HR: 08:45h
AN: T41A-04    [PDF]
TI: Fluid Pressure in the Shallow Plate Interface at the Nankai Trough Subduction Zone
AU: * Tobin, H J
EM: tobin@nmt.edu
AF: Earth and Environmental Science Dept., New Mexico Tech, Socorro, NM 87801 United States
AU: Saffer, D
EM: dsaffer@uwyo.edu
AF: Dept. of Geology and Geophysics, University of Wyoming, Laramie, WY 82071 United States
AB: The factors controlling the occurrence, magnitude, and other characteristics of great earthquakes is a fundamental outstanding question in fault physics. Pore fluid pressure is perhaps the most critical yet poorly known parameter governing the strength and seismogenic character of plate boundary faults, but unfortunately cannot be directly inferred through available geophysical sensing methods. Moreover, true in situ fluid pressure has proven difficult to measure even in boreholes. At the Nankai Trough, several hundred meters of sediment are subducted beneath the frontal portion of the accretionary prism. The up-dip portion of the plate interface is therefore hosted in these fine-grained marine sedimentary rocks. ODP Leg 190 and 196 showed that these rapidly-loaded underthrust sediments are significantly overpressured near the deformation front. Here, we attempt to quantitatively infer porosity, pore pressure, and effective normal stress at the plate interface at depths currently inaccessible to drilling. Using seismic reflection interval velocity calibrated at the boreholes to porosity, we quantitatively infer pore pressure to $\sim$ 20 km down-dip of the deformation front, to a plate interface depth of $\sim$ 6 km. We have developed a Nankai-specific velocity-porosity transform using ODP cores and logs. We use this function to derive a porosity profile for each of two down-dip seismic sections extracted from a 3-D dataset from the Cape Muroto region. We then calculate pore fluid pressure and effective vertical (fault-normal) stress for the underthrust sediment section using a compaction disequilibrium approach and core-based consolidation test data. Because the pore fluid pressure at the fault interface is likely controlled by that of the top of the underthrust section, this calculation represents a quantitative profile of effective stress and pore pressure at the plate interface. Results show that seismic velocity and porosity increase systematically downdip in the underthrust section, but the increase is suppressed relative to that expected from normally consolidating sediments. The computed pore pressure increases landward from an overpressure ratio ($\lambda$* = hydrostatic pressure divided by the lithostatic overburden) of $\sim$ 0.6 at the deformation front to $\sim$ 0.77 where sediments have been subducted 15 km. The results of this preliminary analysis suggest that a 3-dimensional mapping of predicted effective normal stress in the seismic data volume is possible.
DE: 3025 Marine seismics (0935)
DE: 5102 Acoustic properties
DE: 5114 Permeability and porosity
DE: 8105 Continental margins and sedimentary basins
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2003 Fall Meeting