Seismic azimuthal anisotropy of New Zealand revealed by adjoint-state traveltime tomography

Abstract

The omission of seismic anisotropy in current reference models covering the entire New Zealand has been an obstacle to achieving a comprehensive understanding of deformation and dynamics along the complex Pacific-Australian plate boundary segment. Here we present a 3D azimuthally anisotropic model that encompasses both the North and South Islands of New Zealand to a depth of 40 km, using over 1 million local P-wave arrival times and a newly developed adjoint-state traveltime tomography technique. This model is built upon the New Zealand-wide 3D isotropic velocity model, serving as an essential and incremental update to the existing model. Our new model highlights significant variations in anisotropy across the plate boundary region, indicating distinct deformation states between tectonic blocks. In the North Island, pronounced along-strike changes in anisotropy are evident beneath the Hikurangi forearc, which could be attributed to variations in stress regime associated with the oblique plate convergence and changes in interseismic coupling of the subduction megathrust. The oblique plate motion further induces pure shear deformation in the middle to lower crust of the southern backarc, resulting in strong anisotropy with fast axes perpendicular to the principal axes of maximum horizontal compression. In contrast, seismic anisotropy in the central South Island primarily stems from the preferential alignment of minerals, notably within the Haast schist in the Otago block. However, anisotropy in the middle to lower crust of the northern South Island may represent inherited structures that originated during past southward subductions along the Gondwana margins at ∼100 Ma. Our new model offers valuable insights into the intricate geological processes occurring within the plate boundary region.