The effect of loading rate on static friction and the rate of fault healing during the earthquake cycle

The seismic cycle requires that faults strengthen (heal) between earthquakes, and the rate of this healing process plays a key role in determining earthquake stress drop1,2,3,4, rupture characteristics5,6 and seismic scaling relations2,3,4,7. Frictional healing (as evidenced by increasing static friction during quasi-stationary contact between two surfaces1,8,9,10,11,12) is considered the mechanism most likely to be responsible for fault strengthening2,3,13,14. Previous studies, however, have shown a large discrepancy between laboratory and seismic (field) estimates of the healing rate2,3,4,14,15; in the laboratory, rock friction changes by only a few per cent per order-of-magnitude change in slip rate, whereas seismic stress drop increases by a factor of 2 to 5 per order-of-magnitude increase in earthquake recurrence interval. But in such comparisons, it is assumed that healing and static friction are independent of loading rate. Here, I summarize laboratory measurements showing that static friction and healing vary with loading rate and time, as expected from friction theory16,17,18. Applying these results to seismic faulting and accounting for differences in laboratory, seismic and tectonic slip rates, I demonstrate that post-seismic healing is expected to be retardedfor a period of several hundred days following an earthquake, in agreement with recent findings from repeating earthquakes13,14,19,20.