3-D Imaging of the Northern Hikurangi Subduction Zone, New Zealand: Subducted Sediment Near Shallow SSEs

We examine 3-D Vp and Vp/Vs along the northern and central Hikurangi subduction zone, New Zealand, where the subducting Pacific plate offshore is heterogeneous with many seamounts and the overlying Australian plate has rotation and extension. The downdip limit of interseismic coupling is shallower than 15-km, indicating that much of the plate interface (PI) is slipping aseismically, and large shallow slow-slip events (SSEs) and small deep SSEs have been observed. We selected 2600 spatially distributed earthquakes, incorporating upgraded permanent network, and temporary networks from 1993-94, 2001, and 2011-12. Onshore-offshore marine-seismic data were also included, helping to constrain the shallow velocities and the PI gradient, where sampled. Our method used earthquake differential times and receiver differential times with gradational inversions. The results show major variations along strike and regions of subducted sediment. Above the shallow plate interface (less than 20-km depth) and north of Gisborne there is a 70-km long zone of high Vp/Vs and low Vp, which is interpreted as subducted sediment with high fluid-pressure. This section also showed subducted sediment in shallower depth offshore seismic reflection data, and corresponds to an area of shallow SSE's. It is bounded to the north by a high Qp, higher Vp, moderate Vp/Vs feature which may be a seamount, and to the south by high Qp, high Vp features that may correspond to Cretaceaous volcanics or a subducted seamont. At the northeastern tip of the North Island, low Vp above the PI is also consistent with thick sediment. In deeper zones, where the PI is 25-45 km depth, there are northern and central zones of thick low Vp, low Qp material related to underplated sediments, which are uplifting the Raukumara and Kaimanawa Ranges. Small deep (25-45 km) SSEs are related to the central deep underplated sediment zone, but no SSEs have been observed in the northern underplated zone.