Seismic reflection character of the Hikurangi subduction megathrust, North Island, New Zealand

The Hikurangi subduction margin, New Zealand, exhibits marked along-strike changes in the geometry of the likely seismogenic zone, and varies from being a predominantly accretionary margin in the south to predominantly erosional in the north. Many aspects of the northern Hikurangi margin are similar to Costa Rica, whereas the southern Hikurangi margin is an excellent analogue for Nankai and Cascadia. Campaign and continuous GPS measurements reveal deep interseismic coupling and slow slip events (~30-60 km) at the southern Hikurangi margin, while the northern part of the subduction thrust is dominated by aseismic slip, shallow slow slip events (<10-15 km) and shallow co-seismic slip in the form of tsunami earthquakes. Over the last decade a suite of deep seismic reflection data have been acquired along the Hikruangi margin imaging the shallow part of the subduction megathrust to depths of 15-20 km. Together, these surveys reveal variations in the geometry and seismic reflection character of the megathrust in regions that have different seismic behaviours. The southern Hikurangi shallow megathrust, which experiences strong interseismic locking, is smoothly dipping and underlain by a sediment channel. The weakly coupled northern Hikurangi subduction interface is significantly more irregular in terms of its geometry and seismic character. The northern Hikurangi margin is the one of the only locations on Earth where parts of the megathrust that undergo slow slip have been imaged in detail by seismic reflection data. Along the northern Hikurangi margin these data reveal the presence of multiple subducting seamounts. Downdip of the seamounts we observe high-amplitude multi-layered reflectivity, which we suggest is related to sediments and eroded overriding plate material which have been entrained and underthrust. Slow slip events occur on parts of the interface with this bright reflection character. We suggest shallow slow slip events are promoted along the northern Hikurangi margin due to high fluid pressures within this underthrust material and/or associated with shearing within this mixture of competent and incompetent material in the subduction channel ahead of subducted seamounts. The largest megathrust earthquakes to have occurred on the Hikurangi margin in historical times (post 1840) occurred in March and May 1947 along the northern Hikurangi margin. These events were unusual in that they ruptured the megathrust very close to the trench (depth of < 10 km), had low ML (5.6 and 5.9) compared to MW (7 - 7.1 and 6.9 - 7.1), and produced larger than expected tsunami (runups of up to 10m and 6m respectively). Seismic reflection data have revealed these events occurred in locations where subducted seamounts are interpreted from seismic reflection and magnetic data. We suggest along the northern Hikruangi margin seamounts are providing nucleation points for very shallow co-seismic rupture.