Significance of the 1982-2014 Campi Flegrei seismicity: Preexisting structures, hydrothermal processes, and hazard assessment

We find that the double-difference relocated seismicity, which occurred over the last 30years at Campi Flegrei, was triggered by the uprising of fluids preferentially concentrated along onshore and offshore NW striking preexisting caldera faults. Focal volumes of the 2005-2014 seismicity do not overlap that of the 1982-1984 period, when a major uplift of 1.8m occurred in the central sector of the caldera. This indicates a transition from an elastic to a plastic behavior due to fluid saturation and heating of the rocks in the hydrothermal reservoir. The 2012-2014 deeper earthquakes are located in a low V-P/V-S zone at the western boundary of the hydrothermal reservoir, where a volume increase from a magmatic body at 3.5km depth has been recognized. The progressive rheological change from elastic to plastic in the upper 4km of the crust implies that a slow upward migration of magma may not necessarily be preceded by earthquakes or swarms.