Imaging of 3-dimensional structure of the subduction zone

Technical Abstract

A three-dimensional P-velocity structure of the subduction zone under central New Zealand is imaged using 32,512 first P-arrivals and 8,025 S-arrivals from 3,340 earthquakes that occurred in this region since 1986 and using a seismic tomographic method. Detailed structures in the overlying Australian plate and in the subducted Pacific plate are studied based on the obtained seismic tomographic results. The subduction interface is determined to vary in the direction parallel to the strike of the subducted plate and to be broken under northern Cook Strait. On the southern North Island, the Pacific plate not only is subducted northwest, but also dips down towards northeast at a shallow dipping angle. This northeast dipping structure is found to be the reason for the high apparent Pn velocities that have been observed in the Cape Palliser region. The shallowest subduction interface is determined to be at the depth of approximately 13km under Cape Palliser. From a three-dimensional finite-difference flexural analysis on a significant change in depth of the interface, a torn structure in the subducted crust is found to be located under northern Cook Strait and to be approximately perpendicular to the strike of subduction. This torn structure is shown to be extended downwards to as deep as 40km. It is presumed to be the largest fault trending NW-SE under central New Zealand.

The subduction interface is shown to have very different shape on each side of the torn structure. On the southern side the subduction plate is found to be approximately 7-10km deeper than that on the northern side. Under the top of the South Island the interface appears to have a southwest down-dipping curvature. Evidence from seismic images shows that the Wairau fault extends northeast to the North Island along the east coast of the Marlborough Sounds and between the offshore of the west coast and east of Kapiti Island towards onshore at the northwest of Otaki. An offset or bent structure of this fault is shown near the western offshore of the Wellington Peninsular where the torn structure is found. The velocity structure suggests that the northern extension of the Wairau fault extends downward to at least 20km. It is presumed to be another major fault and geological boundary trending NE-SW in central New Zealand.

A narrow high velocity zone is obtained above the subducted interface, along the east coast near Cape Palliser and presumed to be the reason for a horst-like subduction structure or the intrusion of fast volcanic rocks in the bottom of the overlying plate. Offshore of east Cape Palliser, a low velocity region is found. The Australian crust under Cook Strait is characterised by nearly square regions of both high and low velocity perturbations. An extensive faulting system dominated by faults in NE-SW and NW-SE trends is suggested in the Cook Strait crust.