Evidence for Deeply Subducted Lower-Plate Seamounts at the Hikurangi Subduction Margin: Implications for Seismic and Aseismic Behavior

Seamounts are found at many subduction zones and act as seafloor heterogeneities that affect slip behavior on megathrusts. At the Hikurangi subduction zone offshore the North Island, New Zealand, seamounts have been identified on the incoming Pacific plate and below the accretionary prism, but there is little concrete evidence for seamounts subducted beyond the present-day coastline. Using a high-resolution, adjoint tomography-derived velocity model of the North Island, we identify two high-velocity anomalies below the East Coast and an intraslab low-velocity zone up-dip of one of these anomalies. We interpret the high-velocity anomalies as previously unidentified, deeply subducted seamounts, and the low-velocity zone as fluid in the subducting slab. The seamounts are inferred to be 10–30 km wide and on the plate interface at 12–15 km depth. Resolution analysis using point spread functions confirms that these are well-resolved features. The locations of the two seamounts coincide with bathymetric features whose geometries are consistent with those predicted from analog experiments and numerical simulations of seamount subduction. The spatial characteristics of seismicity and slow slip events near the inferred seamounts agree well with previous numerical modeling predictions of the effects of seamount subduction on megathrust stress and slip. Anomalous geophysical signatures, magnetic anomalies, and swarm seismicity have also been observed previously at one or both seamount locations. We propose that permanent fracturing of the northern Hikurangi upper plate by repeated seamount subduction may be responsible for the dichotomous slow slip behavior observed geodetically, and partly responsible for along-strike variations in plate coupling on the Hikurangi subduction interface.