The 1 400 km long Vanuatu subduction zone marks the subduction of the oceanic Australia plate beneath the North-Fijian microplate. Seismic and volcanic activity are both high, and several morphologic features enter into subduction, affecting seismicity and probably plate coupling. The Northern d’Entrecasteaux Ridge, West-Torres plateau, and Bougainville seamount currently enter into subduction below the forearc islands of Santo and Malekula. This subduction/collision coincides with a strongly decreased local convergence velocity rate at the trench (35 mm/yr compared to 120-160 mm/yr to the north and south). It is following by significant uplift on the overriding plate, indicating a high degree of deformation. The close proximity of Santo and Malekula islands to the trench (20-30 km) provides excellent coverage of the megathrust seismogenic zone for a seismological study.
We used an earthquakes selection of 2 863 events, from 10 months of seismological data recovered by the 30-instrument amphibious ARC-VANUATU seismology network, to construct a 3D velocity model using the LOTOS joint location/model inversion software. Then, we locate 11 709 earthquakes, from this project, beneath the central Vanuatu subduction zone using the NonLinLoc software.
The 3D model reveals low P and S velocities in the uppermost tens of kilometers beneath both of the large forearc islands. The Northern d’Entrecasteaux Ridge and the Bougainville Guyot face these velocity anomalies. This may be due to water infiltration in heavily faulted subducted features but can also reflect the amount of faulting. Moreover, we can also identify the presence of a possible seamount entered into subduction at West Malo. Then, the spatial distribution of the earthquake suggests a complex interaction of faults and stress zones related to high and highly variable stress. The depth limit of the seismogenic zone varies about 20-22 km and 30-36 km of depth along-slab. Its presence and limits are quite heterogeneous.