The ”Salse di Nirano” mud volcanoes: hints from gravity data

Mud volcanoes are distributed throughout the globe, both on- and offshore. Mud volcanism has been widely investigated from the geological, geophysical, and geochemical points of view. The study of mud volcanoes has important implications in energy resource exploration, geohazard identification, and greenhouse gas emissions assessment (mainly CH₄ and CO₂). Mud volcano eruptions are mainly driven by a gravitative instabilities and fluid overpressure, due to the overall low density of clay/water/gas mixtures with respect to surrounding units. The geohazard of mud volcanoes is to date underrated despite the violent eruptive examples occurred in the past. For instance, the eruption of the Piparo mud volcano (1997, island of Trinidad) damaged electrical and water infrastructures and killed animals and livestock. In 2014, the eruption of the Macalube di Aragona (Italy) mud volcano killed two children. The understanding of the mechanisms regulating mud volcanoes is, therefore, important also in terms of hazard evaluation. To date, a physical conceptual model of the Nirano Salse, Italy, ascribes the eruptions to the presence of over-pressurized fluids that are expelled from a main deep reservoir. The latter is put into communication with the surface due to the episodically reactivation of pre-existing faults or pipes. The debate about this conceptual model is still open. To improve our current understanding, a new high-resolution dataset of gravimetric data was acquired. Our goal is to provide an insight about the subsurface structure of the investigated domain. The gravimetric inversion aims to identify the structural setting of Nirano and the presence of gas traps and faults. The gravity inversion results indicate the existence of a low-density zone (1200-1500 m long, 100-200 m wide, 800 m deep) with an almost planar shape aligned along a NW-SE structural trend, typical of the Northern Apennines chain. This zone likely represents the intrusion of mud/gas in the damage zone of a sub-vertical fault, which feeds shallow fluid reservoirs.