Sand volcano generated by a violent degassing from methane-saturated aquifers: the case study of Medolla (Modena, Italy).

Sand volcano is an unusual and remarkable geological feature which forms when water-saturated sand deposits are set in motion by liquefaction and are ejected onto the surface. Commonly it is generated during earthquakes, as a result of liquefaction ofwaterlogged bodies at shallow depth, often causing wide damages. The generation of a sand volcano however can also occur unrelated to a seismic event. On the 10th of October 2014, near Medolla (Italy) during a Cone Penetration Test, a large amount of natural gas (CO2 and CH4) together with a mixture of water and sandwere erupted, creating a sand volcano. The study of this event gives the possibility of understanding the dynamics of sand volcano generation, may enable to prevent other anomalies during future CPT tests and, more importantly, underlines the role that natural gas, stored in a sand aquifer,may play in triggering a liquefaction phenomenon. Our results suggest that episodes of gas eruptions require the onset of very peculiar conditions within the reservoir that feeds the emission. The simulations suggest that a geyser discharging a mixture of gas and water, capable of building a sand volcano, requires the presence of a shallowpressurized reservoir (1.2 MPa) where water coexists with a small amount of exsolved gas (a volume fraction of 0.05).The violent degassing occurred in Medolla confirms the role that a free gas phase may have in favoring the mobilization of liquid water and loose deposits, even in the absence of a seismic event.