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TitleFracture alignments in marine sediments off Vancouver Island from Ps splitting analysis
AuthorTonegawa, T; Obana, K; Yamamoto, Y; Kodaira, K; Wang, KORCID logo; Riedel, M; Kao, HORCID logo; Spence, G C
SourceBulletin of the Seismological Society of America vol. 107, no. 1, 2017 p. 387-402, Open Access logo Open Access
Alt SeriesEarth Sciences Sector, Contribution Series 20160105
PublisherSeismological Society of America (SSA)
Mediapaper; on-line; digital
File formatpdf
ProvinceOffshore region
NTS92C; 92E; 92F; 92K; 92L; 92M; 92N; 102
Lat/Long WENS-132.0000 -124.0000 52.0000 47.0000
Subjectsgeophysics; sedimentology; structural geology; marine geology; fractures; fracture zones; fracture analyses; marine sediments; faulting; s waves; p waves; earthquakes; earthquake studies; shear fractures
Illustrationslocation maps; graphs; formulae; tables; geological sketch maps
ProgramPublic Safety Geoscience Western Canada Geohazards Project
Released2016 11 15
AbstractAlignments of fractures and cracks in marine sediments may be controlled by various mechanisms such as horizontal compaction/extension and basement faulting. The orientation of these alignments can be estimated through analyses of S wave splitting. If sensors in ocean bottom observations are deployed through freefall, sensor orientation needs to be determined in order for the recorded data to be used for such analyses. Here, we estimate the sensor orientation from the linear particle motions of P-to-s (Ps) phases converted at the sediment-basement interface and also T waves which are excited by earthquakes and propagate in the seawater. We examined waveforms of local earthquakes recorded by 32 ocean bottom seismometers (OBSs) that were deployed through freefall for three months in 2010 off Vancouver Island where the strike-slip Nootka fault zone (NFZ) intersects the deformation front of the Cascadia subduction zone. Since the particle motion of the Ps wave was corrected by estimating splitting parameters, the fast polarization direction, which reflects S-wave anisotropic structure within the sediment, can also be evaluated. Consequently, we could estimate the fast polarization direction at OBSs deployed near the NFZ and west of the deformation front. The obtained fast directions appeared to correspond to alignments of shear fractures in the marine sediments associated with the left-lateral motion of the fault in the basement along the NFZ, margin-normal cracks due to horizontal compression west of and slightly away from the deformation front, and frontal thrust faults within the accretionary prism near the deformation front.
Summary(Plain Language Summary, not published)
Shear wave anisotropy is the property of a rock formation that causes seismic shear waves to travel at different speeds in accordance with the direction of particle motion. It is controlled by stresses applied to the formation or rock fabrics inherited from ancient deformation. It is therefore one of the seismic observables that contribute to the understanding of the capacity of the crust to produce earthquakes. Ocean Bottom Seismometer (OBS) observations under the Canada-Japan collaborative project SeaJade have not only defined seismicity offshore of Vancouver Island in high resolution, but also yielded information on shear wave anisotropy of the seafloor sedimentary formation. This paper reports the analysis of the SeaJade OBS data to retrieve such information and discuss its tectonic implications.

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