HR: 0800h
AN: S51B-0146    [Abstracts]
TI: Modeling Seismically Induced Deformation And Fluid Flow In The Nankai Subduction Zone
AU: * Ge, S
EM: ges@spot.colorado.edu
AF: University of Colorado, Department of Geological Sciences, Boulder, CO 80309 United States
AU: Screaton, E J
EM: screaton@ufl.edu
AF: University of Florida, Department of Geology, Gainesville, FL 32611 United States
AB: Fluid pressure changes induced by seismic strains in the Nankai subduction zone were investigated through numerical modeling. Seismic strains resulting from dislocations along fault surfaces were coupled to pore pressure generation, and subsequent transient fluid flow was simulated. This study is distinct from previous efforts that modeled homogeneous systems. Effects of variable mechanical and hydrologic properties on the coupled hydromechanical system were investigated by assigning different mechanical and hydrological properties to marine sediments, the decollement zone, and the upper oceanic crust. Seismicities of varying magnitude were simulated by discrete dislocation events. Model sensitivity studies suggest that for a reasonable range of parameter scenarios, transient pressure head signals caused by discrete dislocations of a few meters in the updip region of the seismogenic zone can be observed over large areas of the margin from near the seafloor to deep in the crust. Compressibility, a parameter responsible for the co-seismic response of pore pressures to strain, was varied from 10E-11 1/Pa for the crust below the decollement to 10E-9 1/Pa for the sediments above the decollement. Pressure head changes on the order of tens of meters were observed, while larger head anomalies exist near the tips of the dislocated segments. The duration for the seismically induced pressure head to dissipate depends on the hydraulic diffusivity of the system, which is a function of permeability as well as storage. Permeability in sediments wedge was varied with depth from 10E-15 m2 near the seafloor to approximately 10E-21 m2 at 10 km below seafloor. Upper oceanic crust below the decollement was assumed to have permeability on the order of 10E-12 m2. A scheme was adapted to allow permeability of the decollement to increase after dislocation. Preliminary modeling results suggest that post-seismic permeability changes involving a small region of the dislocation zone would have a limited effect on the overall picture of the hydrologic regime. Permeability increases in larger areas of the decollement, however, would shorten the pressure dissipation periods. Most pressure head dissipation takes place within the first tens to thousands of years following the seismic events. As more monitoring data become available, the coupled mechanical and hydrologic model can be further utilized to assess hydrologic responses of the seismogenic zone processes and provide an additional means to infer large-scale mechanical and hydrogeological parameters of the subduction zone.
DE: 3022 Marine sediments--processes and transport
DE: 3210 Modeling
DE: 1829 Groundwater hydrology
SC: Seismology [S]
MN: 2004 AGU Fall Meeting