initiation and propagation of submarine sediment failure 14 july 2009 schlanger fellowship...
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Initiation and propagation of submarine sediment failure
14 July 2009Schlanger Fellowship presentation
at USAC summer meeting
Robert ViescaAdvisor: Prof. James R. Rice
Harvard University
What have been some remarkable events?
Offshore Morocco, within 200kyr (Talling et al., 2007) debris flow deposit extending 1500km carrying ten-times annual river sediment output to ocean
Papua New Guinea 1999 (e.g., Tappin et al., 2008) submarine landslide implicated in tsunami following earthquakeGulf of Mexico (e.g., Hurricane Ike, 2008) submarine landslides disrupting pipeline connections
(Modified from MBARI bathymetry perspective)
What do submarine landslides look like?
Santa Barbara Basin: Gaviota slide and Goleta landslide complex
~5km~10km
For such shallow slopes, how can these slides exist?
Is the origin from complex 2D fluid flow?
Expedition 308 (Gulf of Mexico): Sedimentation model
Color contour: pore pressure,Line contour: effective stress
0 4P (MPa) (Modified from Behrmann et al. 2006)
012
5 4 30.5
km
5 km
Site U1323
Site U1324
Here, low permeability lens brings high pore pressure near seafloor.
strength there reduced below local shear stress
σ , σ = σ − p (strength ~ effective stress )
total stress pore pressure
Force
Blo
ck s
ize
(sed
imen
t de
pth
)
Fx
Fz T
Under rapid sedimentation, gravity overcomes strength at some depth.
This is possible because strength becomes constant.
In “dry friction” (or submerged & no flow) strength grows with depth.
no flowrapidsedimentation
Or can failure occur in a simple 1D model?
More specifically…
zq
Depth of failure predicted by 1D model
k =koexp(−σ /σ*)
ko
Permeability, k(σ )
Effective stress, σ
With a strongly reducing permeability, trapped fluid creates overpressure in underlying sediments with steady flow q
zx depends on
α =μq
′γ ko
fluid flux
seafloor permeability
fluid viscosity
~sediment weight
0.01–10mm/yr
10–(11–14) m2
σ * perm. stressdependence
′x
Presume some sediment shears:sediment compacts flow barrier.
Flow barrier weakens underlying sediment further shear.
Further shear compacts more sediment more weakening. Weakening (pore pressure increase)
How can movement start & grow?
z
Δp ~ Dμq
k
D
Precisely how does weakening (pore pressure increase) enlarge a shear zone?
Will that shear zone become unstable?
Force weakening with variable curvature K and magnitude W.
Two regions in frictional contact.
Frictional strength decays with slip.
Use fracture mechanics concepts to estimate critical length and loading:
When does movement become unstable?f
fpf = fp−wδ
w1
δ
W =0
W =1
WK
Firstmovement
Total loss of shear strength
′x
Weakening
′x
Cra
ck le
ngth
: a
/ L
W (=Rtfpτo
)
K =10 (=κL2fpτo
)1 5
L =Gτo
fpw
Unstable
lengths
Firstmovement
Total loss of shear strength
τo
σo
slipping region
Potential slip surfaceH
Which direction will instability propagate?
Slight downslope preference for rupture propagation.
y
x'
′x / H
Δσ ( ′x , t)
σ o
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