Seismic velocity variation at Mt. Etna calculated by seismic ambient noise interferometry (EtnadVv)

The raw seismic data is band-pass filtered between 1.0-2.0 Hz, down-sampled from 100 to 20 Hz, and de-trended. Daily traces is segmented into 1-hr windows, winsorizing (clipping) at 3 Root Mean Square (RMS), and spectral whitening in the 1.0–2.0 Hz frequency band is applied before cross-correlation (Bensen et al., 2007; Lecocq et al., 2014). The resulting daily CCFs are then stacked using the time-frequency phase-weighted stacking method (tf-PWS; Schimmel & Paulssen, 1997) for 1,5,10,30 days for each station pair to increase the signal-to-noise-ratio. The reference period spans between 2017-01-01 and 2017-12-31. Daily variations in seismic velocity are computed using the Moving Window Cross-Spectrum analysis (MWCS; Ratdomopurbo & Poupinet, 1995) between a daily and a reference CCF. The resulting time series for each possible pair of stations on Etna volcano are finally combined in a single median variation of seismic velocity for the network.