[S-CG45] Science of slow-to-fast earthquakes

[SCG45-24] Fluid pressure and frictional-viscous transition in megathrust accounting for dehydration fluid production: Case study in Cascadia and implications for slow earthquakes

*小澤 創^1,2、Dunham Eric²、Condit Cailey³ (1.東京大学地震研究所、2.スタンフォード大学、3.ワシントン大学)

Relative plate motion in subduction zones is accommodated by frictional slip in the shallower part and viscous flow in the deeper part. The frictional-viscous transition can control the depth extent of megathrust earthquakes and episodic tremor and slip (ETS). Fluid pressure is a critical component controlling the transition, but is commonly assumed as a tuning parameter to explain the observational data due to large uncertainties. Here, we present a modeling framework to calculate the steady-state fluid pressure and shear stress profile along the subduction interface. We consider fluid production from dehydration reactions of the oceanic lithosphere, which is calculated from thermodynamic equilibrium using the temperature-pressure path of the subducting slab. The fluid pressure is calculated from Darcy's law, and the permeability depends on the effective stress, temperature, and slip rate. The shear stress and the fraction of friction and viscous slip are calculated from the rate-state friction and thermally activated linear viscous flow law. We apply the modeling framework to the Cascadia subduction zone. Our results show that the effective stress in the seismogenic zone is nearly uniform, below which it decreases with depth. We can constrain the range of several parameters, such as the width of the fluid transport zone and the hydrated oceanic crust, with some trade-offs to match the observationally constrained shear stress in the seismogenic zone. The friction-viscous transition spans a wide range of depths, including the ETS source depth. Fluid loss at the mantle wedge corner or the presence of locally low-permeability rocks produces a non-monotonic friction-viscous transition, which may explain the gap between the seismogenic zone and the ETS zone in Cascadia. Our results provide important insight into the earthquake hazard and mechanism of ETS in Cascadia, but the modeling framework is applicable to global subduction zones.