Modelling seismic anisotropy variations across the Hikurangi subduction margin, New Zealand

Anisotropy measurements across subduction zones have produced controversial interpretations related to the mechanisms of plate tectonics. Here we use anisotropic teleseismic wave propagation in two-dimensional finite difference models and four types of analytical modelling to explain previously-determined shear-wave splitting along the Hikurangi subduction margin that suggested high anisotropy with rapid lateral changes. scale models of the subduction zone (hundreds of kilometres) incorporating four main anisotropic domains; the subslab, the slab, the mantle wedge and the far backarc region, result in synthetic shear-wave splitting measurements that closely resemble all large-scale features of real teleseismic observations across the central North Island of New Zealand. The preferred model constrains high (15%) anisotropy to the mantle wedge down to about 100 km under the Central Volcanic Region (CVR), bound to the west by an isotropic region under western North Island; the slab is isotropic and the subslab region has average (3.5%) anisotropy, down to 300 km. The unusually high anisotropy in the mantle wedge is attributed to the presence of aligned melt. fluence of melt on seismic anisotropy is examined with different small-scale (tens of kilometres) analytical modelling approaches calculating anisotropy due to melt occurring in aligned inclusions, such as cracks or bands. The models confirm that small amounts (1–2%) of melt can account for the strong anisotropy required for the wedge region in the preferred large-scale model. Additionally, we show that aligned melt inclusions on the order of tens of meters can provide a suitable explanation for the observed weak trend towards decreasing delay times with higher filtering frequency for teleseismic phases and the differences in local and teleseismic fast polarisations, which both sample the highly anisotropic mantle wedge.