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cnrs I international 30 | In Images magazine to be able to install  tide gauges  on two shallow seamounts located on two different plates, directly beneath the path of altimetry satellites. This is an ideal layout for measuring the relative vertical motion of both plates.” More data was collected by a large network of permanent GPS and seismic stations set up in the archipelago, and by researchers who spent months locating earthquakes and recalculating the position of around forty  geodetic survey markers  present in Vanuatu since the mid-1990s. “By completing the measurements and analyses initiated since 1999 by Stéphane Calmant at the IRD, we highlighted a vertical submarine motion of a few millimeters per year,4” Ballu explains. A world first, since no one had ever managed to observe such slight movement in a submarine environment. For the scientists, this is evidence that stress is building up, heralding a massive earthquake in the coming century. “We are not questioning the effects of climate change,” Ballu concludes. “We are simply trying to raise people’s awareness of the different factors that can affect an environment.” 01. V. Ballu et al., “Comparing the role of absolute sea-level rise and vertical tectonic motions in coastal flooding, Torres Islands (Vanuatu),” PNAS, 2011. 108(32): 13019-22. 02. Institut de Physique du Globe de Paris (CNRS / UPM C / Université Paris-Diderot-Paris-VII / Université de La Réunion). 03. Littoral, environnement et sociétés (CNRS / Université de La Rochelle). 04. V. Ballu et al., “Using altimetry and seafloor pressure data to estimate vertical deformation offshore: Vanuatu case study,” Advances in Space Research, 2012. Doi: 10.1016/j.asr.2012.06.009. A photo gallery on Vanuatu is available on the online version of the magazine. > www.cnrs.fr/cnrsmagazine geo detic survey . marker. A marker in the ground which is used to monitor the motion of the Earth’s surface at this precise location. 08 The researchers use tide gauges to monitor sea level change on both sides of the boundary between the Australian and Pacific plates. 09 GPS buoys placed directly above the tide gauges make it possible to determine variations in sea level. Combined with the data from the tide gauges, this information is used to infer the vertical motion of the tectonic plates. 10 Vanuatu is also exposed to volcanic hazards. On Tanna Island, the inhabitants live under the constant threat of the Yasur volcano, prone to violent explosions. 11 The researchers help to inform the population about the risks they are facing, and play an essential role in devising appropriate prevention plans. 08 09 10 11 Contact i nformation: Littoral, environnement et sociétés, La Rochelle. Valérie Ballu > valerie.ballu@univ-lr.fr © IRD-CNRS © T. VERGOZ/CNRS Phoott hèque © Campagne EG ODEVA IRD-CNRS © Fclo her/EG ODEVA/IRD-CNRS tide gauge. A device that records changes in sea level.


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