The state of pore fluid pressure and 3D megathrust earthquake dynamics

The importance of pore fluid pressure (Pf) for fault strength, stress state and slip behavior holds promise for explaining spatio-temporal subduction zone megathrust behavior, but the coseismic state of Pf and its distribution with depth are poorly constrained. Here, we analyze fault stress states and 3D rupture dynamics of six scenarios based on the 2004 Mw 9.1 Sumatra-Andaman earthquake. We vary Pf from hydrostatic to lithostatic under two different gradients that result in depth-dependent versus constant effective normal stress on the seismogenic part of the megathrust. As Pf magnitude increases, fault strength, moment magnitude, cumulative slip, peak slip rate, dynamic stress drop and rupture velocity decrease. When Pf follows the lithostatic gradient, depth-constant effective normal stress results, as theoretically proposed. We find that such a near-lithostatic pore fluid pressure gradient shifts peak slip and peak slip rate up-dip. We study the dynamically modeled apparent co-seismic principal stress rotations and absolute post-seismic stress state. In all earthquake dynamic rupture scenarios, the mean apparent stress rotations are larger in the accretionary wedge than below the megathrust. Scenarios with higher Pf exhibit lower mean apparent principal stress rotations in the accretionary wedge. By comparison against observations of the 2004 Sumatra-Andaman earthquake, two preferred scenarios emerge. These support the presence of very high coseismic pore fluid pressure at 97 % of the lithostatic pressure, producing average shear and effective normal traction magnitudes of 4-5 MPa and 22 MPa, respectively. The mean dynamic stress drop for both scenario earthquakes is 3 MPa and the mean rupture velocity is 2400-2600 m/s, comparable to observations of the 2004 Sumatra earthquake. The heterogeneous post-seismic stress states in these scenarios are consistent with the variety of aftershock focal mechanisms observed after the 2004 earthquake. These two preferred scenarios differ in pore fluid pressure gradient and thus in slip on the shallow megathrust. Under conditions of very high pore fluid pressure that lead to weak megathrusts in terms of the low static shear strength and low dynamic friction during rupture, near-trench strength and constitutive behavior are crucial for megathrust hazard, as peak slip and peak slip rate occur at shallower depths. This condition also is consistent with observations that the stress drops of small earthquakes in subduction zones are only weakly depth-dependent.