UPPER PLATE RIGIDITY AND SHALLOW SUBDUCTION ZONE SLIP STABILITY IN DATA-CONSTRAINED SEISMIC-CYCLE MODELS OF THE CENTRAL HIKURANGI MARGIN, NEW ZEALAND

As near-trench geodetic resolution has improved globally, transient slow-slip events have been observed on the shallowest portions of many subduction megathrusts, including the Hikurangi subduction zone in New Zealand. Laboratory friction experimental studies have suggested that slow-slip events occur near stability transitions in the rate-and-state frictional properties of the fault rocks (velocity-strengthening to velocity-weakening). Numerical models of seismic-cycle-timescale fault slip in a rate-and-state frictional framework have reproduced slow-slip behavior on highly overpressured faults with near-velocity-neutral frictional properties in a homogeneous elastic medium, however these models typically do not incorporate heterogeneous upper plate structure or strength. Here, we incorporate heterogeneous upper plate shear rigidity and pore fluid pressures derived from full-waveform inversion of seismic reflection data from central Hikurangi into rate-and-state friction numerical models to explore the influence of realistic upper plate mechanical properties on shallow subduction fault slip. These high-resolution seismic images highlight downdip variations in pore fluid pressure (estimated λ* = 0.5 – 0.89) in the shallow subduction interface that align with splay faults in the wedge, suggesting that splay fault-modulated dewatering controls the distribution of pore fluids in the shallow fault zone. Seismic inversions also show that the upper plate stiffens with depth and distance from the trench, with shear moduli ranging from 2-13 GPa in the upper 7 km. Faults in rate-and-state friction continuum models based on these properties tend to slip unstably due to the relatively low overpressures and low shear rigidity. Only modeled faults with nearly velocity-neutral properties produce episodic slow-slip events, suggesting that rate-and-state friction alone may not explain observed shallow aseismic slip transients at subduction zones. Finally, we discuss implications of these models and some other processes that may influence shallow subduction zone slip stability, such as cyclic fluid pressurization.