Azimuthal anisotropy and 3-D shear-wave velocity structures of North Island, New Zealand.

Abstract:

We invert Rayleigh-wave group-velocity dispersion data to calculate isotropic and anisotropic anisotropy of wavespeeds between 6 50 second periods in North Island, New Zealand. This is a region with a subduction system including back-arc spreading as well as upper plate block rotation. We generate interstation empirical Green's Functions from stacked ambient noise cross-correlations recorded from station pairs of the GeoNet network and temporary experiments. Measurements of fundamental-mode Rayleigh-wave dispersion are made using multiple-filtering and frequency-time analysis and manual selection of the dispersion curves. This collection of dispersion curves is then inverted for lateral variations in both isotropic and azimuthally anisotropic wavespeeds at discrete periods using an LSQR method with lateral smoothing and slight norm and gradient damping. Each discrete period provides a different depth sensitivity range, yielding information on depth dependence of the velocity variations. We then use the isotropic solutions to develop a 3D shear wave velocity model for each node of a 50 km triangular grid. We define an anomalous region of slow velicities accompanying trench-perpendicular fast-propagation under the east coast, near the shallow Hikurangi subduction zone. The structure appears at periods of about 21 seconds and is most prominent at periods of about 28 seconds. Toward the western North Island, farther from the trench, the isotropic group velocity and shear-wave velocity is relatively fast with the fast-propagating axis parallel to the trench direction. The isotropic dispersion curves extract from our model are strongly suggestive of a thin (<10km) low-velocity zone within or above the subducting slab which is also present in our 3D shear-wave model.