Imaging of 3-Dimensional Structure of the Subduction Zone of Central New Zealand

Abstract

A 3-dimensional seismic tomographic method has been developed for the simultaneous determination of velocity perturbations, earthquakes locations and station terms. This method comprises a 3-dimensional ray tracing technique and a LSQR inversion procedure. The ray tracing method is able to determine accurate ray paths in a complicated 3-dimensional velocity model which is parameterized in grid points and in which velocity discontinuities can be introduced. It can also produce a travel-time for any seismic phase of body waves and deal with the inversion problems in either local or global scales under several coordinate systems conventionally used in seismic tomography. The LSQR inversion procedure used in this study is demonstrated to produce solutions of comparable quality to the direct SVD method, even for singular and ill-conditioned problems, and yield better solutions than those obtained using simple LSQR iterations. A 3-dimensional P-velocity structure of the subduction zone under central New Zealand is imaged using 32,512 first P-arrivals and S-arrivals from 3,340 earthquakes and using the seismic tomographic method developed in this study. The study region, which includes the southern North Island, Cook Strait and the northern South Island, lies at the frontal margin of the Australian plate under which the Pacific plate is being subducted obliquely northwest. The obtained velocity perturbations reveal significant variations in P-wave velocity in the crust of the Australian plate. Above 50 km, the velocities distribute predominately along a northeast trend. Above 20 km, the velocities generally increase from the east coast towards the western edge of the study area. At the shallowest inversion depth, 5 km, the velocity perturbations appear to have good correlations with the geological structures exposed on the ground surface. Offshore of Cape Palliser lies a low velocity region which extends to 12 km depth. This low velocity region may correspond to the accreted or underplated sediments of accretionary margins. A high velocity region of approximately 20 km in width is found at Cape Palliser along the east coast, above the subducted Pacific plate. An extensive high velocity zone exists to the west of Kapiti Island" It is considered to correspond to Triassic schists under that region. 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. Under 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 main reason for the high apparent Pn velocities which have been observed in the Cape Palliser region. The shallowest subduction interface is determined to be at the depth of approximately 13 km under Cape Palliser. From a 3-dimensional finite difference flexural analysis, a tear in the subducted crust is determined to be located under northern Cook Strait and to be approximately perpendicular to the strike of subduction. This tear is shown to be extended downwards to as deep as 40 km. It is considered to be the largest fault trending NW-SE under central New Zealand. The subduction interface is shown to have different shape on each side of the tear. On the southern side the subduction plate is found to be approximately 7-10 km deeper than that on the northern side. Under the northern end 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 the 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 Peninsula where the tear is found. The velocity images suggest that the northern extension of the Wairau Fault extend downward to at least 20 km. The northern extension of the Wairau Fault is considered to be another major fault and geological boundary trending NE-SW in central New Zealand.