Abstract
Abstract. This work is devoted to the study of both earthquakes and background seismic noise at Ischia Island (Italy) recorded pre and post the Md 4.0 earthquake occurred on 21 August 2017 (18:57 UTC). We compare and characterize noise and earthquakes in terms of Independent Component Analysis, energy and polarization properties. The earthquakes' waveforms and the background noise are decomposed into a few independent components with two main common signals peaked around 1–2 and 3–4 Hz, respectively. A slight increase of the energy of the background seismic noise is observed comparing samples recorded in 2016 and 2017, whereas no variations are detected in 2017 pre and post the main earthquake. The polarization analysis, performed in the frequency bands individuated by Independent Component Analysis and applied to the background seismic noise, indicates a shallow propagation and the azimuthal pattern is mainly controlled by the local structural features. These results suggest that noise and earthquakes are ascribable to a common phenomenon of fluid-solid interaction in the hydrothermal system of Ischia Island.
Highlights
The interaction between fluid flow and solid rock of the shallow plumbing system of a volcano is one of the most common mechanisms that drives the dynamics of the volcanic areas
The coupling between a fluid phase and the vibrations of the solid matrix generates those seismic signals typically observed on volcanoes and, at larger time scale, it may induce synchronization mechanisms between seismic signals and solid earth tides (De Lauro et al, 2012, 2013, 2018; Falanga et al, 2019)
Starting from the seminal work of Aki et al (1977) who proposed a fluid-filled crack as resonating source of the volcanic tremor observed at Kilauea, several physical models based on the fluid-rock interaction have been proposed to interpret the seismic signals in volcanic areas
Summary
The interaction between fluid flow and solid rock of the shallow plumbing system of a volcano is one of the most common mechanisms that drives the dynamics of the volcanic areas. Depending on the pressurization state, common source processes can generate different styles of activity, from continuous and sustained low-energy tremor up to higher magnitude seismic events (Chouet and Matoza, 2013; Fazio et al, 2017). This interplay between pressure-induced changes in the fluid circulations and variations of the seismic background patterns, represents one of the most useful observables in tracking the evolution of volcanic systems (Madonia et al, 2008; Chiodini et al, 2017; Petrosino et al, 2018, 2020). We used the signals recorded by the digital broadband seismic station IOCA (Fig. 1a), equipped with a velocimeter and an accelerometer, which belongs to the Permanent Seismic Network (PSN) managed by Istituto Nazionale di Geofisica e Vulcanologia – Osservatorio Vesuviano
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