The whisper of the hydrothermal seismic noise at Ischia Island.

• A study of the seismic noise at Ischia Island shows two persistent sources. • The principal source is related to the hydrothermal activity. • The fluid-solid interaction mechanism is modelled using organ-pipe theory. 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 anthropogenic 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. [ABSTRACT FROM AUTHOR]