GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.2 GNGTS 2024 Seismic monitoring of gas emissions at mud volcanoes: the case of Nirano (northern Italy) N. Carfagna 1 , A. Brindisi 1 , E. Paolucci 2 , D. Albarello 1,3 1 Department of Physical Sciences, Earth and Environment, University of Siena, Siena, Italy 2 Department of Physics and Astronomy, University of Bologna, bologna, Italy 3 Consiglio Nazionale delle Ricerche, Isttuto di Geologia Ambientale e Geoingegneria, Rome, Italy The Salse di Nirano site is a Natural Reserve in Italy distnguished by the presence of mud volcanic phenomena that have manifested in the formaton of four main volcanic vents situated within its central part close to pools flled by low viscosity mud. The mud volcanoes of Nirano are relatvely small, reaching a maximum of three meters height and exhibit minimal volcanic actvity with weak, persistent surface degassing. Seismic monitoring was conducted at the Nirano Mud Volcanoes during two distnct feld campaigns in April 2021 and July 2023, with the aim of exploring the subsurface structure and the dynamics of the volcano. The seismic survey campaigns involved deploying small 'L-shape' seismic arrays composed of vertcal geophones and three-directonal sensors. Both actve and passive seismic acquisiton methods were also employed to characterize the frst subsoil, focusing on shear wave velocity profles. During the recordings, a peculiar seismic patern emerged, characterized by sequences of short impulsive events (0.1-0.2s) occurring at regular and irregular tme intervals. These transient events, commonly referred to as 'drumbeats' by various authors ( Iverson et al., 2006 ; Lupi et al., 2016 ), exhibit a spectral structure dominated by frequencies ranging between 10 and 30 Hz and by the presence of subsequent sharp peaks, for which a realistc explanaton has been provided in this study. A rapid automatc locaton procedure was deployed to identfy seismic sources corresponding to these impulsive events, assuming a straight ray path from source to receiver. Approximately 5000 sources were identfed over the 7-hour recording period (seven asynchronous arrays recording for one hour each) within a search area of few hundred meters. Most of the sources have been localized at shallow depths (<10m) with a minor contributon of deeper events (within few tens of meters from the surface). The propagaton velocites derived from the locaton procedure, coupled with the results of the conducted polarizaton analysis, suggest shear waves are responsible for most of the energy transmited by these seismic signals.