We use adjoint tomography to invert for three-dimensional structure of the North Island, New Zealand and the adjacent Hikurangi subduction zone. Due to a shallow depth to the plate interface below the North Island, this study area offers a rare opportunity for imaging material properties at an active subduction zone using land-based measurements. Starting from a ray tomography initial model, we perform iterative model updates using spectral element and adjoint simulations to fit waveforms with periods ranging from 4–30s. In total we perform 28 L-BFGS updates, improving data fit and introducing Vp and Vs changes of up to ±30%. Resolution analysis using point spread functions show that our measurements are most sensitive to heterogeneities in the upper 30km. The most striking velocity changes coincide with areas related to the active Hikurangi subduction zone. Lateral velocity structures in the upper 5km correlate well with New Zealand geology. The inversion recovers increased along-strike heterogeneity on the Hikurangi subduction margin with respect to the initial model. In Cook Strait we observe a low-velocity zone interpreted as deep sedimentary basins. In the central North Island, low-velocity anomalies are linked to surface geology, and we relate velocity structures at depth to crustal magmatic activity below the Taupo Volcanic Zone. Our velocity model provides more accurate synthetic seismograms, constrains complex velocity structures, and has implications for seismic hazard, slow slip modeling, and understanding of volcanic and tectonic structures related to the active Hikurangi subduction zone.

Seamounts are found at many global 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 past the present day coastline. Using a high-resolution, adjoint tomography-derived velocity model of the North Islan, New Zealand 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 two previously-unidentified, deeply-subducted seamounts, and the low-velocity zone as fluid in the subducting slab. The seamounts are inferred to be 10--30km wide and on the plate interface at 12--15km depth. Resolution analysis using point spread functions confirm that these are well-resolved features. The locations of the two seamounts correlate with bathymetric features whose geometries are consistent with those predicted from analog seamount subduction experiments. The spatial characteristics of seismicity and slow slip events near the inferred seamounts agree well with previous finite element modeling predictions on 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 geodetic behavior observed, and partly responsible for along-strike variations in plate coupling on the Hikurangi subduction interface.