Seismicity and geodynamics in the Vanuatu subduction zone

The Vanuatu Arc (VA), located in the southeast part of the Pacific ocean (167°E, 16°S), is an highly seismically active region with more than 40 events of magnitude Mw > 7 since 1973 (USGS catalog). It is dominated by the east-dipping subduction of the Australian Plate (AP) under the North Fiji basin microplate. The convergence rates with respect to the AP are estimated to be between 130 and 170 mm/yr, except in the central part of the VA where the convergence rates are between 30 and 40 mm/yr. This is mainly caused by the subduction of the d'Entrecasteaux ridge. The aim of the study is to analyze and understand the complex tectonic processes of the central section of the VA. To achieve this we first located hypocenters using P-S phase picks obtained by a Kurtosis based automatic picker procedure we developed (Baillard et al, 2014). Characterization of the onsets is performed through a derived polarization function. By analyzing the hypocenters from our local microseismic catalog (~29000 earthquakes in 2008-2009) and global catalogs we show that the subduction interface is steep and can reach 70° at great depth, however in the first 50 km depth, it presents a small dipping angle where the ridge is subducting. This bump highlights the buoyancy of the ridge associated to the excess of fluids present in the seamount. This underplating could explain 20% to 60% of the vertical displacement estimated on the forearc islands from corals datations and that can reach a maximum of 6 mm/yr. The high concentration of hydrous minerals in the subducting ridge might also explain the important activity of intermediate depth earthquakes (half of the total activity in the studied region), we observed a very good correlation between the supposed extension of the ridge in depth and the location of these earthquakes. We propose that they are associated to crust minerals dehydration that causes hydrous fracturation trough preexistent faults. This dehydration process is maintained to a maximum depth of 190 km due to the high thermal parameter of the australian plate. Using the geometry of the Wadati-Benioff plane derived from earthquakes locations, we established a 2D mechanical model to explain the horizontal interseismic displacement observed by GPS on islands of the upper plate. We show that the subduction interface alone cannot explain the GPS velocities observed, the system of thrust faults located below the back arc islands of Maewo and Pentecôte, plays a major role in the region geodynamics and accommodate as much convergence as the subduction interface (between ~16 and ~34 mm/yr). Using the model we were also able to explain the closing of the Aoba basin during interseismic phase (~25 mm/yr). Finally, the mechanical model suggests the existence of a 23 km wide locked patch that reaches the trench.