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Séminaire GM/LGO d’Océane FOIX (GM, Ifremer, Brest)

The 3 ‐ D Velocity Models and Seismicity Highlight Forearc Deformation

Due to Subjecting Features (Central Vanuatu)


The central Vanuatu forearc is characterized by a reduced convergence rate at the trench, signifcant uplif of the overriding plate, and the presence of large forearc islands. Volcanic actvity and intermediate ‐ depth seismicity behind the forearc are among the highest on Earth. These features are presumed to be associated with the subducton 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 frst 3 ‐ D velocity model of the region and to relocate earthquakes beneath the forearc. The 3 ‐ D model reveals a highly heterogeneous velocity distributon in the frst 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 subducton and suggest highly fractured and probably water ‐ infltrated features. Trench ‐ parallel high ‐ velocity zones at 5–15 ‐ km depth, further to the east, may be part of a contnuous consolidated rock structure that acts as a backstop. Thick overriding plate crust (29 ± 3 km) in the forearc is consistent with the presence of contnental remnants. The earthquake distributon is generally heterogeneous, suggestng a complex fault structure and variable stress. Earthquakes are, however, well aligned at the plate interface in between the subductng features, where they constrain the angle of subducton to be 15° on average, down to 10–15 ‐ km depth.