Abstract
In this paper, we analyse the seismic noise at Ischia Island (Italy) with the objective of detecting the hydrothermal source signals taking advantage of the Covid-19 quiescence due to lockdown (strong reduction of anthropogenic noise). We compare the characteristics of the background noise in pre-, during and post-lockdown in terms of spectral content, energy release (RMS) and statistical moments. The continuous noise is decomposed into two independent signals in the 1−2 Hz and 2−4 Hz frequency bands, becoming sharpened around 1 Hz and 3 Hz respectively in lockdown. We propose a conceptual model according to which a dendritic system of fluid-permeated fractures plays as neighbour closed organ pipes, for which the fundamental mode provides the persistent whisper and the first higher mode is activated in concomitance with energy increases. By assuming reasonable values for the sound speed in low vapor–liquid mass fraction for a two-phase fluid and considering temperatures and pressures of the shallow aquifer fed by sea, meteoric and deep hydrothermal fluids, we estimate pipe lengths in the range 200–300 m. In this scheme, Ischia organ-like system can play both continuous whisper and transients, depending on the energy variations sourced by pressure fluctuations in the hydrothermal fluids.
Highlights
In this paper, we analyse the seismic noise at Ischia Island (Italy) with the objective of detecting the hydrothermal source signals taking advantage of the Covid-19 quiescence due to lockdown
We studied the seismic noise acquired at Ischia (Italy), an island belonging to the Phlegrean Volcanic District, Southern Italy[12], which normally is a travel destination for many Italians and foreign tourists, attracted by its wonderful natural landscape and therapeutic thermal springs
Following the results of Cusano et al.[13], the first band contains the signature of a permanent hydrothermal source, whereas not a definitive indication was ascertained on the nature of the second band because affected by the anthropogenic noise
Summary
We analyse the seismic noise at Ischia Island (Italy) with the objective of detecting the hydrothermal source signals taking advantage of the Covid-19 quiescence due to lockdown (strong reduction of anthropogenic noise). From a seismological point of view, the lockdown period represented a suitable circumstance to tackle the study of the seismic noise from a new perspective; recent researches have shown that the adoption of this restrictive measure caused a global reduction of anthropogenic noise of up to 50%1,4 and a consequent improvement of the detection thresholds of seismic stations[5,6] Such a long time period of quiescence offered the chance to detect faint signals normally buried in the background noise and to separate anthropogenic and natural components in an easier way. The first one is the most energetic, persistent, and well correlated at the seismic stations located in the Casamicciola gmail.com
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