3-D imaging of the northern Hikurangi subduction zone, New Zealand: Variations in subducted sediment, slab fluids and slow slip

Abstract

We obtain 3-D Vp and Vp/Vs from 8 to 70 km depth along the northern Hikurangi subduction zone, New Zealand, where the downdip limit of interseismic coupling is shallower than 15 km, and where both large shallow slow-slip events (SSEs) and small deep SSEs have been observed. Onshore-offshore marine-seismic data were incorporated, which greatly improved constraint of shallow velocities and the plate interface (PI) zone velocity structure. We also selected 2600 spatially distributed earthquakes, including seismic data from the upgraded permanent seismometer network, as well as seismic data from temporary networks deployed in 1993-1994, 2001 and 2011-2012. Our method used earthquake differential times and receiver differential times with gradational inversions. The results show extensive regions of subducted sediment, but with major variations along strike. Above the shallow PI (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. Subducted sediment is also observed at shallower depth offshore in seismic reflection data, in the vicinity of the shallow SSEs. The SSE patch with highest slip occurs where a zone of high seismicity connects the high Vp/Vs upper oceanic crust to the slab upper mantle such that the oceanic crust may serve as a reservoir below the SSE slip zone and enhance dilatant strengthening. 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.