How is Volcano Seismicity Different from Tectonic Seismicity?

We analyze the temporal patterns of volcano seismicity using the statistics of waiting times between subsequent earthquakes. We compare waiting time distributions of seismicity at Mt. Etna and Mt. Vesuvius volcanoes during (1) intereruption phases and (2) dyke propagations, with those of tectonic seismicity using the southern California catalog. For inter-eruption phases, no matter their duration, statistics of inter-event times are well approximated by the gamma distribution. This allows us to compute the proportion of background uncorrelated events (Molchan, 2005; Hainzl et al., 2006), which is recovered in the range 20%-40% for Vesuvius, three Etna inter-eruptive periods, and the southern California catalog. It argues for roughly 70% of earthquake activity to be cascades of aftershocks for both volcano inter-eruptive and tectonic seismicity. On the contrary, statistics of inter-event times recorded during both the 2001 and 2002 intrusive episodes at the Etna volcano reject the gamma distribution to describe the observations. These seismic crises are characterized by an average seismicity rate about 2 orders of magnitude larger than that of inter-eruptive periods. It suggests that the origin of the specificity of waiting time patterns during dyke injections is driven by the external forcing rate. Using the epydemic type aftershock sequences model simulations we explore the effect of seismicity rate increases on inter-event time distributions. Departures from the gamma law progressively emerges from both (1) an increase of the background seismicity rate and (2) a screening effect. It prevents us from quantifying the portion of uncorrelated seismicity within the considered catalog and from clearly quantifying the forcing rate that characterizes the volcano dynamics during dyke intrusions.