Characterization of the seismic dynamical state through joint analysis of earthquakes and seismic noise: the example of Ischia Volcanic Island (Italy)

摘要

This work is devoted to the study of both earthquakes and background seismicnoise at Ischia Island (Italy) recorded pre and post the M _(d) ?4.0 earthquakeoccurred on 21?August?2017 (18:57 UTC). We compare and characterize noiseand earthquakes in terms of Independent Component Analysis, energy andpolarization properties. The earthquakes' waveforms and the background noiseare decomposed into a few independent components with two main commonsignals peaked around 1–2 and 3–4 Hz , respectively. A slight increase ofthe energy of the background seismic noise is observed comparing samplesrecorded in 2016 and 2017, whereas no variations are detected in 2017 preand post the main earthquake. The polarization analysis, performed in thefrequency bands individuated by Independent Component Analysis and appliedto the background seismic noise, indicates a shallow propagation and theazimuthal pattern is mainly controlled by the local structural features.These results suggest that noise and earthquakes are ascribable to a commonphenomenon of fluid-solid interaction in the hydrothermal system of IschiaIsland.