PORE PRESSURE RECOVERY FOLLOWING THE 2004 SUMATRA-ANDAMAN EARTHQUAKE ADVANCED THE TIMING OF THE 2005 NIAS EARTHQUAKE

The 2004 M9.2 Sumatra-Andaman earthquake (SAE) and 2005 M8.7 Nias earthquake (NE) ruptured adjacent portions of the plate boundary interface of the Sumatra-Andaman subduction zone. The SAE ruptured an approximately 1200-km-long segment of the plate boundary interface, from near Simeulue Island northward through the Andaman Islands. About three months later, the NE ruptured an adjacent segment, just to the south of the M9.2 rupture. The spatiotemporal separation of the SAE and NE suggests a poroelastic delay and triggering mechanism. The spatial and temporal proximity of these events suggests that the first earthquake (SAE) affected the timing of the second earthquake (NE). We use 3D finite element models (FEMs) to simulate coseismic and postseismic deformation and predict the evolution of Coulomb stress following the SAE in an effort to assess the advance in timing of the NE due to the SAE. The FEMs are designed to account for the distribution of rheological properties of the subduction zone, as constrained by seismic tomography and gravity data. The slip distribution of the SAE is calibrated to near-field GPS stations and recovers the general pattern of slip suggested by seismologic data. Coulomb stress is the change in tendency for slip to occur along a fault and is defined as Δσ_c = Δσ_s + f(Δσ_n + ΔP), where σ_c is Coulomb stress, σ_s is shear stress, f is friction, σ_n is normal stress, and P is pore pressure. Conceptually, the coseismic slip of the SAE relaxes accumulated stress along the fault and introduces relatively instantaneous changes in the stress and pore pressure fields in the near-field region. Following the earthquake, the pore pressure re-equilibrates via fluid flow and generates postseismic poroelastic deformation. This calibrated FEM predicts that the SAE initially increased Coulomb stress near the hypocenter of the NE. Pore pressure increased by about 2 MPa during the three-month interval after the SAE and leading up to the NE. This rapid increase in pore pressure (and Coulomb stress) advanced the timing of the NE.