Seismicity rate before eruptions on Piton de la Fournaise volcano: Implications for eruption dynamics

Individual eruptions are difficult to foresee using daily seismicity at Piton de la Fournaise (PdlF) volcano, except within a few hours from the eruption time. However, when using a superposed epoch analysis for all of the 15 eruptions in the 1988-2001 period, we find that the seismicity rate increases 2 weeks before the eruption time as (N)overdot(t) similar to (t(e) - t)(-a), t(e) being the eruption time. Such an increase mimics the pattern of stacked foreshock sequences, i.e., a power law increase in seismicity rate before earthquakes that is proposed to emerge from aftershocks properties [Helmstetter et al. 2003]. It argues for the bursts of seismicity rate before eruptions to both result from the volcano dynamics ( volume or pressure increase induced by magma) and from the cascades of earthquake interactions. This latter process produces stochastic fluctuations in the seismicity rate that perturb any single pre-eruption time series. The PdlF seismicity highlights a brittle pre-eruption damage process which starts at least 2 weeks prior to eruption time. Because this increase is only measured upon averaging, it suggests that during the brittle damage phase the direct effect of volcano dynamics on the seismicity rate remains on the same order of magnitude as the seismicity rate triggered by volcano earthquake interactions.