First, a general comment on strategy for offshore observatories

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First, a general comment on strategy for offshore
observatories

1. Choose best location
2. Choose appropriate and robust instrumentation
3. Make continuous observations
4. Be patient

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Using pressure as a proxy for strain A review of
ODP/IODP CORK technology and observationsEarl
DavisPacific Geoscience Centre, Geological
Survey of CanadaCollaborators, K. Becker, B.
Carson, J. Cowen, A. Fisher, H. Jannasch, M.
Kastner, M. Kinoshita, R. Macdonald, R. Meldrum,
H. Mikada, T. Pettigrew, D. Saffer, E. Screaton,
R. Thomson, H. Villinger, K. Wang, G. Wheat
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Objectives- determine natural-state
temperatures, pressures, and fluid compositions-
driving forces for fluid flow- routes and rates
of flow- formation elastic and hydrologic
properties (compressibility, permeability)-
temporal variations in formation state and
properties
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Hydrologically active targetsridge axes ridge
flanks gas hydrates subduction zones
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Examples of CORK configurations(Circulation
ObviationRetrofitKit)
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Original CORK single-level monitoring
inpre-existing or new cased holes
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AdvancedCORKformulti-levelmonitoring
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Wireline installationsin pre-existing holes
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Early experiment Constraining the vigorof
hydrothermal circulation in oceanic crust
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hence CORK monitoring experiment
Tides filtered, Hole 1027C
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Equilibrated temperatures and pressures
c. 3K
c. 2 kPa
Bottom line permeability gt 10-10 m2, lateral
fluid flux gt 30 m yr-1, both very high
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Oceanographic pressure variabilityA very
broad-band loading signal
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Formation response to variable loading
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Strain / pressure conversion efficiency(few kPa
gt 10-6 strain)
Sensitivity of pressure to volumetric strain

• Controlling parameters
• - porosity
• - matrix compressibility
• - fluid/gas
• compressibility
• - Poissons ratio
• solid compressibility
• permeability
• viscosity

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Examples of co-seismic strain events observed in
the Juan de Fuca region
04
99
04
01
857
05
99
1024
1025
1026/1027
100 km
04
99
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Co-seismic strain associated withseafloor
spreading event,Juan de Fuca Ridge flank,June
1999
raw record
seafloor loading removed
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Dislocation model with lateral drainagequantifies
seismic energy deficiency and confirms high
permeability
Instantaneous (elastic) strain for moment 4.3 x
1017 N m (Mw 5.8) vs. Observed Seismic
moment 2.6 x 1016 (Mw 4.6, 4.8)
Post-deformation drainage for k 10-9 m2
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Near-field strain associated witha seafloor
spreading event,Juan de Fuca Ridge axis (Middle
Valley)
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Episodic signals at subduction zones expected
from modeling results(LaBonte, Brown, and
Fialko, 2008)
Seafloor deformation, fluid flux
Instantaneous volumetric strain, fluid pressure
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Observations of seismic and tectonic loading at
subduction zones
ODP Site 1200
Mariana forearc
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Strain from seismic momentc. 10-8Strain from
pressure at 5 kPa / µstrainc. 10-6
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Onshore deformation and contemporary CORK events
at Costa Rica subduction zoneno seismicity
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Nankai subduction zoneHoles 1173B and 808I
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Secular change at Nankai subduction
zoneInterseismic secular strain?
Accretionary prism toe - screens 1 and 2
low permeability - screen 6 high
permeability (drained)
Incoming Philippine Sea plate - screens 1 - 4
isolated or low permeability - screen 5
high permeability (drained)
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2003 VLF seismic activity off Cape Murotoand
transient borehole response
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Sense of strainmight make sense,but
magnitude(0.6 kPa 10-7 strain at 1173)is once
again too large (moment 1019 N m, or Mw 6.8)
Hypothetical slip event
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A subsequent event 2005 intra-slab earthquakes
off Kii Peninsula

Sites 808/1173
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Observed and modeled pressures for off-Kii
earthquakes
Strain estimated from observed pressure 2 x
10-7 Strain predicted by dislocation model 2.5
x 10-8
Strain for Mw 7.5 dislocation
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A fringe benefit Observations of pressure
response to seismic waves Sumatra seen at Juan
de Fuca Ridge flank Hole 1026B
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A fringe benefit seafloor pressure as a proxy
for vertical deformationRecords from northern
Juan de Fuca Ridge
Expanded view of formation pressure reveals
coherent temporal hydrologic variations
Expanded view of seafloor pressure reveals
history of subsidence and uplift
c. 2 m in 3 years
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Thoughts for the future
S-CORK simple single-trip deployment system
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opportunistic approaches
Strap-CORK
Super-simple CORK
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and cable connections to shore will obviate
submersible servicing,permit high-frequency
real-time monitoring, and facilitate systematic
observatory transects and arrays
NEPTUNE Juan de Fuca
DONET - Nankai
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Some concluding thoughts

• -- Pressure changes in hydrologically isolated
  formations serve as an easy-to-measure
  quantitative proxy for strain
• -- Resolution now c. 10 x 10-9 full-scale (c. 0.5
  Pa) at frequencies up to 1 Hz
• -- P / strain response 5 kPa / ustrain
• -- Thus P resolution of few Pa yields ideal
  strain resolution 0.1 nannostrain
• -- Real limits are imposed by oceanographic
  loading (10-20 Pa) 10 nannostrain
• -- Examples of strain generated by seismic,
  sub-seismic, and
• aseismic events observed to date show clear and
  puzzlingly large signals
• -- Near-field complexities demand multiple
  observation points
• -- Opportunities for future observations include
  cable network connections and simplified or
  opportunistic installation schemes

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Caution must be used to minimize system compliance
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and watch out for leakage !!(monitor
temperature)
effect of variable vertical flow along
geothermal gradient 1 kPa / mK
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