Abstract
The majority of earthquakes are aftershocks, yet aftershock physics
is not well understood. Many studies suggest that static stress changes
trigger aftershocks, but recent work suggests that shaking (dynamic
stresses) may also play a role. Here we measure the decay of aftershocks
as a function of distance from magnitude 2-6 mainshocks in order
to clarify the aftershock triggering process. We find that for short
times after the mainshock, when low background seismicity rates allow
for good aftershock detection, the decay is well fitted by a single
inverse power law over distances of 0.2-50 km. The consistency of
the trend indicates that the same triggering mechanism is working
over the entire range. As static stress changes at the more distant
aftershocks are negligible, this suggests that dynamic stresses may
be triggering all of these aftershocks. We infer that the observed
aftershock density is consistent with the probability of triggering
aftershocks being nearly proportional to seismic wave amplitude.
The data are not fitted well by models that combine static stress
change with the evolution of frictionally locked faults.
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