Role of faulting and gas hydrate in deep- sea landslides off Vancouver Island George Spence Collaborators include: Carol Lopez Ross Haacke Tark HamiltonMichael Riedel + many others Recipe for slumping: Lift, cut, shake, but maybe freeze first or

Storegga Slide : mother of all landslides mass failure area equiv to Iceland headwall ~250 km long runout ~800 km Multiple events (3?) oldest, biggest 250 ka most recent 8.2 ka

1929 Grand Banks earthquake (M 7.2), slump and tsunami tsunami : 28 deaths; observed in Portugal undersea cable breaks out to 500 km (turbidity currents) failure area 20,000 km 2, sed vol km 3 (thickness ~5 m) (Fine et al. 2005)

1998 Papua New Guinea earthquake (M 7.1) and tsunami tsunami : 2200 deaths tsunami source : motion on low-angle fault plus slump

slump amphitheatre Papua New Guinea slide (Synolakis et al. 2002) slump sediment volume only 1-4 km 3 (max thickness 600 m)

Cascadia margin, Vancouver Island Swath bathymetry, U Washington 2004

U1326 U1326 : IODP drilling, 2005

U1326

Deformation front Basin Sediments Accretionary Prism Sediments Oceanic Crust BSR (Bottom Simulating Reflector) Base of gas hydrate Cascadia margin setting

Methane Hydrate Structure Carbon + hydrogen (centre) trapped in ice lattice

Multichannel seismic Hz BSR

U1326 : array of ocean bottom seismometers

U1326 downhole log high-vel: hydrate BSR at mbsf OBS high-vel ( mbsf) OBS velocities

Depth (km) Final Velocity Model – Line 2 U1326 : High vel: hydrate? high-vel shallow hydrate layer extends laterally for 4-6 km BSR depth well-constrained at mbsf BSR

seismic reflection lines slump

Line 13Line 21 slump NW SE 2.4 s 3.0 s BSR Scarps: up to 75 m high

Margin-perpendicular faults : extensional, with motion parallel to least-compressive stress direction

What produced these margin-perpendicular faults? extension cracks

Expansion cracks on ridge are due to longitudinal flexure, i.e. tension on outside edge tension compression Better analogy : bend a baguette

Lateral extent of slump controlled by margin-normal faults

Reconstruct original ridge by interpolating across slump: C to A

Vertical extent of slump coincident with base of hydrate Volume of slumped material : 0.6 km 2

Slump Mechanisms 1.Gas hydrate dissociation 2.High pore fluid pressures 3.Contrasting seds & physical properties, e.g. glacial vs. de-glacial vs. interglacial 4.Earthquakes

Hydrate may increase sediment strength by cementing grains (but increase depends on how hydrate is distributed, and how much hydrate is present) Is there coincidence between glide plane and base of hydrate? 1.

High fluid flux (e.g. high sed rates; compaction at convergent margin) produces high pore pressures High pore pressure reduces sed strength (i.e. reduces grain-to-grain contact) Frontal ridge is region of greatest deformation and greatest fluid flux 2. High pore fluid pressures

Overpressure at decollement decollement slope seds Mounds and slumps, offshore Nicaragua (Talukder et al. 2008)

3. Contrasting sed properties Coring program Aug 2008 : Haacke, Riedel, Pohlmann, Hamilton, Enkin, Rose, and others key core

Key core at intersection of headwall and glide plane Bottom of core contains older seds, much stiffer and stronger than overlying seds found found everywhere else, which are likely weak de-glacial deposits (~14 kyr) Top of stiff sediments may provide the glide plane.

4. Earthquakes acceleration-induced sliding earthquakes may produce excess pore pressures Coring cruise Aug 2008 : series of turbidites found overlying the slumped deposits, which is comparable to the number of earthquakes since last glacial period, i.e. consistent with slumps occuring at de-glacial time

Ridge on slope off Van Is Original data bubble pulse BSR

Predictive deconvolution bowtie

Migration