Collaborative research: Unraveling coseismic and postseismic deformation: A prerequisite for analyses of stress-coupling and tsunami genesis

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The December 26, 2004 M9.2 Sumatra-Andaman Earthquake (SAE), the third largest earthquake ever recorded, ruptured the boundary separating the subducting Indo-Australian Plate from the overriding Burma Plate and triggered a devastating tsunami that significantly impacted 9 countries bordering the Indian Ocean. Near the epicenter, the tsunami caused over 30 meters of run-up in some coastal areas of Northern Sumatra, almost instantly killing over 200,000 people. The rupture of the SAE included more than 20 meters of fault-slip, based on associated seismologic data and GPS measurements of seafloor and ground deformation. This magnitude of deformation provides a rare opportunity to conduct a regional-scale in-situ rheological experiment, in which the coseismic fault-slip is the impulse and the subsequent deformation is the response. Modeling these measured perturbations can test hypotheses of coseismic (including tsunami-genesis) and postseismic (including earthquake-coupling and tsunami run-up) behavior. Specifically, Finite Element Models (FEMs) of the subduction zone near the epicenter allow for a quantitative evaluation of the role of rheologic partitioning and processes, on the stress, strain, and pore pressure that govern coseismic and postseismic behavior. Tsunami propagation models can then use FEM-generated seafloor deformations to predict coastal run-up. This modeling and interpretive study of the SAE will address the following scientific questions: (1) How does the distribution of material properties (i.e., structure, density, porosity, and stiffness of rock formations in the subduction zone) affect fault-slip estimations ? (2) How does this distribution influence seafloor deformation, tsunami genesis, and run-up predictions ? (3) What is the timing and distribution of poroelastic and viscoelastic postseismic deformation ? (4) What afterslip is required ? (5) Do Coulomb stress and pore pressure transients correlate to aftershock occurrence ? These questions are underpinned by a more fundamental question: How do we construct and constrain models of coseismic and postseismic behavior as a synthesis of processes, all of which contribute to the deformational system ? Accordingly, the primary goal of the proposed research is to determine the distribution and calibration of rheologic properties that describe coseismic and postseismic behavior of the SAE. More specifically, FEM simulations will address aftershock occurrence in both space and time (including stress-coupling between the SAE and the March 25, 2005 M8.7 Nias earthquake that occurred 350 km away from the SAE epicenter). FEM-generated seafloor deformation predictions will drive tsunami propagation simulations. Because we expect that variations in material properties (and possibly secondary splay faulting) will cause both long and shorter scale seafloor deformations, tsunami generation and propagation simulations will be performed with the dispersive long wave model FUNWAVE. Tsunami hazards will be expressed in terms of simulated run-up and inundation for the most affected areas of the Indian Ocean (e.g., Northern Sumatra), and compared to the observed impact of the 12/26/04 tsunami. This synoptic approach to simulating coseismic and postseismic deformational systems may significantly advance tectonic and tsunami coastal hazard assessment capabilities for the SAE and impact future assessments of similar mega-thrust earthquakes for other subduction zones hosting high population densities, such as the upper US West Coast (Cascadia) and Japan. Techniques for designing and implementing FEMs will be disseminated to the scientific community during a workshop in the latter stages of this project. Students will use Abaqus software to construct FEMs that simulate fault-slip, which can be used in forward and inverse models of deformation and drive of postseismic processes, including poroelastic and viscoelastic deformation.