Earthquakes illuminate the incipient collision and subduction of Loyalty Ridge at Vanuatu subduction zone

Bathymetric highs resist subduction producing large- to small-scale change in the morphology of the subduction zone: Increase of the outer-rise curvature, trench indentation and large-scale slides and slumps in the fore-arc region. At the plate interface, bathymetric highs induce geometrical and frictional changes that can produce increase or decrease of the local coupling, and thus having an effect on the likelihood of occurrence of large megathrust earthquakes. Numerical models predict complex strain and stress patterns arising from subduction of such relief mainly driven by the size and relative geometry of trench and subducted high. However, the collision and subduction of bathymetric highs is investigated mainly via geophysical and geological surveys since seismic sequences have rarely illuminated the subduction of seafloor relief. Here, we report of a year-long and very energetic earthquake activity (10 Mw 6.5-7.5) at the Loyalty Ridge – Vanuatu trench at both the plate interface and in the outer-rise region. The spatio-temporal and magnitude of the earthquakes revealed complex release of the accrued flexural strain along the outer-rise and a pronounced segmentation of the interface with repeating M7 earthquakes, low aftershock activity and a large “aseismic” zone. The collision and subduction of the Loyalty Ridge along the Vanuatu trench seem to indicate a frictionally segmented interface where large megathrust earthquakes are unlikely to occur.