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http://hdl.handle.net/2122/12889
Authors: | Cusano, Paola* Petrosino, Simona* De Lauro, Enza* Falanga, Mariarosaria* |
Title: | The whisper of the hydrothermal seismic noise at Ischia Island | Journal: | Journal of Volcanology and Geothermal Research | Series/Report no.: | /389 (2020) | Issue Date: | 2020 | DOI: | 10.1016/j.jvolgeores.2019.106693 | Keywords: | Ischia Island Hydrothermal noise ICA |
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 3−4 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 500−600m/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. |
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