P-wave velocity structure and stress state of shallow subduction faults in the northern Hikurangi margin, New Zealand, from NZ3D OBS data

Abstract:

We present a high-resolution three-dimensional anisotropic P-wave velocity (Vp) model in the northern Hikurangi margin offshore Gisborne, New Zealand, constructed by tomographic inversion of over 430,000 first arrivals recorded during the NZ3D OBS experiment. The study area covers a region where shallow slow slip events occur repeatedly (Wallace et al., 2016) and the subduction of a seamount has been proposed (Barker et al., 2018). This makes it an ideal location to link our understanding of structural and hydrogeologic properties at megathrust faults to slip behavior. We find evidence for fault-bound anisotropy where Vp azimuthal anisotropy is larger in the vicinity of the backstop interface, splay faults and the deformation front than it is in other areas. The localized anisotropy of over 5 % and its regional pattern suggest that the primary cause of the anisotropy is both preferentially oriented cracks and fault-bound clay-rich sedimentary layers. While the fast axes of Vp are mostly oriented in the trench-normal direction in the accretionary wedge, they are rotated to the trench-parallel direction on the seaward side of the trench and in the landward backstop. This regional variation is consistent with the results of shear-wave splitting analysis (Zal et al., 2020) and the directions of maximum horizontal stress inferred from the borehole breakouts at two drilling sites (U1518 and U1519) of IODP Exp372/375 (Wallace et al., 2019) located on the lower slope and mid slope of the accretionary wedge. We also found that most segments of the splay faults and the decollement produce high-amplitude wide-angle seismic reflections, suggesting that there may be low-velocity zones along the faults. Such fine-scale structures will be further examined by full-waveform inversion analysis.