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TitleDual-vergence structure from multiple migration of widely spaced OBSs
AuthorYelisetti, S; Spence, G D; Scherwath, M; Riedel, M; Klaeschen, D
SourceTectonophysics 2017., https://doi.org/10.1016/j.tecto.2017.04.005
Year2017
Alt SeriesEarth Sciences Sector, Contribution Series 20140460
PublisherSociety of Exploration Geophysicists
Documentserial
Lang.English
Mediapaper; on-line; digital
File formatpdf
ProvinceWestern offshore region
AreaCascadia Basin; Pacific Ocean
Lat/Long WENS-127.0667 -126.8667 48.6000 48.4333
Subjectsseismic velocities; seismic profiles, marine; seismic surveys, marine; faults, thrust; folds; deformation; Ocean Bottom Seismometer (OBS); dual-vergence; tomography; inversion
Illustrationslocation maps; seismic sections; graphs; seismic velocity profiles
ProgramMarine Geohazards, Public Safety Geoscience
AbstractThe detailed structure of the northern Cascadia basin and frontal ridge region was obtained using data from several widely spaced ocean bottom seismometers (OBSs). Mirror imaging was used in which the downgoing multiples (mirror signal) are migrated as they provide information about a much larger area than imaging with primary signal alone. Specifically, Kirchhoff time migration was applied to hydrophone and vertical geophone data. Our results indicate remarkable structures that were not observed on the northern Cascadia margin in previous single-channel or multi-channel seismic (MCS) data. Results show that, in these water depths (2.0–2.5 km), an OBS can image up to 5 km on either side of its position on the seafloor and hence an OBS spacing of 5 km is sufficient to provide a two-fold migration stack. Results also show the top of the igneous oceanic crust at 5–6 km beneath the seafloor using only a small airgun source (120 in.3). Specifically, OBS migration results clearly show the continuity of reflectors which enabled the identification of frontal thrusts and a main thrust fault. These faults indicate, for the first time on this margin, the presence of a dual-vergence structure. These kinds of structures have so far been observed in < 0.5% of modern convergent margins and could be related to horizontal compression associated with subduction and low basal shear stress resulting from over-pressure. Reanalysis of previous MCS data from this region augmented the OBS migration results and further suggests that the vergence switches from seaward to landward around central Vancouver Island. Furthermore, fault geometry analyses indicate that the total amount of shortening accommodated due to faulting and folding is about 3 km, which suggest that thrusting would have started at least ~ 65 ky ago.
Summary(Plain Language Summary, not published)
The detailed structure of the frontal ridge region of the northern Cascadia subduction zone was obtained using data from widely spaced ocean bottom seismometers deployed as part of the SeaJade-I experiment. Mirror imaging was used in which the downgoing multiples are used as they provide information about a larger area than imaging with the primary signal alone. Results indicate remarkable structures that were not previously observed. The top of the igneous oceanic crust was imaged at 5 to 6 km beneath seafloor and the frontal thrusts reveal a dual-vergence structure. Fault geometry indicates that the total amount of shortening due to faulting and folding is about 3 km, which suggest that thrusting would have started at least 65 ky ago.
GEOSCAN ID295869