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. Plain Language Summary Seamounts are large volcanic edifices on the seafloor that eventually make their way into subduction zones. Seamounts have been identified at various stages of subduction and are thought to either promote or suppress the occurrence of large earthquakes. It is difficult to track seamounts far into a subduction zone due to the decreasing sensitivity of most geophysical measurements with depth. In this study, we identify several distinctive seismic velocity anomalies in a high-resolution 3D velocity model of the North Island, New Zealand. The model is derived using adjoint tomography, a form of seismic imaging that optimizes the match between observed and simulated seismic waveforms. We interpret the anomalies to indicate the presence of two deeply subducted seamounts and fluid in the downgoing plate. The two seamounts are inferred to be at interface depths, with horizontal dimensions of about 10-30 km. These features are well resolved and our interpretations are supported by independent evidence including seafloor bathymetry data and the presence of nearby geophysical anomalies. We associate these seamounts with variations in slip behavior observed along the Hikurangi subduction margin and propose that they have caused permanent damage to the upper plate, thereby reducing its ability to store energy and host large earthquakes.