Slow slip along the Hikurangi margin linked to fluid-rich sediments trailing subducting seamounts

Large seamounts and basement relief cause permanent deformation when they collide with the overriding plate at subduction zones. The resulting structural and compositional heterogeneities have been implicated as controlling factors in megathrust slip behaviour. Subducting seamounts may temporarily lock plates, favouring subsequent large earthquakes. Alternatively, seamounts may redistribute stress, reducing seismic slip. Here we present three-dimensional seismic data from the seamount-studded subducting Hikurangi Plateau along New Zealand’s North Island. We find that one well-imaged seamount, the Pāpaku Seamount, locally uplifts the overriding plate and leaves a tube-shaped lens of sediment trailing in its wake. Anomalously low seismic velocities within and below the Pāpaku lens and along the megathrust fault are consistent with the presence of unconsolidated, overpressured fluid-rich sediments. Similar observations from an older sediment lens, which corresponds to the location of a 2014 slow-slip rupture event, suggest that such overpressures can persist along the megathrust due to delayed drainage out of the subducting plate. The collocation of the 2014 slow-slip earthquake with this sediment lens suggests that these fluid-rich regions define zones that enable slow slip. We hypothesize that sediment lenses left behind by subducting seamounts can create low-effective-stress patches within transitionally stable marine sediment along the megathrust that are conducive to slow slip.