O. Foix W.C. Crawford I. Koulakov
C. Baillard M. Regnier B. Pelletier E. Garaebit

: Journal of Geophysical Research: Solid Earth

The central Vanuatu forearc is characterized by a reduced convergence rate at the trench, significant uplift of the overriding plate, and the presence of large forearc islands. Volcanic activity and intermediate-depth seismicity behind the forearc are among the highest on Earth. These features are presumed to be associated with the subduction of a large seamount chain and an immersed ridge. We used a catalog of P and S arrivals from a local seismological network to construct the first 3-D velocity model of the region and to relocate earthquakes beneath the forearc. The 3-D model reveals a highly heterogeneous velocity distribution in the first 40 km beneath the surface. Trench-parallel low P and S velocity zones in the upper tens of kilometers beneath the western edges of the two largest forearc islands correlate to the major features entering into subduction and suggest highly fractured and probably water-infiltrated features. Trench-parallel high-velocity zones at 5-15-km depth, further to the east, may be part of a continuous consolidated rock structure that acts as a backstop. Thick overriding plate crust (29 ± 3 km) in the forearc is consistent with the presence of continental remnants. The earthquake distribution is generally heterogeneous, suggesting a complex fault structure and variable stress. Earthquakes are, however, well aligned at the plate interface in between the subducting features, where they constrain the angle of subduction to be 15 on average, down to 10-15-km depth.