Small-magnitude earthquakes triggering fluid vents in a pressurised CO2 system, Caprese Michelangelo (Central Italy)

The CO2-(N2)-rich pressurised Caprese Reservoir (Northern Apennines, central Italy) is a structural system controlled by a subsurface fold anticline. Mud volcano-like structures and cold gas seeps lie along steep, -NE-trending faults, which are intimately connected to the deep fluid reservoir. In August 2010, five vents erupted mud flows after a local, small magnitude seismic sequence (MLmax3.2). Although the exact dates of such paroxysmal episodes are unknown, they occurred within 55 days after the main shocks. These eruptions are evidence of triggered responses of mud volcano-like vents induced by earthquakes with small magnitude (ML3.0-3.2). Static stresses at the erupted vents are negligible, thereby dynamic strain is inferred to be the sole triggering mechanism. The main earthquakes produced a near-vent peak ground velocity (PGV) of-0.5 cm s-1 (ML3.2) and -0.2 cm s-1 (ML3.0). Although small, they are similar to other PGV values that triggered eruptive events in similar systems worldwide. A potential triggering mechanism can be searched in the enhanced permeability of fluid pathways produced by the passage of seismic waves. Fluid pressurization within the antiformal-shaped Caprese Reservoir by seismic wave focusing can also have played a significant role. This implies that seismic waves carrying even small perturbing stresses have the ability to stimulate hydrogeological systems to produce eruptions. Furthermore, our investigations allowed to assess the relationship between dis-tance and earthquake magnitude for an M -3 event, and corroborate the validity of some existing empirical triggering thresholds of mud volcano-like systems even in the case of small earthquake magnitudes.