Variations in rupture process with recurrence interval in a repeated small earthquake

IN theory and in laboratory experiments, friction on sliding surfaces such as rock, glass and metal increases with time since the previous episode of slip¹. This time dependence is a central pillar of the friction laws widely used to model earthquake phenomena^2,3. On natural faults, other properties, such as rupture velocity^4,5, porosity and fluid pressure^6-11, may also vary with the recurrence interval. Eighteen repetitions of the same small earthquake, separated by intervals ranging from a few days to several years, allow us to test these laboratory predictions in situ. The events with the longest time since the previous earthquake tend to have about 15% larger seismic moment than those with the shortest intervals, although this trend is weak. In addition, the rupture durations of the events with the longest recurrence intervals are more than a factor of two shorter than for the events with the shortest intervals. Both decreased duration and increased friction are consistent with progressive fault healing during the time of stationary contact.