Response of Fogo volcano (Cape Verde) to lunisolar gravitational forces during the 2014–2015 eruption
Introduction
Periodicities corresponding to lunisolar tides have for long been observed in seismic and volcanic activity (Mauk and Johnston, 1973; Heaton, 1975; Sparks, 1981; Rymer and Brown, 1984; Bodri and Iizuka, 1989; Cigolini et al., 2009; Bredemeyer and Hansteen, 2014; Delorey et al., 2017; Petrosino et al., 2018; Ricco et al., 2019; Varga and Grafarend, 2019). In addition, recent studies have evidenced presence of tides at time scales ranging from a few days to decades, in the polar motion (Lopes et al., 2017), plate motion (Zaccagnino et al., 2020) and even in climate indexes as a response to the temporal evolution of atmospheric pressures (Le Mouël et al., 2019a). At volcanoes, periodic variations have been revealed by various kind of observations such as lava lake height, degassing, micro-seismicity and Long-Period (LP) events, volcanic tremor, ground tilt, temperature of fumarolic fields, energy radiated by lava or strength of eruptive phases (Golombek and Carr, 1978; Berrino and Corrado, 1991; Williams-Jones et al., 2001; Custodio et al., 2003; Sottili et al., 2007; Cigolini et al., 2009; De Lauro et al., 2012, De Lauro et al., 2013, De Lauro et al., 2018; Sottili and Palladino, 2012; Bredemeyer and Hansteen, 2014; Girona et al., 2018; Dinger et al., 2018; Caputo et al., 2020; Dumont et al., 2020; Petrosino et al., 2020). Lunisolar gravitational forces have also been evoked as triggers of volcanic eruptions (Mauk and Johnston, 1973; Dzurizin, 1980; Jentzsch et al., 2001; Dumont et al., 2020).
Different approaches have been considered to demonstrate the correlation between tidal action and variations in volcanic activity, from statistics to spectral or principal component analysis (see examples in Mauk and Johnston, 1973; Patanè et al., 1994; Girona et al., 2018; De Lauro et al., 2013; Dumont et al., 2020). The main and most frequent periodicities detected in time-series acquired at volcanoes are those of the fortnightly, which is induced by the Moon's declination, the semi-diurnal caused by the Sun and lunar elliptic trajectory, as well as the diurnal component which is related to the Sun (Berrino and Corrado, 1991; Custodio et al., 2003; Sottili and Palladino, 2012; De Lauro et al., 2018; Girona et al., 2018; Le Mouël et al., 2019b). These tidal oscillations are, however, not the only to modulate activity at volcanoes, as suggested by a couple of studies (Sottili and Palladino, 2012; Bredemeyer and Hansteen, 2014; Caputo et al., 2020; Dumont et al., 2020). In spite of all these studies, providing clear evidence on the correlation between processes that occur at very different time scales has proved to be particularly challenging. In addition and beyond the techniques used, the cause-effect relationship between tidal action and volcanic activity has remained elusive (Sparks, 1981; Neuberg, 2000). Tidal stresses and tidal acceleration have been suggested as the main drivers of tidal forcing although tidal stresses are about 3–5 orders of magnitude smaller than tectonic stresses (Mauk and Johnston, 1973; Sparks, 1981), which makes them too low to induce fracturing (McMillan et al., 2019; Dumont et al., 2020). The complexity of volcanic systems, which is mainly due to their eruptive history, composition, internal structure and tectonic setting, also plays an important role on how volcanoes respond to the quasi-permanent tidal oscillations explaining also why all volcanoes do not show a similar sensitivity to Earth tides (Mauk and Johnston, 1973; Dzurizin, 1980). Additionally, the specific positions of volcanoes on the planet, means they are not influenced similarly by the different Earth tides, as the zonal, tesseral and sectorial components of the tidal potential have a specific spatial distribution on Earth (see Jobert and Coulomb, 1973; chapter 18). The spatial distribution and especially the latitudinal position of the different gravitational bodies that interact with the volcano is determinant in the way the tidal potential applies on Earth (Mauk and Johnston, 1973; Hamilton, 1973; Jobert and Coulomb, 1973) .
