IN theory and in laboratory experiments, friction on sliding surfaces
such as rock, glass and metal increases with time since the previous e
pisode of slip1. This time dependence is a central pillar of the frict
ion laws widely used to model earthquake phenomena2,3. On natural faul
ts, other properties, such as rupture velocity4,5, porosity and fluid
pressure6-11, may also vary with the recurrence interval. Eighteen rep
etitions of the same small earthquake, separated by intervals ranging
from a few days to several years, allow us to test these laboratory pr
edictions in situ. The events with the longest time since the previous
earthquake tend to have about 15% larger seismic moment than those wi
th the shortest intervals, although this trend is weak. In addition, t
he rupture durations of the events with the longest recurrence interva
ls 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 stationa
ry contact.