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 Here we review a large set of published and unpublished experiments (similar to 300) performed in rotary shear apparatus at slip rates of 0.1-2.6 ms(-1).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

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] 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,