HR: 1340h
AN: T13B-0465    [Abstracts]
TI: Hydrologic Monitoring in the Nankai Accretionary Prism
AU: * Hucks, A
EM: ahucks@geosc.psu.edu
AF: Penn State University, Department of Geosciences 305 Deike Building, University Park, PA 16802 United States
AU: Flemings, P B
EM: flemings@geosc.psu.edu
AF: Penn State University, Department of Geosciences 305 Deike Building, University Park, PA 16802 United States
AU: Becker, K
EM: kbecker@rsmas.miami.edu
AF: University of Miami, Rosenstiel School of Marine and Atmospheric Science 4600 Rickenbacker Causeway, Miami, FL 33149 United States
AU: Kinoshita, M
EM: masa@jamstec.go.jp
AF: IFREE, Japan Agency for Marine-Earth Science & Technology 2-15 Natsushima-cho, Yokosuka, Kanagawa, 237-0061 Japan
AB: Pressures have been monitored since 2001 at two Ocean Drilling Program borehole hydrologic observatories in and near the Nankai Accretionary prism off southwestern Japan: the Site 1173 observatory lies in the Philippine Sea plate, and the Site 808 observatory lies in the seaward part of the Nankai accretionary prism. The 730-meter deep installation at Site 1173 has five screened intervals. Interval 1 (722 - 727 mbsf) is the deepest zone and communicates with the ocean crust. Intervals 2 (563 - 569 mbsf), 3 (439 - 444 mbsf), 4 (396 - 401 mbsf), and 5 (353 - 358 mbsf) lie within the lower Shikoku mudstone. The 970-meter deep installation at Site 808 has six intervals. Interval 1 (919 - 925 mbsf) lies in the lower Shikoku unit, about 10 meters above the d‚collement zone. Intervals 2 (876 - 881 mbsf) and 3 (830 - 836 mbsf) also lie in the lower Shikoku unit. Interval 4 (785 - 790 mbsf) lies in the upper Shikoku cemented mudstone, and intervals 5 (531 - 537 mbsf) and 6 (368 - 374 mbsf) lie in trench deposits. The pressure records at all screened intervals oscillate due to tidal loading. After valve closure at Site 1173, mean pressures at intervals 2, 4, and 5 rose above hydrostatic by 65 kPa over one week (1 to 3% of hydrostatic effective stresses). The tidal signals at these intervals all lead the tidal signal at the seafloor by a phase shift of ~5 degrees, whereas the tidal signals at intervals 1 and 3 lag by 25 and 20 degrees, respectively. The tidal loading efficiencies (interval amplitude divided by sea floor amplitude) are ~1 except in interval 2 (~0.55) and interval 3 (~0.25). After valve closure at Site 808, mean pressures at intervals in the lower Shikoku unit all rose above hydrostatic. Interval 2 saw the greatest increase: 100 kPa above hydrostatic, or ~1% of hydrostatic effective stress. The tidal signal at Interval 1 is in phase with the seafloor tidal signal. At intervals 2 and 3, it leads by about 20 and 40 degrees, respectively. The tidal signals at intervals 4, 5, and 6 (above the lower Shikoku) lag the seafloor signal by 35, 10, and 5 degrees, respectively. The loading efficiencies of Intervals 1-6 are, respectively, ~1, ~0.1, ~0.1, 0.6, 1, and 1. At Site 808, the monitoring intervals in the lower Shikoku unit behave unlike the other intervals - their tidal signals have significant phase leads, and the loading efficiencies are low. A major pressure transient was observed at these intervals and at interval 4 in the upper Shikoku unit in summer 2003. These transients are characterized by a near-instantaneous increase in fluid pressures followed by a gradual decay to background pressures. The intervals with low tidal loading efficiencies have the highest peak pressure responses during this event, suggesting that intervals with low tidal loading efficiency are in good communication with the formation.
DE: 8104 Continental margins: convergent
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8164 Stresses: crust and lithosphere
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8194 Instruments and techniques
SC: Tectonophysics [T]
MN: Fall Meeting 2005