  The acoustic emission activity associated with recent rock fracture
experiments under different conditions has indicated that some features of
event-event triggering are independent of the details of the experiment and the
materials used and are often even indistinguishable from tectonic earthquakes.
While the event-event triggering rates or aftershock rates behave pretty much
identical for all rock fracture experiments at short times, this is not the
case for later times. Here, we discuss how these differences can be a
consequence of the aftershock identification method used and show that the true
aftershock rates might have two distinct regimes. Specifically, tests on a
modified Epidemic Type Aftershock Sequence model show that the model rates
cannot be correctly inferred at late times based on temporal information only
if the activity rates or the branching ratio are high. We also discuss both the
effect of the two distinct regimes in the aftershock rates and the effect of
the background rate on the inter-event time distribution. Our findings should
be applicable for inferring event-event triggering rates for many other types
of triggering and branching processes as well.
