GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.2 GNGTS 2023 Seismic Observation at Nirano Mud Volcanoes A. Brindisi 1 , N. Carfagna 1 , E. Paolucci 2 , D. Albarello 1,2 1 Dipartimento di Scienze Fisiche, della Terra e dell’Ambiente, Università degli Studi di Siena, Italia. 2 CNR-IGAG, Montelibretti (Roma), Italia. Mud volcanoes are geological structures originated by the rising of overpressured fine sediments at depths of hundreds of meters. Fast depositional process during their genesis is the main reason for these upward phenomena. High sedimentation rate hampers fine sediments to dehydrate before being buried and, because of this, fluids are trapped at depth with consequential creation of isolated geological reservoirs marked by elevated fluid pressure. The rising material phenomena from reservoirs toward the surface, the so-called Sedimentary Diapirism, in the last decades has increasingly interested the scientific community. Mud volcanoes and their activity have been studied as a marker of the presence of hydrocarbon reservoirs, for hazard induced by their rarely explosive events and because of their close connection with the regional seismotectonic activity. Many studies, in the last few years, also tried to find a relationship between mud volcanoes and gases emissions, in particular CO 2 and CH 4 , two of the most important greenhouse gases. Among the Italian mud volcanoes, those of Nirano (MO), have reservoirs located within the permeable Epi-Ligurian Units and capped by low permeability clays (Bonini, 2008). This natural reserve represents a typical example of mud volcanoes field, in which the main structures are three cones lined up along the NE-SW direction, close to small pools with less thick clay materials, called “salse”. The subsurface structure of the Nirano mud volcanic field has been investigated by geoelectrical, gravimetrical and seismic surveys (Lupi et al., 2016). In the present study, dynamic behaviour of Salse di Nirano is focused on aiming at monitoring gas outflow and locating eventual secondary reservoirs at shallow depth. In particular, seismic signals possibly associated with gas outflow are investigated by deploying seismic arrays and three directional velocimetric stations. Outcomes of these measurements show that subsonic seismic emissions of these structures present analogies with those of active volcanoes, possibly due to similar dynamic mechanisms probably associated with gas bubbling. Three kinds of seismic activity have been identified: background ambient vibrations, short periodic energy bursts (drumbeats) and high energy paroxysmic phases. All the observed events, compared to that of active volcanoes, present higher frequencies range.