Shallow morphology of the subducted Pacific plate along the Hikurangi margin, New Zealand

Abstract

Along the east coast of the North Island, New Zealand, the Pacific plate is subducted beneath the Australian plate. The seismicity associated with this subduction has been determined using the results from a number of temporary and permanent microearthquake networks. A synthesis of the microearthquake seismicity, the depths of thrust events, the depth of S to P conversions, and deep reflection seismic studies leads to a coherent picture of the subducting slab, both in terms of its mechanical structure and in terms of the general morphology. The slab is initially subducted at a low angle of a few degrees, is bent through a circular arc of few hundred kilometre radius and then becomes planar again below 120 km depth. The radius of curvature varies along strike from about 280 km, just north of Wellington, to 240 km, north of Hawke Bay. The slab shape, between depths of 15–45 km, is thus approximately conical. It also approximates, within the errors of hypocentre location, a surface of constant Gaussian curvature, as would be geometrically expected from the simple bending of a thin spherical lithospheric shell. Observed seismicity is primarily concentrated within what is interpreted as the subducted crust of the Pacific plate, with a lower level of activity within the subducted mantle. Normal fault mechanisms at the top of the plate probably reflect tensional stress associated with the plate bending whereas thrust events observed at the lower bound of the observed seismicity probably reflect compression below the neutral plane.