GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.2 GNGTS 2023 In this work we use the arrival times of located seismic events to perform a double-difference relative relocation of earthquakes that occurred in 1982-84 years and after 2005. The resulting distribution of hypocenters suggests constraints for the identification of preferential pathways of rising fluids and for the definition of structural barriers, which can be interpreted jointly in light of previous works and available tomographic models (De Siena et al., 2017; Calò e Tramelli, 2018) for the definition of possible scenarios of unrest evolution. Methods and Results We analysed P-wave and S-wave arrival times extracted from the OV database for the two time frames 1982-1984 and 2005-2022 to perform a relative double-difference location with the HypoDD algorithm (Waldhauser and Ellsworth, 2000; Waldhauser, 2001). For the processing, we derived a layered one-dimensional velocity model from the P-wave velocity structure of the CFc proposed by Vanorio et al. (2005), with Vp/Vs ratio fixed at 1.8. We used event-pair differential catalog phase times, calculated from the absolute P and S arrival time data, for all pairs of events separated by less than 5 km. The inversion involved 8142 and 5037 linked events for in the years 1982-84 and 2005-2022, respectively. We obtained relative location for 3062 and 2522 events, respectively for the two time spans. Moreover, we used calibration laws for magnitude scales to infer the moment magnitude Mw estimation from available catalogs of duration magnitude Md (Petrosino et al., 2008) and, following Yokoyama (1988), we estimated the spatial distribution of the cumulative seismic energy released during the two time spans. The seismicity distribution of the 1980s was widely scattered throughout the entire volume of the caldera. EW-oriented vertical sections show that hypocenters were mainly located between 1.5 and 3 km depth across the entire extension of the caldera, with notable concentration below the Solfatara (Fig. 1). In the context of a widespread background seismicity that was affecting the entire hydrothermal volume, the seismic swarm of April 1 (1984) identified the major activation of a structure oriented NW-SE, roughly aligned with La Starza marine terrace, and ∼ 60°-dipping towards SW. Unlike what was observed in the 1980’s, recent seismicity has been almost completely absent west of the Solfatara and in particular below 1 km depth. The relative locations show some near-vertical fracture alignments under the Solfatara bending eastward, and indicate that some of the preferred fluid rising paths through the hydrothermal system originate at depths greater than 2.5 km where major events occur (Md>2) and where an important share of the seismic energy release is concentrated. The observed reactivation of structural patterns below the Solfatara-Pisciarelli area, as in the 1982-84 unrest, also suggests the presence of a weakened crust which could easily allow the upwelling of fluids from the deeper layers of the hydrothermal system.