Abstract
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 1−2Hz and 3−4Hz, 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 500−600m/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 1−2Hz 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 1−2Hz seismic signal and, eventually, higher modes.
Published Version
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