Plate coupling and the hazard of large subduction thrust earthquakes at the Hikurangi subduction zone, New Zealand

Recent dense deployments of portable seismographs along the Hikurangi subduction zone have provided insights into the structure and seismic strain regime of the subducted and overlying plates, and the nature of plate coupling at the shallow part of the plate interface. Beneath Marlborough, the plates appear to be permanently locked, and large subduction thrust events are not expected. In the Wellington and Wairarapa regions, the plates appear to be strongly coupled over a downdip width of the plate interface of c. 70 km. Subduction thrust earthquakes of about M-W 8.0 are estimated for this region. Farther to the northeast, the downdip width of the inferred locked portion of the plate interface progressively decreases, and subduction thrust events of about M-W 6.9 are estimated for the northern part of the Raukumara Peninsula. In the south of the subduction zone, changes in coupling arise principally from changes in the thickness of the subducted plate, whereas in the north they are mainly due to changes in thickness of the overlying plate. Tectonic rotations within the overlying plate observed paleomagnetically appear to be a natural consequence of changes in plate coupling along the subduction zone.