Hikurangi megathrust slip behavior influenced by lateral variability in sediment subduction

Subduction megathrusts exhibit a range of slip behaviors spanning from large earthquakes to aseismic creep, yet what controls spatial variations in the dominant slip mechanism remains unresolved. We present multichannel seismic images that reveal a correlation between the lithologic homogeneity of the megathrust and its slip behavior at a subduction zone that is world renowned for its lateral slip behavior transition, the Hikurangi margin. Where the megathrust exhibits shallow slow-slip in the central Hikurangi margin, the protolith of the megathrust changes ~10 km downdip of the deformation front, transitioning from pelagic carbonates to compositionally heterogeneous volcaniclastics. At the locked southern Hikurangi segment, the megathrust forms consistently within pelagic carbonates above thickened nonvolcanic siliciclastic sediments (unit MES), which subduct beyond 75 km horizontally. The presence of the MES layer plays a key role in smoothing over rough volcanic topography and establishing a uniform spatial distribution of lithologies and frictional properties that may enable large earthquake ruptures.