The State of Pore Fluid Pressure and 3‐D Megathrust Earthquake Dynamics

We study the effects of pore fluid pressure (Pf) on the pre‐earthquake, near‐fault stress state, and 3‐D earthquake rupture dynamics through six scenarios utilizing a structural model based on the 2004 Mw 9.1 Sumatra‐Andaman earthquake. As pre‐earthquake Pf magnitude increases, effective normal stress and fault shear strength decrease. As a result, magnitude, slip, peak slip rate, stress drop, and rupture velocity of the scenario earthquakes decrease. Comparison of results with observations of the 2004 earthquake support that pre‐earthquake Pf averages near 97% of lithostatic pressure, leading to pre‐earthquake average shear and effective normal tractions of 4–5 and 22 MPa. The megathrust in these scenarios is weak, in terms of low mean shear traction at static failure and low dynamic friction coefficient during rupture. Apparent co‐seismic principal stress rotations and absolute post‐seismic stresses in these scenarios are consistent with the variety of observed aftershock focal mechanisms. In all scenarios, the mean apparent stress rotations are larger above than below the megathrust. Scenarios with larger Pf magnitudes exhibit lower mean apparent principal stress rotations. We further evaluate pre‐earthquake Pf depth distribution. If Pf follows a sublithostatic gradient, pre‐earthquake effective normal stress increases with depth. If Pf follows the lithostatic gradient exactly, then this normal stress is constant, shifting peak slip and peak slip rate updip. This renders constraints on near‐trench strength and constitutive behavior crucial for mitigating hazard. These scenarios provide opportunity for future calibration with site‐specific measurements to constrain dynamically plausible megathrust strength and Pf gradients. Plain Language Summary Large volumes of fluid can lead to high‐pressures that weaken rocks in fault zones and influence earthquake rupture. While fluids are critical to understanding behavior at subduction zones, where the largest earthquakes in the world occur and where tsunami generation increases hazard, measuring fluid and fluid pressure directly across an entire megathrust currently is not possible. Here, we use supercomputers to model the devastating 2004 Mw 9.1 Sumatra‐Andaman earthquake in 3‐D in order to isolate the role of fluid pressure on earthquake behavior. By first building a reliable base model and then varying fluid pressure to generate six earthquake scenarios, we find that fluid pressure is likely very high, and als