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Journal of Geophysical Research, 2010

1:08am D R A F T tual steps in velocity and the final decelerating phase). A system of coupled 11 equations is derived and solved numerically. The resulting transient friction 12 and wear evolution yield a satisfactory fit (1) with experiments performed 13 under variable sliding velocities (0.9-2 m/s) and different normal stresses (0.5-14 20 MPa) for various rock types and (2) with estimates of slip weakening ob-15 tained from observations on ancient seismogenic faults that host pseudotachylite 16 (solidified melt). The model allows to extrapolate the experimentally observed 17 frictional behavior to large normal stresses representative of the seismogenic 18 Earth crust (up to 200 MPa), high slip rates (up to 9 m/s) and cases where 19 melt extrusion is negligible. Though weakening distance and peak stress vary 20 widely, the net breakdown energy appears to be essentially independent of 21 either slip velocity and normal stress. In addition, the response to earthquake-22 like slip can be simulated, showing a rapid friction recovery when slip rate 23 drops. We discuss the properties of energy dissipation, transient duration, 24 velocity weakening, restrengthening in the decelerating final slip phase and 25 the implications for earthquake source dynamics.

Journal of Geophysical Research, 2008

Accurate descriptions of strength evolution are required in predictive models of fault zone behavior during earthquakes. At low sliding rates, frictional resistance between fault rocks is much higher than the shear stress that is typically inferred to be present during earthquakes. Laboratory experiments confirm that the friction coefficient drops at high sliding rates, and it is suggested that strengthening, possibly related to an increase in the area of viscous melt patches, may occur after this initial weakening stage. Most existing weakening mechanisms do not predict such strengthening, which may exert an important control on the thickness of the shear zone. We propose a micromechanical model of flash heating that describes how shear resistance evolves at the asperity scale as a result of distributed deformation over a weak layer that grows during the brief lifetime of each asperity contact. Beyond a threshold weakening velocity, our model predicts that friction should decrease with slip rate since higher sliding speeds cause the weak layer to thicken more rapidly. A comparison with published experimental data from a range of mineral systems shows good agreement with the model predictions. The parameter choices that ensure good model fits to the laboratory friction data are consistent with a priori estimates for the onset of asperity melting at high contact normal stresses. At higher sliding rates and/or elevated temperatures, our model predicts that the frictional rate dependence should transition from velocity weakening to become velocity strengthening because decreases in the contact lifetime with slip rate cause the average asperity strength to increase.

Frontiers in Earth Science, 2020

Geophysical Research Letters, 2005

Rotary friction experiments have been performed on Westerly granite at start velocities of 2.0-4.0 m s À1 , under loads of 250-500 N until the system stalls (up to $4 s). Interface temperature, velocity and force were measured at intervals of 0.2 milliseconds. The results show that kinetic friction increases in direct proportion to temperature up to a critical value T cr , whereupon the system changes from boundary lubrication (slip strengthening) to hydrodynamic lubrication (slip weakening) with an associated decrease in friction and temperature. For Westerly granite T cr is $1150°C. This corresponds to the melting point of feldspar, which constitutes $50% of the rock. If T cr is exceeded in natural slip systems, which will depend on rock type, this can result in the generation of friction melt. Melt lubrication can explain the scaled energy of large earthquakes being 10 to 100 times more than that of small earthquakes.

Journal of Geophysical Research, 2005

Field observations of pseudotachylites and experimental studies of high-speed friction indicate that melting on a slipping interface may significantly affect the magnitude of shear stresses resisting slip. We investigate the effects of rock melting on the dynamic friction using theoretical models of shear heating that couple heat transfer, thermodynamics of phase transitions, and fluid mechanics. Results of laboratory experiments conducted at high (order of m/s) slip velocities but low (order of MPa) normal stresses suggest that the onset of frictional melting may give rise to substantial increases in the effective fault strength, presumably due to viscous effects. However, extrapolation of the modeling results to in situ conditions suggests that the efficiency of viscous braking is significantly reduced under high normal and shear stresses. When transient increases in the dynamic fault strength due to fusion are not sufficient to inhibit slip, decreases in the effective melt viscosity due to shear heating and melting of clasts drastically decrease the dynamic friction, resulting in a nearly complete stress drop ("thermal runaway"). The amount of energy dissipation associated with the formation of pseudotachylites is governed by the temperature dependence of melt viscosity and the average clast size in the fault gouge prior to melting. Clasts from a coarse-grained gouge have lower chances of survival in a pseudotachylite due to a higher likelihood of nonequilibrium overheating. The maximum temperature and energy dissipation attainable on the fault surface are ultimately limited by either the rock solidus (via viscous braking, and slip arrest) or liquidus (via thermal runaway and vanishing resistance to sliding). Our modeling results indicate that the thermally activated fault strengthening and rupture arrest are unlikely to occur in most mafic protoliths but might be relevant for quartz-rich rocks, especially at shallow (<5-7 km) depths where the driving shear stress is relatively low.

Nature Communications

The triggering and magnitude of earthquakes is determined by the friction evolution along faults. Experimental results have revealed a drastic decrease of the friction coefficient for velocities close to the maximum seismic one, independently of the material studied. Due to the extreme loading conditions during seismic slip, many competing physical phenomena occur (like mineral decomposition, nanoparticle lubrication, melting among others) that are typically thermal in origin and are changing the nature of the material. Here we show that a large set of experimental data for different rocks can be described by such thermally-activated mechanisms, combined with the production of weak phases. By taking into account the energy balance of all processes during fault movement, we present a framework that reconciles the data, and is capable of explaining the frictional behavior of faults, across the full range of slip velocities (10−9 to 10 m/s).

2011

Earthquakes have long been recognized as resulting from a stick–slip frictional instability. The development of a full constitutive law for rock friction now shows that the gamut of earthquake phenomena—seismogenesis and seismic coupling, pre-and post-seismic phenomena, and the insensitivity of earthquakes to stress transients—all appear as manifestations of the richness of this friction law.

Meso-Scale Shear Physics in Earthquake and Landslide Mechanics, 2009

Field observations of maturely slipped faults show a generally broad zone of damage by cracking and granulation. Nevertheless, large shear deformation, and therefore heat generation, in individual earthquakes takes place with extreme localization to a zone <1-5 mm wide within a finely granulated fault core. Relevant fault weakening processes during large crustal events are therefore likely to be thermal. Further, given the porosity of the damage zones, it seems reasonable to assume groundwater presence. It is suggested that the two primary dynamic weakening mechanisms during seismic slip, both of which are expected to be active in at least the early phases of nearly all crustal events, are then as follows: (1) Flash heating at highly stressed frictional micro-contacts, and (2) Thermal pressurization of fault-zone pore fluid. Both have characteristics which promote extreme localization of shear. Macroscopic fault melting will occur only in cases for which those processes, or others which may sometimes become active at large enough slip (e.g., thermal decomposition, silica gelation), have not sufficiently reduced heat generation and thus limited temperature rise. Spontaneous dynamic rupture modeling, using procedures that embody mechanisms (1) and (2), shows how faults can be statically strong yet dynamically weak, and operate under low overall driving stress, in a manner that generates negligible heat and meets major seismic constraints on slip, stress drop, and self-healing rupture mode.