The figures and movie in this electronic supplement show the variation of seismic noise amplitudes depending on type of sensor (Fig. S1), time of day (Fig. S2), and day of week (Fig. S3). Figure S4 shows the temporal evolution of seismic noise amplitudes for different periods of ground acceleration. Figure S5 and Animation S1 present a comparison between oceanic and atmospheric parameters and the seismic noise amplitudes recorded on the West Iberia Lithosphere and Astenosphere Structure (WILAS) project’s network.
Figure S1. Median noise curves divided according to instrument type: (a) STS2 120 s, (b) CMG-3T 120 s, (c) CMG-3ESP 120 s, (d) CMG-3ESP 60 s, and (e) CMG-40T 30 s. Gray thin lines show noise levels at individual stations. Colored thick lines show the median noise levels of all stations equipped with a given sensor. The thick black line is the median noise curve of all stations in the network. Only east–west recordings are considered in this plot. The curves for the north–south noise levels are very similar to those shown in this plot. The colored vertical lines show the cutoff frequency of each sensor. Instrument type does not significantly affect seismic noise amplitudes up to 10 s. Only CMG-40T (30 s) presents higher noise amplitudes at short periods. Long-period noise amplitudes are strongly dependent on instrument type.
Figure S2. Difference between day (1 p.m. to 5 p.m.) and night (1 a.m. and 5 a.m.) noise levels for stations in mainland Portugal in the short-period (SP) band (0.3 s): (a) east–west (EW), (b) north–south (NS), and (c) vertical (Z). The difference between day and night noise levels goes up to ~18 dB in one extreme case (PW18), is on the order of ~8 to ~10 dB at most stations that are strongly affected by this variability, and is less than ~2 dB at the stations less affected.
Figure S3. (a) Spectrograms of ambient noise recorded at PW15-E, a temporary station deployed near an urban region. (b) Power spectral densities (PSDs) recorded at the same site. Weekends appear as periods of lower SP noise.
Figure S4. Smoothed variations of vertical PSDs recorded between 1 January 2010 and 31 December 2012 at the ground-motion periods of (a) 0.3 s, (b) 4 s, (c) 7 s, (d) 17 s, (e) 33 s, and (f) 100 s. Thin lines show smoothed PSDs at given periods for individual stations. Thick lines at the top of the plots show the stack of smoothed PSDs of all stations. The strongest seasonal signature is observed at 7 s. Note that noise amplitudes recorded across the network are very coherent at the microseismic periods of 4 s, 7 s, and 17 s. The limited operation period of temporary stations (upper traces) is evident.
Figure S5. Relation between atmospheric and oceanic variables and seismic noise amplitudes. (a) The gray lines (top) show variation of the mean wave period (MWP) and sea wave height (SWH) at a chosen reference location on the edge of the continental platform offshore of Portugal (10° W and 40° N, red stars in Figure 11 of the main article), extracted from European Center for Medium Range Weather Forecasting (ECMWF) reanalysis. The colored lines (bottom) show ambient noise amplitudes in the microseismic band: ground acceleration periods of 17 s (red), 7 s (yellow), and 4 s (cyan). Noise amplitudes are shown for all three components of ground motion (EW, NS and Z, from top to bottom). The thick lines are obtained after smoothing the raw time series, which are shown by semitransparent lines behind thick lines. Although amplitude variations are different for different microseismic periods, most noise peaks appear simultaneously at more than one microseismic period. They often coincide with increased SWH and MWP offshore of Portugal. (b) The gray lines (top) show variation of atmospheric pressure and wind at the same reference location according to ECMWF reanalysis. The blue lines (bottom) show a stack of 100-s ambient noise amplitudes (EW, NS and Z, from top to bottom). Many, but not all, peaks of 100-s seismic noise find a correspondence in the atmospheric pressure and/or wind time series. Watch Animation S1 for a better perception of the temporal evolution of parameters.
Animation S1 [h.264-encoded MP4; 142.7 MB]. Evolution of oceanic and atmospheric variables and of seismic noise amplitudes recorded at different ground-motion periods and components between 2010 and 2012. (Upper left) Worldwide distribution of sea wave height (SWH) and (lower left) mean wave period (MWP) according to ECMWF reanalysis. The red star marks a reference location on the edge of the continental platform offshore Portugal, at 10° W and 40° N. (Right) The atmospheric and oceanic variables for the chosen reference location (gray lines, top) and stacked seismic noise amplitudes (colored lines, bottom). Atmospheric pressure, wind intensity, MWP, and SWH (gray lines, top) were all obtained from ECMWF reanalysis. Seismic noise amplitudes at the ground-motion periods of 100 s (blue), 33 s (green), 17 s (red), 7 s (yellow), 4 s (cyan), and 0.3 s (pink) are smoothed stacks of ambient noise recorded across the network. The vertical red line marks the date and time of each frame shown in the animations on the left. Strong microseismic peaks can be observed at all periods, from 0.3 to 100 s.
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