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TitleRole of gas hydrates in slope failure on frontal ridge of northern Cascadia margin
AuthorYelisetti, S; Spence, G D; Riedel, M
SourceGeophysical Journal International vol. 199, 2014 p. 441-458, (Open Access)
Alt SeriesEarth Sciences Sector, Contribution Series 20130502
PublisherRoyal Astronomical Society
Mediapaper; on-line; digital
File formatpdf
ProvinceWestern offshore region
AreaCascadia margin
Lat/Long WENS-127.1000 -126.8167 48.5667 48.4167
Subjectsfossil fuels; marine geology; geophysics; hydrocarbons; gas; hydrate; slope failures; slope stability; slope stability analyses; submarine features; submarine transport; landslides; structural features; faults; fractures; seismic interpretations; seismic surveys; seismic surveys, marine; tomography
Illustrationslocation maps; profiles; plots; photographs
ProgramMarine Geohazards, Public Safety Geoscience
Released2014 08 12
Several slope failures are observed near the deformation front on the frontal ridges of the northern Cascadia accretionary margin off Vancouver Island. The cause for these events is not clear, although several lines of evidence indicate a possible connection between the occurrence of gas hydrate and submarine landslide features. The presence of gas hydrate is indicated by a prominent bottom-simulating reflector (BSR), at a depth of ?265-275 m beneath the seafloor (mbsf), as interpreted from vertical-incidence and wide-angle seismic data beneath the ridge crests of the frontal ridges. For one slide, informally called Slipstream Slide, the velocity structure inferred from tomography analyses shows anomalous high velocities values of about 2.0 km s?1 at shallow depths of 100 mbsf. The estimated depth of the glide plane (100 ± 10 m) closely matches the depth of these shallow high velocities. In contrast, at a frontal ridge slide just to the northwest (informally called Orca Slide), the glide plane occurs at the same depth as the current BSR. Our new results indicate that the glide plane of the Slipstream slope failure is associated with the contrast between sediments strengthened by gas hydrate and overlying sediments where little or no hydrate is present. In contrast, the glide plane of Orca Slide is between sediment strengthened by hydrate underlain by sediments beneath the gas hydrate stability zone, possibly containing free gas. Additionally, a set of margin perpendicular normal faults are imaged from seafloor down to BSR depth at both frontal ridges. As inferred from the multibeam bathymetry, the estimated volume of the material lost during the slope failure at Slipstream Slide is about 0.33 km3, and ?0.24 km3 of this volume is present as debris material on the ocean basin floor. The 20 per cent difference is likely due to more widely distributed fine sediments not easily detectable as bathymetric anomalies. These volume estimates on the Cascadia margin are approaching the mass failure volume for other slides that have generated large tsunamis-for example 1-3 km3 for a 1998 Papua New Guinea slide.
Summary(Plain Language Summary, not published)
Slope failures are observed near the deformation front at the northern Cascadia margin off Vancouver Island. Mapping using seismic data revealed a wide-spread layer of gas hydrate occurrence at shallow depth around 100 meter below seafloor. The glide plain of the slope failure is at the depth of the shallow the gas hydrate layer. This suggests that the contrast in sediment strength due to gas hydrate could be a critical boundary at which slope failures are initiated. This is a new finding as previous work worldwide suggested that slope failures are mostly initiated at the base of gas hydrate occurrence zone and not within the sediment package hosting gas hydrates.