GEOPHYSICAL RESEARCH LETTERS

Transcription

1 GEOPHYSICAL RESEARCH LETTERS Supporting Information for Observation of deep water microseisms in the North Atlantic Ocean using tide modulations Éric Beucler, 1 Antoine Mocquet, 1 Martin Schimmel, 2 Sébastien Chevrot, 3 Olivier Quillard, 1 Jérôme Vergne, 4 Matthieu Sylvander 3 1 Laboratoire de Planétologie et Géodynamique, CNRS UMR 6112, Université de Nantes, France 2 Department of Structure and Dynamics of the Earth, Institute of Earth Sciences Jaume Almera, Consejo Superior de Investigaciones Científicas, Barcelona, Spain 3 Institut de Recherche en Astrophysique et Planétologie, CNRS UMR 5277, Toulouse, France 4 EOST, CNRS UMR 7516, Strasbourg, France

2 X - 2 Contents of this file 1. Figures S1 to S7. Introduction This file contains 7 additionnal figures discussed in the article. The data used are freely available at (for the seismic data), and (for the buoy data). All figures are produced using GMT [Wessel et al., 2013]. Fig. S1 and Fig. S2 are the same figures as Fig.2, shown in the main text, but for horizontal components. Fig. S3 shows the swell spectra at the four buoys, three of them (BI, GA and PN) are added to the BY buoy discussed in the main text. Histograms of swell periods for the whole year 2012 are realized with bins of 0.25 s, they enhance the behaviour difference of oceanic swells between coastal buoys (BI and PN) and the deep ocean (BY and GA). Fig. S4 explains how the modulation curves shown in the main text (Fig.3) are computed and shows the strong effects of ocean tide at different frequency except at 0.33 Hz. Fig. S5 shows that, even with normalized modulation curves, the lack of energy modulation between 2.5 and 5 s (observed in the main text Fig.3) is remarkable. For each curve, the normalization coefficient applied is given in the caption. Fig. S6 is realized using 225 days of continuous signal. Only short duration PSD in the 90 length time windows, centered around the official high and low tide times, are used

3 X - 3 to compute Probability Density Functions, for each component. They are not displayed here for the sake of clarity. Only the differences between the two PDF are displayed, they show that, at PY41, the ocean tides largely increase the seismic energy at high tides for the whole frequency range except between 2.5 and 5 s. Fig. S7 outlines the similarity between the vertical seismic energies recorded near the coast (PY41) and inland (ECH), at 3.33 s period during 115 days. The swell variations at BY buoy is added and shows a large simultaneity between oceanic events (such as storms) and the increases of seismic energies. They are computed by extracting the PSD variations at 3.33 s, as shown in Fig. S4, and by removing the trends. References Peterson, J. (1993), Observations and modelling of seismic background noise, US Geological Survey, open-file report, Wessel, P., W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe (2013), Generic mapping tools: Improved version released, Eos, Transactions American Geophysical Union, 94 (45), , doi: /2013eo

4 X-4 Figure S1. Horizontal EW short-duration power spectrograms during 17 days in April Periods and frequencies are plotted on the horizontal axes (bottom and top, respectively). PSD are computed between 0.2 and 70 s period. No data selection has been performed to remove earthquakes. The local tidal ranges at PY41 and PY48 are represented by magenta curves on the right. The wave heights at Brittany (BY) and K4 buoys (Fig. 1) are plotted in brown and green, respectively. The dashed lines highlight some noticeable wave events discussed in the text. January 15, 2015, 8:38pm

5 X-5 Figure S2. Horizontal NS short-duration power spectrograms (same legend as Fig. S1). January 15, 2015, 8:38pm

6 X-6 Figure S3. Swell spectra at four buoys in the North-East Atlantic ocean. BY and GA (dark red and orange) are located in deep ocean and PN and BI (dark and light green) are coastal. a, Swell period curves for the whole year of The inserted panel zooms on the 17 days in April detailed in the main text. The time labels correspond to the swell height maxima at BY, shown in Figs. 2, S1 and S2. b, Buoy locations. BY is located at (47.5, -8.5 ). GA is the meteorological Gascogne buoy located at (45.2, -5 ) above approximately 4560 m of water height. PN and BI are Candhis (French National Center for Archiving Swell Measurements) Pierres Noires and Belle-I le buoys, both located above approximately 55 m of water height. c, Histograms of swell period for 2012 realized with bins of 0.25 s. January 15, 2015, 8:38pm

7 X - 7 Figure S 4. Ocean tide harmonic components in PSD time series. a, Four PSD time series of the PY41 vertical power spectrograms (Fig. 2) are extracted at four different periods. For the sake of clarity, we show a quiet week with no earthquake to enhance modulations of the PSD. b, For each time series the amplitude of modulation is plotted in the spectral domain. The annotations at the spectral peaks refer to the most energetic tidal harmonics (inlcuding shallow water components).

8 X - 8 Figure S 5. Normalized energy of Power Spectral Density modulations by the principal semi-diurnal tidal component M 2. Same legend as Fig. 3 but each curve is normalized by its maximum value in the s period band. The normalization coefficients are given in 10 log 10 ((m/s 2 ) 2 /Hz)/Hz : PY41 (8.92 for EW, 8.29 for NS, and 5.79 for Z), PY48 (4.06 for EW, 4.49 for NS, and 3.36 for Z), ECH (0.34 for EW, 0.36 for NS, and 0.6 for Z) and VAL (1.03 for EW, 1.06 for NS, and 0.43 for Z).

9 X - 9 Figure S6. Probability Density Functions of PSD at High and Low tides. For each component of PY41, the PSD are retained only for time windows of 1h30, centered around the official High and Low tide times given by the French Naval Hydrographic and Oceanographic Service (SHOM). The differences between Probability Density Functions (PDF) at High and Low tides are plotted using red-green color scale. The PDF are computed for 225 days of the year 2012 (April, 10th Nov., 23rd) with bins of 3 log(ν), where ν is the frequency. The effect of ocean tides are noticeable over the whole frequency range except in the s period range. One may also notice the characteristic signature of the cultural noise for periods lower than 0.5 s.

10 X - 10 Figure S 7. Comparison of the vertical ECH and PY41 PSD time series and ocean swell variations. Two PSD time series are extracted from the vertical power spectrograms (Fig. 2) at 3.33 s period and during 115 days of 2012 and the trends are removed (mean values are of -141 db and -148 db for PY41 and ECH, respectively). The large similarity between the two curves (normalized correlation coeff. of 0.92) shows that open-sea-generated microseisms can be detected more than 700 km in land. The energy is however attenuated and therefore becomes lower than the LPSM noise level as propagating in the continental crust. The comparison with the swell variations at BY (brown curve) also suggests that Atlantic ocean activity contributes for a large part to the short-period noise recorded at ECH.