Intermittent lab earthquakes in dynamically weakening fault gouge.

Large and destructive earthquakes on mature faults in Earth's crust occur as slip in a layer of a fine granular material-fault gouge-produced by comminution during sliding 1,2 . A range of insights into the frictional resistance of faults-one of the main factors controlling earthquake nucleation, dynamic propagation and arrest, and hence the destructive ground shaking of earthquakes 2,3 -has been obtained in experiments with spatially uniform slip imposed in small samples 2,4-21 . However, how various features of gouge friction combine to determine spontaneous progression of earthquakes is difficult to study in the lab owing to substantial challenges with sample sizes and adequate imaging 22 . Here, using lab experiments, we show that spontaneously propagating dynamic ruptures navigate a fault region with fine rock gouge through complex, intermittent slip processes with dramatic friction evolution. These include repeated arrest of rupture propagation caused by friction strengthening at lower slip rates and dynamic earthquake re-nucleation enabled by pronounced rapid friction weakening at higher slip rates consistent with flash heating 8,12,23 . The spontaneous repeated weakening and strengthening of friction in fine rock gouge highlights the fundamental dependence of friction on slip rate and associated processes, such as shear heating, localization and delocalization of shear, and dilation and compaction of the shear layer 6,7,9-21 . Our findings expand experimental support 9,11 of the concept that co-seismic weakening may enable earthquake rupture to break through stable fault regions 24,25 , with substantial implications for seismic hazard.