigate the possibility that rate-weakening rock-on-rock friction between sediments frozen to the bottom of the glacier and the underlying water-saturated sediments or bedrock may be responsible for subglacial shear seismicity along temperate glacier beds. We test this ‘entrainment-seismicity hypothesis’ using targeted laboratory experiments and simple models of glacier sliding, seismicity and sediment entrainment. These models suggest that sediment entrainment may be a necessary but not sufficient condition for the occurrence of basal shear seismicity. We propose that stagnation at the Whillans Ice Stream, West Antarctica may be caused by the growth of a frozen fringe of entrained sediment in the ice stream margins. Our results suggest that basal shear seismicity may indicate geomorphic activity.

Stable creeping fault segments can become destructive as a result of dynamic weakening

Faults in Earth's crust accommodate slow relative motion between tectonic plates through either similarly slow slip or fast, seismic-wave-producing rupture events perceived as earthquakes. These types of behaviour are often assumed to be separated in space and to occur on two different types of fault segment: one with stable, rate-strengthening friction and the other with rate-weakening friction that leads to stick-slip. The 2011 Tohoku-Oki earthquake with moment magnitude M(w) = 9.0 challenged such assumptions by accumulating its largest seismic slip in the area that had been assumed to be creeping. Here we propose a model in which stable, rate-strengthening behaviour at low slip rates is combined with coseismic weakening due to rapid shear heating of pore fluids, allowing unstable slip to occur in segments that can creep between events. The model parameters are based on laboratory measurements on samples from the fault of the M(w) 7.6 1999 Chi-Chi earthquake. The long-term slip behaviour of the model, which we examine using a unique numerical approach that includes all wave effects, reproduces and explains a number of both long-term and coseismic observations-some of them seemingly contradictory-about the faults at which the Tohoku-Oki and Chi-Chi earthquakes occurred, including there being more high-frequency radiation from areas of lower slip, the largest seismic slip in the Tohoku-Oki earthquake having occurred in a potentially creeping segment, the overall pattern of previous events in the area and the complexity of the Tohoku-Oki rupture. The implication that earthquake rupture may break through large portions of creeping segments, which are at present considered to be barriers, requires a re-evaluation of seismic hazard in many areas.

Coupled afterslip and viscoelastic flow following the 2002 Denali Fault, Alaska earthquake

Models of postseismic deformation following the 2002 M 7.9 Denali Fault, Alaska earthquake provide insight into the rheologic structure of the Alaskan lithosphere and the physical processes activated following a large earthquake. We model coseismic GPS displacements and 4 yr of postseismic GPS position time-series with a coupled model of afterslip on the fault in the lithosphere and distributed viscous flow in the asthenosphere. Afterslip is assumed to be governed by a simplified version of a laboratory-derived rate-strengthening friction law that is characterized with a single parameter, σ(a − b), where σ is the effective normal stress on the fault and a − b is a dimensionless empirical parameter. Afterslip is driven by coseismic stress changes on the fault generated by the main shock. The lithosphere is modelled as an elastic plate overlying a linear, Maxwell, viscoelastic asthenosphere. We devise a scheme to simultaneously estimate the distributions of coseismic slip and afterslip, friction parameters, lithosphere thickness and asthenosphere viscosity. The postseismic GPS time-series are best reproduced with a 45–85 km thick elastic lithosphere overlying an asthenosphere of viscosity 0.6–1.5 × 1019 Pa s. The 45–85 km elastic lithosphere thickness is greater than or equal to the average crustal thickness in the region suggesting that distributed postseismic flow occurs largely within the mantle while postseismic deformation in the crust is confined largely to slip on a discrete fault penetrating the entire crust and perhaps into the uppermost mantle. For typical laboratory values of a − b of order 10−2 at temperatures corresponding to the inferred depth of afterslip, the estimated effective normal stress on the fault is ∼50 MPa, which is about an order of magnitude lower than effective normal stresses at hydrostatic pore pressure. Previous studies showed that models with linear Newtonian rheology cannot reproduce the observed GPS time-series but that models incorporating non-linear or biviscous flow of the mantle do fit the data. We show that a model with afterslip governed by a non-linear fault zone rheology coupled to Newtonian mantle flow is sufficient to reproduce the GPS time-series.