In this study, we adopted a similar approach to Dumont et al. (2020) and used the Singular Spectrum Analysis to investigate three geophysical time-series, namely the SO2 emission, lava volume flow rate (VFR) and seismic tremor, spanning the 2014–2015 mix eruption at Fogo volcano, Cape Verde. This eruption started on 23 November 2014 and ended on 8 February , after 2.5 months of intense effusive activity (Mata et al., 2017; Richter et al., 2016). The resulting extensive lava field (~40-45 106 m3) covered ~4.5 km2 of Chã das Caldeiras and destroyed two villages (Richter et al., 2016; Jenkins et al., 2017; Bagnardi et al.(2016); Vieira et al. (2020)) . We characterize the lunisolar tidal influence on this dominantly effusive eruption which took place in the Equatorial zone, to further investigate the specific response of Fogo volcano to this permanent external forcing associated with Earth tides.
Section snippets
Fogo volcano and the 2014–2015 eruption
Fogo volcano is located on one of the dozen islands composing the Cape Verde archipelago, which lies ~450 km west of Africa, in the North Atlantic (Fig. 1). The west-open crescent formed by the islands rises on the top of a 1200 km diameter swell characterized by the largest bathymetric (+ 2 km) and geoid (+ 8 m) anomalies in the world which are thought to result from an upwelling mantle plume (Courtney and White, 1986; Grevemeyer, 1999; Carvalho et al., 2019). The rise of the Cape Verde
Retrieval of lava and SO2 emissions
Lava Volume Flow Rate (VFR) and SO2 vertical column densities (VCD) have been retrieved together from the HOTVOLC system. HOTVOLC is a Web-GIS volcano monitoring system using SEVIRI (Spinning Enhanced Visible and Infrared Imager) sensor on-board Meteosat geostationary satellite (https://hotvolc.opgc.fr) and developed at the OPGC (Observatoire de Physique du Globe de Clermont-Ferrand) in 2009 (Gouhier et al., 2016). The spectral bands of the SEVIRI sensor allow us to simultaneously characterize
Results
MSG-SEVIRI satellite images reveal a maximum of lava VFR and SO2 VCD detected between 1 and 2 days respectively, after the onset of the eruption (Fig. 2). Although the seismic tremor data does not cover the first week of the eruption, it still shows significant variations with a maximum reached on 3 December, followed by an overall slow decay. A stronger decline is observed for the SO2 VCD that drops by half in about a week and then slightly varies around an index value of 8 for the rest of the
Discussion
The Singular Spectrum Analysis applied to three geophysical time-series acquired during the 2.5-month Fogo eruption provides a new evidence of the influence of lunisolar gravitational forces on effusive eruptions. We identified between 4 and 5 tidal periodicities in the SO2, lava VFR and seismic tremor time-series with periods ranging from semi-diurnal to fortnightly (Table 1, Fig. 4). These results confirm and complement the observations made by Dumont et al. (2020) showing that the movements
Conclusion
Our study focuses on the 2014–2015 eruption of the Fogo volcano, Cape Verde. We analyze three co-eruptive geophysical time-series, namely the seismic tremor, SO2 emissions and lava volume flow rate (VFR), using the Singular Spectrum Analysis (SSA). By considering the first month of the eruptive activity, we were able to identify between 4 and 5 different tidal periods in each of these volcanological time-series, ranging from semi-diurnal to fortnightly periods. These results clearly show a
Data availability
The SO2 emission and lava volume flow rate (VFR) time-series are available from the HOTVOLC platform (https://hotvolc.opgc.fr). The l.o.d time-series is freely accessible as part of the EOP14C04 data set provided by the International Earth Rotation Service (https://www.iers.org/IERS/EN/DataProducts/EarthOrientationData/eop.html, IERS, Paris, France) as well as that of the sea level, accessible from Permanent Service for mean sea level platform (PSML, 2019). The seismic tremor data is available
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors would like to thank E.P·S Eibl for fruitful discussion on data processing and João Fonseca for making possible the seismic mission to Fogo and its contribution to the FIRE project. The authors also thank two anonymous reviewers and Mark Jellinek, for editorial handling. SD would like to thank the Fundação para a Ciência e a Tecnologia (FCT) for his financial support through the postdoctoral grant (SFRH/BPD/17714/2016), as part of Human Capital Operating Programme (POCH) and the
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