Three-dimensional modeling of slab geometry and shear strength along Hikurangi subduction interface, New Zealand: implications for slow slip events

Subduction zone megathrust faults host about 75% of the large earthquakes (M≥8) worldwide. Geodetic and seismological observations indicate that the subduction zone also exhibits various types of slow earthquakes including slow slip events (SSEs), low-frequency earthquakes (LFEs), very-low-frequency earthquakes (VLFEs), and tectonic tremor. Developing the spatial variations of 3D fault geometries and seismic slip behavior along subduction plate slab are important and contributes to the understanding of the mechanisms of large earthquakes in subduction zones, as well as to the improved forecasting of earthquakes and tsunamis in subduction zones. The Hikurangi subduction zone is located on the eastern margin of North Island, New Zealand, where the Pacific Plate subducts beneath the eastern North Island at a rate of 55 mm/yr. In north Hikurangi margin, the shallow plate boundary megathrust hosted two large magnitude earthquakes (Mw 7.0-7.2) in 1947 that produced 8 to 10 m tsunami along the coast of the North Island. Geodetic observations indicate that slow slip events (SSEs) vary along the margin: in the northern and central segments, they are shallow (<15 km), short (<1 month), and frequent (every 1-2 years), whereas in the southern segment, they are deeper (25-40 km), longer (>1 year), and less frequent (occurring every 5 years).

Here we used an implicit approach to combine multi-sourced data, including seismic reflection profiles, relocated seismicity, focal mechanism solutions and topography profiles to develop a new slab model for Hikurangi subduction margin. The Hikurangi slab model (HSM-1.0) provide the detailed 3d geometry of a ~750 km subducting slab with ~8 km resolution. The geometry of shallow slab varies along-strike, from a steep (5°-10°) southern part, to a gentle (~2°) central segment; and then an irregular (1°-5°) northern margin. The southern margin has the deepest (~110 km) transition zone, longest (~200 km) distance from transition zone to trench, shallowest (~250 km) seismogenic zone, and steepest (~77°) deep slab. The modeling results indicate that the curvature of the Hikurangi slab (10^-4) is two orders of magnitude higher than that of the global slab (10^-6), and it displays a more irregular slab morphology. The slow-slip event (SSEs) source area at the northern margin of the Hikurangi slab exhibits a wide range of curvature values, while the locked region at the southern margin shows relatively less variation. In the SSEs region, the maximum principal stresses (σ₁) to the fault plane are oriented at a high angle (>50–60°), whereas in the southern locked region, the maximum principal stresses (σ₁) are oriented at a lower to moderate angle (30–40°). The shear strength gradients along the subducted slab suggest that the northern SSEs source region is relatively spatially heterogeneous, while the southern locked region demonstrates greater spatial homogeneity. The HSM-1.0 will facilitate fault system analysis, subducted slab reconstructions, and dynamic rupture modeling in the Hikurangi margin.