The determination of rock friction at seismic slip rates (about 1 m s(-1)) is of paramount importance in earthquake mechanics, as fault friction controls the stress drop, the mechanical work and the frictional heat generated during slip(1). Given the difficulty in determining friction by seismological methods(1), elucidating constraints are derived from experimental studies(2-9). Here we review a large set of published and unpublished experiments (similar to 300) performed in rotary shear apparatus at slip rates of 0.1-2.6 ms(-1). The experiments indicate a significant decrease in friction (of up to one order of magnitude), which we term fault lubrication, both for cohesive (silicate-built(4-6), quartz-built(3) and carbonate-built(7,8)) rocks and non-cohesive rocks (clay-rich(9), anhydrite, gypsum and dolomite(10) gouges) typical of crustal seismogenic sources. The available mechanical work and the associated temperature rise in the slipping zone trigger(11,12) a number of physicochemical processes (gelification, decarbonation and dehydration reactions, melting and so on) whose products are responsible for fault lubrication. The similarity between (1) experimental and natural fault products and (2) mechanical work measures resulting from these laboratory experiments and seismological estimates(13,14) suggests that it is reasonable to extrapolate experimental data to conditions typical of earthquake nucleation depths (7-15 km). It seems that faults are lubricated during earthquakes, irrespective of the fault rock composition and of the specific weakening mechanism involved

[1] Frictional melt is implied in a variety of processes such as seismic slip, ice skating, and meteorite combustion. A steady state can be reached when melt is continuously produced and extruded from the sliding interface, as shown recently in a number of laboratory rock friction experiments. A thin, low-viscosity, high-temperature melt layer is formed resulting in low shear resistance. A theoretical solution describing the coupling of shear heating, thermal diffusion, and extrusion is obtained, without imposing a priori the melt thickness. The steady state shear traction can be approximated at high slip rates by the theoretical formunder a normal stress s n , slip rate V, radius of contact area R (A is a dimensional normalizing factor and W is a characteristic rate). Although the model offers a rather simplified view of a complex process, the predictions are compatible with experimental observations. In particular, we consider laboratory simulations of seismic slip on earthquake faults. A series of highvelocity rotary shear experiments on rocks, performed for s n in the range 1-20 MPa and slip rates in the range 0.5-2 m s À1 , is confronted to the theoretical model. The behavior is reasonably well reproduced, though the effect of radiation loss taking place in the experiment somewhat alters the data. The scaling of friction with s n , R, and V in the presence of melt suggests that extrapolation of laboratory measures to real Earth is a highly nonlinear, nontrivial exercise.

Dynamic simulations of earthquakes on dipping faults show asymmetric near-source ground motion caused by the asymmetric geometry of such faults. The ground motion from a thrust or reverse fault is larger than that of a normal fault by a factor of 2 or more, given identical initial stress magnitudes. The motion of the hanging wall is larger than that of the footwall in both thrust (reverse) and normal earthquakes. The asymmetry between normal and thrust (reverse) faults results from time-dependent normal stress caused by the interaction of the earthquake-generated stress field with Earth's free surface. The asymmetry between hanging wall and footwall results from the asymmetric mass and geometry on the two sides of the fault.

[1] We observe the nucleation phase of in-plane ruptures in the laboratory. We show that the nucleation is composed of two distinct phases, a quasi-static and an acceleration stage, followed by dynamic propagation. We propose an empirical model which describes the rupture length evolution: The quasi-static phase is described by an exponential growth while the acceleration phase is described by an inverse power law of time. The transition from quasi-static to accelerating rupture is related to the critical nucleation length, which scales inversely with normal stress in accordance with theoretical predictions, and to a critical surfacic power, which may be an intrinsic property of the interface. Finally, we discuss these results in the frame of previous studies and propose a scaling up to natural earthquake dimensions.

We report macroscopic stick‐slip events in saw‐cut Westerly granite samples deformed under controlled upper crustal stress conditions in the laboratory. Experiments were conducted under triaxial loading (σ1>σ2=σ3) at confining pressures (σ3) ranging from 10 to 100 MPa. A high‐frequency acoustic monitoring array recorded particle acceleration during macroscopic stick‐slip events allowing us to estimate rupture speed. In addition, we record the stress drop dynamically and we show that the dynamic stress drop measured locally close to the fault plane is almost total in the breakdown zone (for normal stress >75 MPa), while the friction f recovers to values of f > 0.4 within only a few hundred microseconds. Enhanced dynamic weakening is observed to be linked to the melting of asperities which can be well explained by flash heating theory in agreement with our postmortem microstructural analysis. Relationships between initial state of stress, rupture velocities, stress drop, and energy budget suggest that at high normal stress (leading to supershear rupture velocities), the rupture processes are more dissipative. Our observations question the current dichotomy between the fracture energy and the frictional energy in terms of rupture processes. A power law scaling of the fracture energy with final slip is observed over 8 orders of magnitude in slip, from a few microns to tens of meters.