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Initiation and propagation of submarine sediment failure 14 July 2009 Schlanger Fellowship presentation at USAC summer meeting Robert Viesca Advisor: Prof. James R. Rice Harvard University

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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 earthquake Gulf of Mexico (e.g., Hurricane Ike, 2008) submarine landslides disrupting pipeline connections

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(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?

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(Total “Know-How” Series, 2007) I’ll focus on passive margins:

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Is the origin from complex 2D fluid flow? Expedition 308 (Gulf of Mexico): Sedimentation model Color contour: pore pressure, Line contour: effective stress 04P (MPa) (Modified from Behrmann et al. 2006) 0 1 2 543 0.5 km 5 km Site U1323 Site U1324 Here, low permeability lens brings high pore pressure near seafloor. strength there reduced below local shear stress ( strength ~ effective stress ) total stresspore pressure

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Force Block size (sediment depth) FxFx FzFz 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 flow rapid sedimentation Or can failure occur in a simple 1D model?

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More specifically… z Depth of failure predicted by 1D model With a strongly reducing permeability, trapped fluid creates overpressure in underlying sediments with steady flow q fluid flux seafloor permeability fluid viscosity ~sediment weight 0.01–10 mm/yr 10 –(11–14) m 2 perm. stress dependence

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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 D

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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? First movement Total loss of shear strength Weakening

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Crack length: a / L Unstable lengths First movement Total loss of shear strength

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slipping region Potential slip surface H Which direction will instability propagate? Slight downslope preference for rupture propagation. y x'x'

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Summary Here we explain how shallow slopes can slide. If movement begins locally, that localization can grow. If that grows to a certain size, the slope catastrophically slides. Catastrophic enlargement proceeds preferentially downslope.

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