Vertical seafloor deformation in a partially blocked subduction zone from tide gauge, altimetry and GPS data
Valerie Ballu(1) , Gwenaele Jan(2) , Marie-Noelle Bouin(3) , Stephane Calmant(4) , Michel Diament(1) , Jean-Michel Bore(5) , and Nicolas Bergeot(1)
4 place Jussieu,
(2) Noveltis, Parc technologique du canal, 31520 Ramonville Saint Agne, France
(3) ENSG/LAREG, 6 et 8, Av. Blaise Pascal , 77455 Marne-la-Vallee , France
(4) LEGOS, 18 Av E Belin, 31000 Toulouse, France
(5) IRD, Centre IRD, BP A5 Noumea, New Caledonia
The Vanuatu archipelago is part of the Pacific « ring of fire », where plates are rapidly converging. In the area, movements are very rapid and the seismic activity is intense, which gives a good opportunity to study deformation and seismic cycle. To get an integrate picture of vertical deformation over one plate and between the two plates, one needs to be able to monitor vertical movements on both underwater and emerged areas. We conducted an experiment in the Vanuatu archipelago, South-West Pacific, to compare measurements from bottom pressure gauges and altimetry satellite data using constraints from kinematic GPS data. Two bottom pressure gauges are immerged since Nov. 1999 on Sabine bank (15.90°S, 166.14°E) and Wusi Bank (15.34°S, 166.55°E), West of Santo island, Vanuatu. Water height data provided by seafloor tide gauges is a combination of sea-level variations and ground motion. Both of these signals are of scientific interest, but they must be separated in order to be useful. One promising method to separate the two contributions and retrieve the ground motion signal we are looking for is to use satellite altimetry data which gives absolute water height. Satellite altimetry data must be calibrated using « ground truth » measurements, such as GPS. On board and buoy kinematic GPS data are also used to link pressure gauge and satellite altimetry data, because the pressure gauge is not located directly beneath the satellites tracks. Once different components of the signal are separated, bottom pressure gauges can be used to detect vertical movements of the seafloor such as co-seismic or slow inter-seismic motions. Our results are analyzed in comparison with vertical movements observed on-land at Wusi GPS station which is located a few kilometers East of the tide gauge.