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 Here we review a large set of published and unpublished experiments (similar to 300) performed in rotary shear apparatus at slip rates of 0.1-2.6 ms(-1).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] To understand how frictional melting affects fault instability, we performed a series of high-velocity friction experiments on gabbro at slip rates of 0.85-1.49 m s À1 , at normal stresses of 1.2-2.4 MPa and with displacements up to 124 m. Experiments have revealed two stages of slip weakening; one following the initial slip and the other immediately after the second peak friction. The first weakening is associated with flash heating, and the second weakening is due to the formation and growth of a molten layer along a simulated fault. The two stages of weakening are separated by a marked strengthening regime in which melt patches grow into a thin, continuous molten layer at the second peak friction. The frictional coefficient decays exponentially from 0.8-1.1 to 0.6 during the second weakening. The host rocks are separated completely by a molten layer during this weakening so that the shear resistance is determined by the gross viscosity and shear strain rate of the molten layer. Melt viscosity increases notably soon after a molten layer forms. However, a fault weakens despite the increase in melt viscosity, and the second weakening is caused by the growth of molten layer resulting in the reduction in shear strain rate of the molten layer. Very thin melt cannot be squeezed out easily from a fault zone so that the rate of melting would be the most critical factor in controlling the slip-weakening distance. Effect of frictional melting on fault motion can be predicted by solving a Stefan problem dealing with moving host rock/molten zone boundaries.Citation: Hirose, T., and T. Shimamoto (2005), Growth of molten zone as a mechanism of slip weakening of simulated faults in gabbro during frictional melting,

High-velocity weakening of faults may drive fault motion during large earthquakes. Experiments on simulated faults in Carrara marble at slip rates up to 1.3 meters per second demonstrate that thermal decomposition of calcite due to frictional heating induces pronounced fault weakening with steady-state friction coefficients as low as 0.06. Decomposition produces particles of tens of nanometers in size, and the ultralow friction appears to be associated with the flash heating on an ultrafine decomposition product. Thus, thermal decomposition may be an important process for the dynamic weakening of faults.

[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.

[1] Subduction zone earthquakes can propagate to the surface causing large seafloor displacements resulting in tsunamis. This requires the earthquake to rupture through clay-rich sediments of the accretionary wedge, which are largely aseismic. As found previously, the frictional properties of a range of wet clays at low slip velocity are velocity strengthening, thus inhibiting earthquake nucleation. However, at high slip velocity the same materials weaken almost immediately resulting in a negligible critical slip weakening distance and fracture energy. We interpret this behaviour as rapid thermal pressurization of the pore fluid within the clay gouge. The lack of fracture energy can explain how a large rupture, propagating from depth, might not be arrested by clay-rich, velocity-strengthening sediments, as is commonly seen. The results suggest that generally, earthquakes may be difficult to nucleate on mature faults dominated by clay, but the propagation of earthquakes through these zones is energetically very favourable. Citation: Faulkner, D. R., T. M.

[1] High-velocity friction experiments on a fault gouge collected from the Nojima fault activated during the 1995 Kobe earthquake showed that the friction coefficient decreased from 0.63 to 0.18 over a slip weakening distance, D c , at high slip rates of $ 1 m/s. The dramatic drop in friction coefficient of more than 0.3 is consistent with that for the Kobe earthquake estimated from seismological observations. Experimentally determined D c becomes 5 m at a higher normal stress of 1.85 MPa, close to the order of magnitude of seismologically determined D c of 0.5 to 1 m. The difference in D c is not significant because the fracture energy consumed during frictional slip is the same order of 10 6 N/m for both cases. Here we show that frictional behavior of a fault during an earthquake can be predicted by conducting high-velocity friction experiments. Citation: Mizoguchi, K., T. Hirose, T. Shimamoto, and E. Fukuyama (2007), Reconstruction of seismic faulting by high-velocity friction experiments: An

[1] High-velocity friction tests were conducted on solid and hollow cylinders of Carrara (calcite) marble, dolomite marble, silicate-bearing calcite marble, and calcite gouge to investigate the strength of carbonate faults during seismic slip. The experiments, performed at normal stresses of 0.6-14.7 MPa, slip rates of 0.03-1.60 m/s, and room temperature in a rotary-shear friction testing machine, yielded an extraordinarily low steady state friction coefficient (<0.1) at slip rates of $1.1-1.2 m/s. The slip-weakening distance of 4-28 m became shorter at higher normal stress or frictional work rate. Strong velocity weakening was observed not only in steady state but also in nonsteady state friction, while the slip rate was changing; thus slip deceleration was accompanied by fault strength recovery. Large, rapid temperature rises in narrow shear localization zones (less than a few micrometers) induced carbonate decomposition, such as the breakdown of calcite into aggregates of CaO nanograins and CO 2 in Carrara marble. Scanning electron microscope observation revealed that the shear localization zone in the highly porous decomposition product was a layer of scattered small grains (mostly <1 mm in diameter). These microstructures and the measured high permeability ($10 À14 m 2 ) of the decomposed marble indicate that the dominant weakening mechanism in our experiments was possibly powder lubrication. Powder rheology at high slip rates is not yet well understood, but the frictional behavior of nanograins appears to be strongly velocity dependent. If decarbonation occurs during seismic slip in natural carbonate faults, powder lubrication may make the faults slippery even under fluid-drained conditions. Citation: Han, R., T. Hirose, and T. Shimamoto (2010), Strong velocity weakening and powder lubrication of simulated carbonate faults at seismic slip rates,

[1] Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has B071031 of 25 produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45°rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises ∼70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.