The whisper of the hydrothermal seismic noise at Ischia Island

This work is devoted to the study of the background seismic noise at Ischia Island. We highlight two persistent independent signals, with dominant frequency peaks in the range 1-2 Hz and 3-4 Hz, respectively. The first signal is the most energetic and persistent and its polarization properties are well defined, revealing a shallow propagation likely controlled by both tectonic/volcanic structures and morphological features. This signal is well correlated at all the stations in the Casamicciola area and its group velocity is estimated as 500-600 m/s. The second signal shows similar polarization properties but the temporal pattern of its amplitude is strongly affected by cultural noise, revealing a modulation on a diurnal scale. On the investigated spatial scales, any clearly correlated signal is not evidenced. We interpret the 1-2 Hz correlated wave-packets as the persistent whisper of the shallow circulation of fluids, which are a mixing of sea and meteoric water and thermal fluids of the hydrothermal reservoir. In the conceptual scheme of the self-sustained musical instruments, we hypothesize that the solid structure of the shallow hydrothermal system of Ischia is formed by a network of channels, continuously excited by the circulating hydrothermal fluids, which produce a whispered sound (i.e. the seismic noise). This mechanism is able to produce persistent self-sustained oscillations that we observe as the 1-2 Hz seismic signal and, eventually, higher modes.