Fault Heating and Lubrication During Earthquakes: Experimental Constraints

The determination of fault strength (rock friction sensu latu) at seismic slip rates (about 1 m/s), is of paramount importance in earthquake mechanics, as fault strength controls rupture properties, stress drop, radiated energy and heat produced during slip. Given the lack of determination through seismological methods, elucidating constraints arise from experimental studies. Here we show that a review of the experiments (~400) performed in rotary shear apparatuses at slip rates of 0.1 - 1.3 m/s indicate a significant decrease in friction (up to one order of magnitude) for cohesive (silicate-, quartz- and carbonate- built) and non-cohesive (clay-rich and dolomite gouges) rocks. Low friction is concurrent to an increase in temperature in the slipping zone which triggers thermally-activated physico-chemical processes responsible for fault lubrication (decarbonation and dehydration reactions, flash heating, melt lubrication, etc.). Extrapolation of experimental data to natural conditions, suggests large coseismic stress drops (> 70 MPa) at earthquake nucleation depths (7 - 10 km), irrespective of fault rock composition and of the specific weakening mechanism involved. Such large stress drop estimates are consistent with dynamic stress drops obtained from seismic inversion data and geological studies.