EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Seismic anisotropy structure of the northern Hikurangi margin, New Zealand, and its significance for subduction fault systems

Ryuta Arai1, Shuichi Kodaira1, Stuart Henrys2, Nathan Bangs3, Koichiro Obana1, Gou Fujie1, Seiichi Miura1, Daniel Barker2, Dan Bassett2, Rebecca Bell4, Kimihiro Mochizuki5, Richard Kellett2, Valerie Stucker2, and Bill Fry2
Ryuta Arai et al.
  • 1Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan
  • 2GNS Science, Lower Hutt, New Zealand
  • 3Institute for Geophysics, University of Texas at Austin, Austin, USA
  • 4Imperial College London, London, UK
  • 5Earthquake Research Institute, University of Tokyo, Tokyo, Japan

The NZ3D OBS experiment performed in 2017-2018 in the northern Hikurangi margin off the east coast of North Island, New Zealand, provided the highest-resolution seismic refraction/wide‐angle reflection data with multi-azimuth ray coverage in subduction zones to date (Arai et al., 2020). The study area extending 60 km in the trench-normal direction and 14 km in the trench-parallel direction covers source regions of a variety of slow earthquake phenomena, such as shallow slow slip events and tectonic tremor (e.g., Wallace, 2020), and thus offers an ideal location to link our understanding of structural and hydrogeologic properties at subduction faults to slip behavior. We applied an anisotropic traveltime tomography analysis to this active-source dataset from 97 ocean bottom seismographs deployed with an average spacing of 2 km on four parallel lines and dense air gun shooting with a 25 m interval, and succeeded in quantitatively constraining the P-wave velocities (Vp) of the upper plate forearc and the subducting slab and their azimuthal anisotropy in three dimensions. The velocity models revealed some locations with significant Vp azimuthal anisotropy over 5 % near the splay faults in the low-velocity accretionary wedge and the deformation front. This finding suggests that the anisotropy is not ubiquitous and homogeneous within the upper plate, but more localized in the vicinity of active thrust faults. While the fast axes of Vp are mostly oriented in the trench-normal direction in the accretionary wedge, which is interpreted as results of preferentially oriented cracks in a compressional stress regime associated with the plate convergence, 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 IODP drilling sites (Wallace et al., 2019). The significant magnitudes of anisotropy may indicate that in addition to the crack orientation, clay-rich sedimentary sequences that stack and form coherent strata along the accretionary wedge also contribute to seismic anisotropy in the subduction margin.


How to cite: Arai, R., Kodaira, S., Henrys, S., Bangs, N., Obana, K., Fujie, G., Miura, S., Barker, D., Bassett, D., Bell, R., Mochizuki, K., Kellett, R., Stucker, V., and Fry, B.: Seismic anisotropy structure of the northern Hikurangi margin, New Zealand, and its significance for subduction fault systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1403,, 2022.