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Supplementary material for
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The 2007 eruptions and caldera collapse of the Piton de la Fournaise
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volcano (La Réunion Island) from tilt analysis at a single very broadband
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seismic station.
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Fabrice R. Fontainea, Geneviève Roultb, Laurent Michona, Guilhem Barruola, Andrea Di
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Murob,c
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a
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Sorbonne Paris Cité, UMR CNRS 7154, Université Paris Diderot, F-97744 Saint Denis, France.
b
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Laboratoire GéoSciences Réunion, Université de La Réunion, Institut de Physique du Globe de Paris,
Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154
CNRS, F-75005 Paris, France.
c
Observatoire Volcanologique du Piton de la Fournaise (OVPF), 14 RN3, 97418 La Plaine des Cafres,
France.
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 Data analysis and method
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Seismic recording
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The acquisition chain has a high dynamic range (140 dB) and a wide frequency band. Two
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recording channels were analyzed in the present work: the raw BH (Broadband and High
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gain) channel (sampling rate 20 Hz) and the VH (Very long period and High gain) channel
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obtained after low-pass filtering (sampling rate 0.1 Hz). We did not use the less sensitive
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OVPF (Observatoire Volcanologique du Piton de la Fournaise) tiltmeters located around the
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summit cone whose long-term subtle signals are masked by intense signals due to magma
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migration below the summit and by high background noise [Peltier et al., 2011]. The higher
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amplitude observed in the N-S component compared to the E-W component at RER station is
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due to its quasi-radial direction to the Dolomieu caldera.
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Tidal corrections
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We used the ETERNA 3.30 software [Wenzel, 1996] and the tidal potential catalogue
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[Hartmann and Wenzel, 1995] to compute the theoretical solid earth tide acceleration
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signature in the vertical component and the tilt signature in the horizontal ones. The
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calculation performed does not take in account ocean tides. The ocean tides are classified as
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microtidal range around La Réunion with spring and neap tidal range of respectively 0.90 and
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0.10 m [e.g. Cordier et al., 2013]. Furthermore, the predicted amplitude of solid earth tide at
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RER
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http://home.comcast.net/~dmilbert/softs/solid.htm) is one order of magnitude higher than
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amplitude expected from the ocean tidal loading effects [Francis and Mazzega, 1990]. In this
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study, we therefore assumed that ocean tide loading is a second order effect compared to the
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solid earth tide contribution.
station
(computed
using
the
code
"solid"
available
at:
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When analyzing the vertical component, the theoretical solid earth tide signal and the
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seismic observations show very good agreement, with both being characterized by similar
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amplitude and phase. Results are more complex for the horizontal components, but the
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general waveforms of the theoretical and observed tides are very similar if we take into
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account several effects:
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i) an azimuth deviation of 7° in our computations, which may result from the combined
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effects of the sensor misorientation (2.3° in our case according to a recent gyrocompass
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measurement), of the Earth surface inclination around RER and of local seismic anisotropy in
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the edifice;
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ii) a short time shift between the theoretical and the observed records (the tidal
component phases have to be retrieved from a very long time series at the RER station);
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iii) an amplification factor applied to the amplitude of the theoretical component: this
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effect may be due to site effects (station – surrounding medium coupling, local velocity
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heterogeneities) that may lead to such discrepancies. Both effects (phase and amplification
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factors) may be explained by the 'cavity effect' [Harrison, 1976; Lambotte et al., 2006]. After
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matching the theoretical tidal and observed signals for periods lacking any volcanic activity,
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we removed the theoretical tide effect from the observed signal to compute the corrected
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signal (Figure 3).
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 Tilt related to tides, atmospheric pressure and temperature changes
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Tilt motions are not only sensitive to changes in the volcano dynamics and other effects
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can influence tilt signals such as tides, atmospheric pressure and temperature variations.
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Tides
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Tilt predicted for the solid earth tide with ETERNA 3.30 show higher amplitude for the E-
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W component than for the N-S component, which is in agreement with our observations
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(Figure 2a). The predicted amplitude is higher for the vertical component than for the
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horizontal components whereas the observations show a similar range of values for the E-W
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and the vertical components.
