Abstract
We study the effects of pore fluid pressure (Pf ) on the pre-earthquake,
near-fault stress state and 3D earthquake rupture dynamics through 6
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 MPa 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 up-dip. 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.