Modelling temporal variation of surface creep on the Chihshang fault in eastern Taiwan with velocity-strengthening friction

SUMMARY The active Chihshang fault at the boundary between the Eurasian and the Philippine Sea plates along the Longitudinal Valley in eastern Taiwan is creeping near the surface but has also produced large earthquakes at mid-crustal depth such as the 2003, M w 6.5, Chengkung earthquake. The creep rate measured at the surface shows strong seasonal fluctuations before the Chengkung earthquake, correlated with groundwater pressure variations measured at nearby wells. The Chengkung earthquake did not rupture the fault near the surface but induced a sudden increase of creep rate that decayed with time during the postseismic period. These observations suggest that the near surface fault obeys a rate-strengthening friction law. We conduct numerical simulations based upon the observed variations of creep rate with regard to a velocity-strengthening friction law and 1-D groundwater diffusion model to investigate the fault rheology behind this phenomenon. The model, which assumes a creeping fault segment extending from the surface to a depth of 5 km, yields a good fit to the creep data when the friction parameters are assumed to vary with depth or to have changed at the time of the Chengkung earthquake. Our best model suggests that the creeping zone is characterized by a rate parameter a = ∂μ/∂log (V )o f 1.3× 10 −2 and a friction coefficient of 0.84, and a long-term slip rate of 25.9 mm yr −1 at depths less than 87 m. By contrast, the lower segment of the creeping zone exhibits a much lower friction coefficient of 0.19, a smaller a of 6.6 × 10 −3 and a higher long-term slip rate of 38.1 mm yr −1 at depths between 0.087 and 5.0 km. This change in rate parameters implies that the lithologies or physical properties of the fault rocks may vary with depth. Alternatively, a model assuming a dramatic increase in rate parameter from 5.6 × 10 −4 to 6.2 × 10 −3 by the strong ground shaking of the Chengkung earthquake can also yield a fair agreement with the observed data. The lack of a notable deceleration in fault creep rate during the dry period from 2002 to mid-2003 suggests that the real recharging system for the fault zone may be more complicated than what we assumed in this study.

Evidence for coupled seismic and aseismic fault slip during water injection in the geothermal site of Soultz (France), and implications for seismogenic transients

We analysed the triggered seismicity recorded near 3 km in depth within granite during the 1993 water injection experiment at Soultz (France). We selected all large multiplets associated with the largest dislocated fault identified on the borehole logging (4.3 cm slip), and showed that each one consisted of the repeated rupture of a single asperity. These asperities were forced to rupture due to fault creep around them, so that their growing, cumulative slip history reveals the creep history on the fault, with a mean slip rate and a seismicity rate both decaying approximately as 1/t. This is consistent with a rate-strengthening friction law on the creeping faults, and differs from the interpretation of Omori type 1/t seismicity rates in terms of a weakening friction control of the unstable fault surfaces. This model of asperity rupture forced by relaxation creep is easily generalized in seismogenic regions, even for single ruptures of asperities on small creeping faults, showing that a significant part of aftershock rates can be controlled by the strengthening friction properties and the creep of faults at all scales. This lead us to introduce the concept of a critical asperity density, above which dynamic interaction between neighbouring asperities can initiate large seismic ruptures. At Soultz, the asperity density is subcritical, but the neighbouring asperities nonetheless interact, at distances up to a few source dimensions, as revealed by their delayed cross-triggering. Considering the seismic cloud in Soultz at a global scale, the strain produced by the injection experiment was mostly aseismic, related to creep on major faults, and causing large permeability changes – influencing in turn the pore pressure diffusion and the creeping process. Our observations demonstrate that the identification and analysis of multiplets can provide accurate images of the geometry and kinematics of transient slip patches, and indirectly detect pore pressure changes, which should contribute to a better understanding of the coupling between transient fluid flow, creep and microseismic activity in seismogenic regions.