SIMULATING THE POSTSEISMIC POROELASTIC AND VISCOELASTIC DEFORMATION OF THE M9 SUMATRA-ANDAMAN EARTHQUAKE: A NEXUS OF GEOLOGY AND GEOPHYSICS

The Great M9 Sumatra-Andaman Earthquake of 2004 ruptured the interface of the subducting Indo-Australian plate and overriding Burma microplate. The earthquake induced a devastating tsunami and generated stress and pore pressure changes that triggered numerous aftershocks, including an M8.7 event, in the near-field region. Finite element models (FEMs) are constructed to simulate the coseismic slip and ongoing postseismic deformation of the M9 earthquake. These FEMs are designed to honor the known geologic structure, results of petrophysical laboratory experiments, seismic tomography, and gravity data associated with the subduction zone. FEM-based inverse analyses of near-field GPS data provide an optimized slip distribution of the earthquake. Forward models, driven by this slip distribution, predict the postseismic poroelastic and viscoelastic deformation of the earthquake. Preliminary results suggest poroelastic deformation may be up to several tens-of-centimeters in offshore regions, although predicted poroelastic displacements for existing GPS sites are generally a few centimeters. Predicted regions of postseismic pore pressure recovery correlate to observed aftershock swarms, in accord with the poroelastic formulation of Coulomb failure theory. Although the predicted magnitude of poroelastic deformation is resolvable by GPS measurements in the near-field region, viscoelastic deformation is the overwhelmingly dominant postseismic deformation mechanism. Results of this study suggest that (1) Assessments of aftershocks of the M9 Sumatra-Andaman earthquake must account for postseismic poroelastic behavior, (2) Quantitative predictions of postseismic deformation for near-field GPS sites must be corrected for poroelastic deformation, and (3) More than a meter of viscoelastic deformation is expected for near-field GPS sites over the next decade. Future challenges to be addressed by this study include calibrating permeability and viscosity parameters to GPS measurements of transient deformation.