Developing a 3D Model Geometry of the Hikurangi Subduction Zone

The Hikurangi subduction zone, located off the east coast of the North Island of New Zealand, has not seen a large (MW ≥8.0) megathrust earthquake in recent history. This does not mean that great earthquakes are not possible on the subduction thrust. The seismic hazard of the Hikurangi margin can be assessed by determining where on the plate contact slip deficit is accumulating during the interseismic period. This can be done by comparing surface velocities produced by a 3D mechanical model to actual GPS data, changing locking configurations to determine the best fit. This study is the first part of a multi-step project on interseismic coupling at the Hikurangi Subduction Zone. This first part focuses on building a 3D model geometry that represents the Hikurangi margin. This geometry was constructed using Gmsh (Geuzaine and Remacle, 2009). The subducting slab is based on Slab2 (Hayes et al., 2018) and varies along both strike and dip. The overriding plate is simplified, with constant thickness and no topography. The geometry also includes a cold nose, mantle wedge and sub slab mantle. To test the geometry, two models are compared to each other, one where the subducting slab is stably sliding without resistance and another where a circular feature on the megathrust with a diameter of 50 km is coupled to the overriding plate or ’locked’. Displacement and slip on the megathrust are considered as well as the accumulation of shear traction on the slab’s surface. Finally horizontal surface velocities produced by locking are compared to GPS velocities. The results show that the locking is functional and that locking has a far-reaching, mechanically continuous influence on displacement in the model. Overall it can be said that the model geometry is ready for the next step of this project.