Seismic structure, gas hydrate, and slumping studies on the Northern Cascadia margin using multiple migration and full waveform inversion of OBS and MCS data

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dc.contributor.authorYelisetti, Subbarao
dc.contributor.supervisorSpence, George D.
dc.date.accessioned2014-11-05T21:26:28Z
dc.date.available2014-11-05T21:26:28Z
dc.date.copyright2014en_US
dc.date.issued2014-11-05
dc.degree.departmentSchool of Earth and Ocean Sciences
dc.degree.levelDoctor of Philosophy Ph.D.en_US
dc.description.abstractThe primary focus of this thesis is to examine the detailed seismic structure of the northern Cascadia margin, including the Cascadia basin, the deformation front and the continental shelf. The results of this study are contributing towards understanding sediment deformation and tectonics on this margin. They also have important implications for exploration of hydrocarbons (oil and gas) and natural hazards (submarine landslides, earthquakes, tsunamis, and climate change). The first part of this thesis focuses on the role of gas hydrate in slope failure observed from multibeam bathymetry data on a frontal ridge near the deformation front off Vancouver Island margin using active-source ocean bottom seismometer (OBS) data collected in 2010. Volume estimates (∼ 0.33 km^3) of the slides observed on this margin indicate that these are capable of generating large (∼ 1 − 2 m) tsunamis. Velocity models from travel time inversion of wide angle reflections and refractions recorded on OBSs and vertical incidence single channel seismic (SCS) data were used to estimate gas hydrate concentrations using effective medium modeling. Results indicate a shallow high velocity hydrate layer with a velocity of 2.0 − 2.1 km/s that corresponds to a hydrate concentration of 40% at a depth of 100 m, and a bottom simulating reflector (BSR) at a depth of 265 − 275 m beneath the seafloor (mbsf). These are comparable to drilling results on an adjacent frontal ridge. Margin perpendicular normal faults that extend down to BSR depth were also observed on SCS and bathymetric data, two of which coincide with the sidewalls of the slump indicating that the lateral extent of the slump is controlled by these faults. Analysis of bathymetric data indicates, for the first time, that the glide plane occurs at the same depth as the shallow high velocity layer (100±10 mbsf). In contrast, the glide plane coincides with the depth of the BSR on an adjacent frontal ridge. In either case, our results suggest that the contrast in sediments strengthened by hydrates and overlying or underlying sediments where there is no hydrate is what causing the slope failure on this margin. The second part of this dissertation focuses on obtaining the detailed structure of the Cascadia basin and frontal ridge region using mirror imaging of few widely spaced OBS data. Using only a small airgun source (120 cu. in.), our results indicate structures that were previously not observed on the northern Cascadia margin. Specifically, OBS migration results show dual-vergence structure, which could be related to horizontal compression associated with subduction and low basal shear stress resulting from over-pressure. Understanding the physical and mechanical properties of the basal layer has important implications for understanding earthquakes on this margin. The OBS migrated image also clearly shows the continuity of reflectors which enabled the identification of thrust faults, and also shows the top of the igneous oceanic crust at 5−6 km beneath the seafloor, which were not possible to identify in single-channel and low-fold multi-channel seismic (MCS) data. The last part of this thesis focuses on obtaining detailed seismic structure of the Vancouver Island continental shelf from MCS data using frequency domain viscoacoustic full waveform inversion, which is first of its kind on this margin. Anelastic velocity and attenuation models, derived in this study to subseafloor depths of ∼ 2 km, are useful in understanding the deformation within the Tofino basin sediments, the nature of basement structures and their relationship with underlying accreted terranes such as the Crescent and the Pacific Rim terranes. Specifically, our results indicate a low-velocity zone (LVZ) with a contrast of 200 m/s within the Tofino basin sediment section at a depth 600 − 1000 mbsf over a lateral distance of 10 km. This LVZ is associated with high attenuation values (0.015 − 0.02) and could be a result of over pressured sediments or lithology changes associated with a high porosity layer in this potential hydrocarbon environment. Shallow high velocities of 4 − 5 km/s are observed in the mid-shelf region at depths > 1.5 km, which is interpreted as the shallowest occurrence of the Eocene volcanic Crescent terrane. The sediment velocities sharply increase about 10 km west of Vancouver Island, which probably corresponds to the underlying transition to the Mesozoic marine sedimentary Pacific Rim terrane. High attenuation values of 0.03 − 0.06 are observed at depths > 1 km, which probably corresponds to increased clay content and the presence of mineralized fluids.en_US
dc.description.proquestcode0373en_US
dc.description.proquestcode0372en_US
dc.description.proquestcode0605en_US
dc.description.proquestemailsubbarao@uvic.caen_US
dc.description.scholarlevelGraduateen_US
dc.identifier.bibliographicCitationYelisetti, S., G. D. Spence, and M. Riedel (2014), Role of gas hydrates in slope failure on frontal ridge of northern Cascadia margin, Geophysical Journal International, 199 (1), 441–458, doi:10.1093/gji/ggu254.en_US
dc.identifier.urihttp://hdl.handle.net/1828/5719
dc.languageEnglisheng
dc.language.isoenen_US
dc.rights.tempAvailable to the World Wide Weben_US
dc.rights.urihttp://creativecommons.org/licenses/by-nc-nd/2.5/ca/*
dc.subjectseismic structureen_US
dc.subjectgas hydrateen_US
dc.subjectsubmarine landslides or slumpsen_US
dc.subjectNorthern Cascadia marginen_US
dc.subjectmultiple migrationen_US
dc.subjectfull waveform inversionen_US
dc.subjectvisco-acoustic wave equationen_US
dc.subjecttomographyen_US
dc.subjectfluid flowen_US
dc.subjectearth quakesen_US
dc.subjecthydrocarbon explorationen_US
dc.subjecttectonicsen_US
dc.subjectbottom simulating reflection or BSRen_US
dc.subjectseismic attenuationen_US
dc.subjectocean bottom seismic dataen_US
dc.subjectmutli-channel seismic dataen_US
dc.subjectsingle-channel seismic dataen_US
dc.subjectairgun sourceen_US
dc.subjectdual-vergence structureen_US
dc.subjectlow velocity zoneen_US
dc.subjecthigh velocity regionen_US
dc.subjectCrescent terraneen_US
dc.subjectPacific Rim terraneen_US
dc.subjectnormal faultsen_US
dc.subjectfrontal thrusts and backthrusten_US
dc.titleSeismic structure, gas hydrate, and slumping studies on the Northern Cascadia margin using multiple migration and full waveform inversion of OBS and MCS dataen_US
dc.typeThesisen_US

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