Coseismic fault lubrication by viscous deformation

Despite the hazard posed by earthquakes, we still lack fundamental understanding of the processes that control fault lubrication behind a propagating rupture front and enhance ground acceleration. Laboratory experiments show that fault materials dramatically weaken when sheared at seismic velocities (>0.1 m s−1). Several mechanisms, triggered by shear heating, have been proposed to explain the coseismic weakening of faults, but none of these mechanisms can account for experimental and seismological evidence of weakening. Here we show that, in laboratory experiments, weakening correlates with local temperatures attained during seismic slip in simulated faults for diverse rock-forming minerals. The fault strength evolves according to a simple, material-dependent Arrhenius-type law. Microstructures support this observation by showing the development of a principal slip zone with textures typical of sub-solidus viscous flow. We show evidence that viscous deformation (at either sub- or super-solidus temperatures) is an important, widespread and quantifiable coseismic lubrication process. The operation of these highly effective fault lubrication processes means that more energy is then available for rupture propagation and the radiation of hazardous seismic waves.