Three-dimensional anisotropic P-wave velocity structure around the shallow plate boundary in the Northern Hikurangi margin, New Zealand

We present three-dimensional P-wave velocity (Vp) model in the northern Hikurangi subduction zone offshore Gisborne, New Zealand, using seismic refraction data from the NZ3D OBS experiment. The study area is well known to host active slow slips at the shallow plate boundary (<10 km below sea level) and provides an ideal location to understand the relationship between the structural and hydrogeologic properties along the megathrust faults and shallow slow slips.

The Vp model constructed by traveltime tomography using >880,000 first arrivals reveals significant structural heterogeneities. In the forearc region, we find a ~30-km-wide low-velocity (Vp=1.8-4.0 km/s) accretionary wedge, within which the previous MCS reflection data suggest that several active thrust faults branching from the plate interface are developed. Further landward, less-deformed, higher-velocity (Vp=2.0-5.0 km/s) geological units form the backstop. Beneath the accretionary wedge, another low-velocity (Vp~5.0 km/s) volume is imaged along the plate interface. Since the velocity values roughly correspond to Cretaceous volcaniclastic rocks sampled from the incoming Pacific plate, the structure may represent thickly underplated sedimentary material.

Additional structural information can be obtained from P-wave anisotropy that is sensitive to fault orientation and stress field. In the study area, most OBSs in the forearc show a consistent azimuthal variation in traveltime residuals, but this feature is not obvious on the seaward side of the trench. A preliminary estimation suggests that there is a significant azimuthal anisotropy of up to ~4% within the forearc wedge. The fast direction of the Vp is roughly oriented to the trench-normal direction, indicating that the Vp anisotropy is primarily controlled by the regional stress regime associated with the plate subduction.