GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 3.2 GNGTS 2024 The audio and electrical monitoring of the Mefte area in ELF band G. Cianchini 1 , C. Fidani 1,2 , A. Piscini 1 , M. Soldani 1 , A. De Sants 1 , L. Perrone 1 , M. Orlando 1 , D. Sabbagh 1 1 Isttuto Nazionale di Geofsica e Vulcanologia, Roma, Italy 2 Central Italy Electromagnetc Network, Fermo, Italy A mult-parameter staton came into operaton last year near the Mefte site, a CO 2 -rich vent located in Irpinia, province of Avellino, Campania Region. The aim of studying fuid migraton in relaton to tectonic actvity at this partcular site falls within the scope of the FURTHER (the role of FlUids in the pReparaTory pHase of EaRthquakes in Southern Apennines) project. In this staton there are two magnetometers for contnuous monitoring, two electrodes for electrical variatons, a microphone, and occasionally other instruments for limited intervals of tme. The Very Low Frequency band of the electric feld sampled at Mefte in relaton to moderate and strong seismic events in the Mediterranean region was previously reported (Cianchini et al., 2023). The Extremely Low Frequency band concerns both the electric and acoustc felds. Electric feld in this band are monitored by an instrument of the Central Italy Electromagnetc Network, and near the earth's surface contains informaton about several phenomena both far and close to the electrode sensors (Fidani, 2011). They are natural and anthropogenic phenomena. Anthropogenic signatures of electric phenomena are regular shapes on spectrograms. For example, the horizontal lines of Figure 1 top represent the everywhere emission by the electrical power distributon network. The main frequency of this emission occurs at 50 Hz and a series of harmonics at multple frequencies. Such anthropogenic emissions occur around the electrodes up to hundreds of m. Instead, the horizontal bands on the black background barely appearing in grey and blue of the spectrum under 50 Hz, are a natural phenomenon known as Schumann Resonances. These are eigenfrequencies of the cavity between the earth's surface and the ionosphere that behave like conductors. They are global phenomena of radiaton sustained by lightning actvity and selected by interference at 8, 14, 19, 25, 31 Hz, and more, that are litle afected by small and local causes within 100 km. Spectrograms shown in Figure 1 are 147-minute records in the evening of December 22, 2022, with stable meteorological conditons indicated by a low density of vertcal coloured lines, which are more evident at the low part of the spectra. Vertcal lines are electrical discharges occurring in the atmosphere mainly generated by meteorological actvity and partally generated by anthropogenic actvity. Lightning can be detected at hundreds of km, as in this case, up to thousands of km. While anthropogenic switching of utlizers can be recorded up to one km from electrodes. Finally, a very evident local phenomenon is observed in the electrical ELF spectrogram (Top). It was appearing only on the E-W electrode, that is the electrode oriented towards the