HR: 1340h
AN: T53B-1426    [Abstracts]
TI: Fluid pressure, sediment compressibility, and secular and transient strain in subduction prisms: Results from ODP CORK borehole hydrologic observatories
AU: * Davis, E E
EM: edavis@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada, Box 6000, Sidney, BC V8L 4B2 Canada
AU: Becker, K
EM: kbecker@miami.edu
AF: Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149-1098 United States
AB: Instruments for long-term hydrogeological monitoring in Ocean Drilling Program boreholes have been installed in five subduction zone settings, including Cascadia, Barbados, Mariana, Costa Rica, and Nankai. Pressure records reveal a wide range of average formation states that are consistent with formation permeability and proximity to sources of formation fluid. For example, near-hydrostatic pressures (excess pore-pressure ratio λ* ~ 0) are observed in the silty parts of the Nankai accretionary prism and in the upper oceanic crust beneath the Costa Rica prism, where well-drained conditions are inferred to be present, and elevated pressures (λ* up to 0.5) have been recorded in finer-grained sedimentary sections near the toe of prisms (e.g., at the level of the decollement in the fine-grained part of the Barbados accretionary prism). In no instances have high pressures (approaching lithostatic, λ* = 1) been observed, although operational difficulties have thus far precluded installations in underthrust sediment sequences where the highest average pressures are expected to be maintained. Records often reveal non-steady behavior, with variations occurring over a broad frequency range. Tidal-frequency variations present in all records are the consequence of oceanographic loading at the seafloor. The amplitude of these signals provide constraints on formation compressibility. Estimated values vary with depth and consolidation state, and range from 5 x 10^-9 to 3.5 x 10^-10 Pa^-1. Once these signals are removed, other transients can be observed, including ones correlated with both seismic and aseismic deformation. Secular strain has been seen in hydrologically isolated parts of the formations at several sites. At the Mariana forearc site, seismic-frequency pressure variations and persistent positive pressure changes were observed at the time of two large (M_b ~ 7.0) deep (~ 70 km) earthquakes located roughly 200 km away; these signals are inferred to reflect local formation response to seismic surface waves. At the Nankai trough, positive and negative transients were observed within the toe of the accretionary prism and in the incoming Philippine Sea plate at the time of a swarm of very-low-frequency earthquakes further landward in the prism. The earthquakes and pressure transients may reflect strain that is the consequence of slow (sub-seismic) slip along a nearby part of the locked plate interface. A similar correlation between strain and pressure transients has been observed in the Costa Rica prism. In this case, deformation of the landward part of the prism, documented by GPS horizontal strain monitoring, generated no detectable earthquakes.
DE: 3021 Marine hydrogeology
DE: 3036 Ocean drilling
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005