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On-shore and off-shore geodesy in Vanuatu


IPGP - Îlot Cuvier


Séminaire de sismologie, de géosciences marines et de géophysique d'exploration

Salle 310

Valérie Ballu

Université de La Rochelle

Vanuatu archipelago in the Southwest Pacific is part of the Pacific Ring of Fire. There, the Australian plate is rapidly subducting eastward beneath the North Fiji basin, on the western border of the Pacific Plate. High topographic features, impacting the subduction, are present on the diving plate. Previous studies around the world have shown the major role of subduction locking in the genesis of destructive earthquakes. GPS network points have been installed in the Vanuatu since the early 90’s. We will show results from GPS data collected from 1996 to 2011, which have been re-processed and combined into ITRF2008. Our results show that vertical deformation rates vary both across and along the archipelago. These variations likely result from variable distance to the plate limit and variable locking parameters (interface dip and locking amount). Although challenging, offshore deformation measurements are particularly important for constraining the amount and limits of plate locking in subduction zones. Understanding the stress accumulation in these zones is mandatory for understanding the processes involved and mitigating potential risks. In Vanuatu, we have augmented the on-land network by two seafloor geodesy sites. The sites –Wusi and Sabine Banks – are installed beneath altimetry satellite tracks, on both sides of the plate limit and are equipped with a tide gauge since 1999. I will present and discuss the results obtained when combining seafloor pressure data with altimetry derived sea surface heights to determine variations of seafloor heights in a global reference frame. The obtained vertical deformation rates are modeled using a 2D elastic finite-element code. Using both on-shore and off-shore geodesy data, we conclude that stress is currently accumulating west of Santo, Central Vanuatu. The work that I will present is a collaborative work involving in particular MN. Bouin, C. Baillard, S. Calmant, P. Bonnefond, B. Pelletier, W.C. Crawford, T. Kanas, E. Garaebiti.