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N°29 I quarterly I april 2013 In Images | 29 w 03 04 the IPGP2 and the Research Institute for Development (IRD)—first quantified the vertical motion of the Torres Islands. Now a member of the LIENSs laboratory, 3 Ballu pays close attention to this phenomenon and its effects. “As recent events in Sumatra and Japan have shown, locked subduction can trigger massive earthquakes since the energy that builds up over the long term is released all at once. And vertical motion is an excellent indicator of locking.” Yet, given the frequent earthquakes throughout the region, measuring such motion is no mean feat. On land, thanks to the ever-improving precision of GPS measurements, the build-up of stress and its associated vertical motion are more frequently detected. Doing this at sea is another matter altogether. In Vanuatu, as Ballu explains, “we were lucky enough 05 This temporary GPS has been set up to measure the vertical and horizontal motion of the island of Efate to the nearest millimeter. 06 This GPS station on the west coast of Santo provides data that is collected every year. It monitors the sinking or rising of the archipelago over time. 07 Vanuatu is located at the exact spot where the Australian plate descends beneath the Pacific plate. 06 05 © Crawford /IRD-CNRS © T. VERGOZ/CNRS Photot hèque © T. VERGOZ/CNRS Photot hèque © Campagne GE ODEVA/IRD-CNRS © Crawford /IRD-CNRS 07 © IRD-CNRS


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