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Cyclone Gamède
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During cyclone Gamède the acceleration in the N-S component is increasing rapidly from
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February 25, 2007 (when the cyclone was approaching La Réunion) whereas the vertical (Z)
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component shows no clear variation suggesting a tilt-dominating signal (Figures S.2a and
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S.2b) during this approaching phase. The fact that the E-W component is less sensitive than
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the N-S component may be due to the source position of the cyclone north to the island. On
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February 25, 2007 at 00:00 UTC the eye of the cyclone (i.e the source of lowest atmospheric
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pressure) is located at approximately 230 km north of La Réunion and at that time the
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atmospheric pressure decrease rapidly at RER station (Figure S.2a). The unfiltered integrated
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seismometer output of the Z component is not expected to be sensitive to tilt. It shows
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however a clear apparent ground displacement during cyclone Gamède. This rapid
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displacement downward correlates with a rapid increase of the atmospheric pressure (Figure
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S.2b), as the cyclone is moving away from La Réunion Island from February 28, 2007. The
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causes of these ultra long period signals mostly related to tilt on February 25, 2007 at 00:00
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and related to ground displacement on February 28 may be complex [e.g. Zürn and Widmer,
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1995; Webb, 1998] and is beyond the scope of this study.
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Influence of temperature variations
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A negative drift of the E-W component occurred during a long-term increase of the
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temperature (Figure S.2c). The temperature varies from 16.5°C on January 25 to 16.66°C on
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April 28, 2007. Interestingly, a similar correlation between the E-W tilt (from the
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volcanological observatory of Piton de la Fournaise tiltmeter installed at the same site) and
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the temperature was already suggested by Peltier et al. [2011] whereas the N-S tilt didn’t
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show similar long-term correlation with the temperature variation. As suggested by these
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authors the E-W component of RER may be sensitive to seasonal temperature variation within
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the vault and rock dilatation due to temperature variation outside the vault.
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 References
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Cordier E., J. Lézé, and J.-L. Join (2013), Natural tidal processes modified by the existence of
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fringing reef on La Reunion Island (Western Indian Ocean): Impact on the relative sea
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level variations, Continental Shelf Research, 55, 119-128, doi:10.1016/j.csr.2013.01.016.
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Francis, O., and P. Mazzega (1990), Global charts of ocean tide loading effects, J. Geophys.
Res., 95, 11411-11424.
Harrison, J. C. (1976), Cavity and topographic effects in tilt and strain measurement, J.
Geophys. Res., 81, 319–328.
Hartmann, T., and H. G. Wenzel (1995), The HW95 tidal potential catalogue, Geophys. Res.
Lett., 22, 3553–3556.
Lambotte, S., L. Rivera, and J. Hinderer (2006), Vertical and horizontal seismometric
observations of tides, Journal of Geodynamics, 41, 39–58.
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Peltier, A., P. Bachèlery, and T. Staudacher (2011), Early detection of large eruptions at Piton
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de La Fournaise volcano (La Réunion Island): contribution of a distant tiltmeter station, J.
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Volcanol. Geotherm. Res., 199, 96-104.
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Webb, S. C. (1998), Broadband seismology and noise under the ocean, Rev. Geophys., 36,
105-142.
Wenzel, H. G. (1996), The Nanogal Software: Earth tide data processing package ETERNA
3.3, Bull. Inf. Marées Terrestres, 124, 9425-9439.
Zürn, W. and R. Widmer (1995), On noise reduction in vertical seismic records below 2 mHz
using local barometric pressure, Geophys. Res. Lett., 22, 3537-3540.
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 Supplementary Figure captions
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Figure S.1. The N-S tilt signals (rad) computed at RER from November 7, 2006 to January
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12, 2007 after applying the first procedure (see main text for details).
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Figure S.2. a) Unfiltered integrated seismometer output of the N-S component and
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atmospheric pressure variations for the whole 2007 eruptive period recorded at station RER.
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b) Unfiltered integrated seismometer output of the vertical (Z) component and atmospheric
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pressure variations for the same period recorded at station RER. c) Unfiltered integrated
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seismometer output of the E-W component and temperature variations for the same period
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recorded at station RER.
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