4.05 - Friction of Rock at Earthquake Slip Rates

Abstract

At seismic slip rates a variety of weakening processes may occur that do not operate at the slow slip rates at which most laboratory rock friction data are acquired. These weakening mechanisms may cause substantial reductions of the coefficient of friction from its typical experimental value of 0.6. These mechanisms include dynamic normal stress reduction or loss of contact due to normal interface vibrations, dynamic normal stress reduction from elastic or permeability mismatch, acoustic fluidization, elastohydrodynamic lubrication, thermal pressurization of pore fluids, local ‘flash’ weakening/melting at asperity contacts, interfacial lubrication by frictional melt, and interfacial lubrication by thixotropic silica gel. Although these mechanisms are understood to varying degrees, based on theoretical and/or experimental studies, much more remains to be understood about all of them. It is not known whether many of these potential weakening mechanisms operate during earthquakes, and some are more likely to be important than others, but it would be surprising if weakening due to one or more of them were not important. If so, the effective dynamic friction coefficient, the shear stress divided by the normal stress, may reach very low values, ranging from essentially 0 to 0.2 during coseismic slip. This would have many important implications for earthquake mechanics, including the magnitude of earthquake stress drops and a greater tendency for ruptures to propagate as self-healing slip pulses, as well as for the magnitude of strong ground motions and the magnitudes and orientations of stress in the crust.