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The deep plate boundary has usually been recognized as an aseismic area, with few large earthquakes occurring at the 60-100 km depth interface. In contrast, we use a finite-fault rupture model to demonstrate that large slip in the 2016 M7.9 Solomon Islands earthquake may have originated from the deep subduction interface and propagated all the way up to the trench. The initial rupture occurred at a depth of about 100 km, forming a deep asperity and then propagating updip to the middle-depth large coseismic slip area. Our proposed source model indicates that the depth-varying rupture characteristics of this event could shift to deeper depths with respect to other subduction zones. This result also implied that the deep subducting plate boundary could also be seismogenic, which might trigger rupture at the typical middle-depth stress-locked zone and develop into rare composite megathrust events.

Plain Language Summary The deep plate boundary has usually been recognized as an aseismic area, with few large earthquakes occurring at the 60-100 km depth interface. In contrast, we use a finite-fault rupture model to demonstrate that large slip in the 2016 M7.9 Solomon Islands earthquake may have originated from the deep subduction interface and propagated all the way up to the trench. The initial rupture occurred at a depth of about 100 km, forming a deep asperity and then propagating updip to the middle-depth large coseismic slip area. This scenario is privileged with respect to the alternative model where rupture initiates on a deep intraslab fault, based on a better data modeling, a careful review of the aftershock distribution, and a better consistency with the overall source centroid. Our proposed source model indicates that the depth-varying rupture characteristics of this event could migrate to deeper depths with respect to other subduction zones. This result also implied that the deep subducting plate boundary could also be seismogenic, which might trigger rupture at the typical middle-depth stress-locked zone and develop into rare composite megathrust events.