GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 1.2 109 RELATIVE EARTHQUAKE LOCATION AND PRELIMINARY SEISMOLOGICAL ANALYSES ON LOW-ENERGY VOLCANIC SEISMICITY AT CAMPI FLEGREI S. Danesi 1 , N.A. Pino 2 , S. Carlino 2 , P. Ricciolino 2 , D. Delle Donne 2 , C. Martino 2 1 INGV Sezione di Bologna, Italy 2 INGV Osservatorio Vesuviano, Napoli, Italy Introduction. The analysis of the distribution and occurrence of seismic activity in a volcanic environment can provide useful constraints for the study of the evolution of active magmatic and hydrothermal processes. The present work intends to contribute to the comprehension of the Campi Flegrei caldera (CFc; Naples, Italy) unrest, through (i) the relative relocation of the diffuse seismicity recorded during the 1982-84 unrest and of the seismicity that occurred after 2005, ​when a new and ongoing unrest started again (ii) the seismological re-analysis of a few significant recent events. The CFc is one of the most monitored volcanic area in the world, with a multi-parametric network of observing stations operating in the area (Bianco et al. , 2022). The key structure of Campi Flegrei caldera is composed by a 1-3 km depth high temperature hydrothermal system with an almost impermeable zone and its base. The temperature gradients measured in the deep borehole, down to a depth of about 3km, are > 150°C/km; a zone of pressurized gas and intrusion of sills at 3-4 km; a long-term magma reservoir at 7-9 km of depth that is persistently supplying CO 2 to the surface ( e.g. , fumaroles at Solfatara-Pisciarelli) (Zollo et al. , 2008; Carlino et al. , 2012). An uncertainty still exists in assessing how any of the mentioned components of the magmatic system have contributed to the sequence of unrest starting from 1950. Tramelli et al. (2021) used the Osservatorio Vesuviano (OV) catalogue of absolute seismic locations to infer statistical correlations between the seismic time series and independent geochemical/geophysical observations for the main phases of unrest at CF, in the time span between 2000 and 2020. They remarked a correlation between the cumulative number of events, the observed ground vertical deformation, and the increment of the CO/CO 2 ratio in the hydrothermal system. It has been proved that fluid pressurization and heating at the CFc hydrothermal system level is the main forcing of the seismicity at 0.5-1 km depth (Chiodini et al. , 2021). The discussion on the interaction mechanisms between the rising of fluids from deeper volumes and the seismicity below the hydrothermal system (2-8 km depth) is still open (Buono et al. , 2022; Calò and Tramelli, 2018). In this work we use the arrival times of seismic events from the OV database to perform a double-difference relative relocation of earthquakes for the main episodes of unrest in the 1982-84 years and after 2005. The resulting distribution of events suggests constraints to the preferential directions of rising fluids or, vice versa, structural barriers that can be interpreted jointly in the light of previous works and available tomographic models (De Siena et al. , 2017; Calò and Tramelli, 2018, and references therein). Moreover, we present some preliminary results for the recent seismic events of magnitude Md≥3, and provide an estimate of some significant seismological parameters to add information about the seismic sources and the local surface deformation processes. Methods and Results. We analyze P-wave and S-wave arrival times 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). After the pre-processing, 8142 and 5037 linked events were selected in the years 1982- 84 and 2005-2022, respectively. For both datasets, we defined a loose inversion scheme and used a 12-layer, one-dimensional velocity model derived from the P-wave velocity structure for the CFc proposed by Vanorio et al. (2005), with Vp/Vs ratio fixed at 1.8. After the hypoDD