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TitleSeismic characteristics of the Nootka Fault Zone: results from the Seafloor Earthquake Array Japan Canada Cascadia Experiment (SeaJade)
 
AuthorHutchinson, J; Kao, HORCID logo; Spence, G; Obana, K; Wang, KORCID logo; Kodaira, S
SourceBulletin of the Seismological Society of America vol. 109, no. 6, 2019 p. 2252-2276, https://doi.org/10.1785/0120190008 Open Access logo Open Access
Image
Year2019
Alt SeriesNatural Resources Canada, Contribution Series 20180462
PublisherSeismological Society of America
Documentserial
Lang.English
Mediapaper; on-line; digital
File formatpdf; html
ProvinceBritish Columbia; Western offshore region
NTS92B/03; 92B/04; 92B/05; 92B/06; 92B/11; 92B/12; 92B/13; 92B/14; 92C; 92D; 92E; 92F; 92G/03; 92G/04; 92G/05; 92G/06; 92G/11; 92G/12; 92G/13; 92G/14; 92J/03; 92J/04; 92J/05; 92J/06; 92J/11; 92J/12; 92J/13; 92J/14; 92K; 92L; 102A; 102H; 102I
AreaVancouver Island; Pacific Ocean; Washington State; Canada; United States of America
Lat/Long WENS-130.0000 -123.0000 51.0000 47.0000
Subjectstectonics; structural geology; geophysics; Science and Technology; Nature and Environment; Health and Safety; earthquakes; bedrock geology; basement geology; structural features; fault zones; faults; faults, strike-slip; faults, normal; faults, thrust; geophysical surveys; seismic surveys, marine; seismic velocities; seismic waves; anomalies; seismicity; earthquake magnitudes; crustal studies; crustal structure; crustal movements; tectonic setting; fracturing; precipitation; veins; Mohorovicic discontinuity; mantle; seismological network; lineations; fluid dynamics; alteration; pore pressures; strain analysis; Nootka Fault Zone; Cascadia Subduction Zone; Juan de Fuca Plate; Explorer Plate; Seafloor Earthquake Array Japan-Canada Cascadia Experiment (SeaJade); Pacific Plate; Juan de Fuca Ridge; Sovanco Fracture Zone; Canadian National Seismograph Network
Illustrationslocation maps; geoscientific sketch maps; profiles; tables; histograms; plots; cross-sections; focal mechanisms; seismograms
ProgramPublic Safety Geoscience Assessing Earthquake Geohazards
Released2019 09 17
AbstractThe Nootka fault zone (NFZ) divides the incoming Explorer and Juan de Fuca plates of the Cascadia subduction zone. Three months of seafloor monitoring using 33 ocean-bottom seismometers off the west coast of Vancouver Island has allowed us to better understand the tectonic configuration and seismogenic characteristics of the NFZ. We have learned that the NFZ is comprised of northern and southern primary bounding faults, and several conjugate faults developed subperpendicular to the primary faults. Earthquakes typically occur over the depth ranges of 15-20 and 6-15 km along the primary bounding and conjugate faults, respectively. Focal mechanisms reveal that the most common modes of failure in this region are left-lateral strike slip, with normal faulting occurring along the southwestern extent of the NFZ and thrust faulting to the northeast before the subduction front. Seismic tomography suggests that the oceanic Moho is at a depth of 12-14 km below sea level (10-12 km below seafloor) just seaward of the Cascadia deformation front, and that it deepens to 19 km (17 km below seafloor) approximately 20 km landward of the deformation front. Converted phase analysis illuminates four velocity-contrasting interfaces with average depths below sea level deepening landward of the subduction front at ~4-6, ?6-9, ~11-14, and ~14-18 km. We interpret them as the sedimentary basement, upper-lower crust boundary, oceanic Moho, and the base of the highly fractured and seawater or mineral enriched veins within mantle. The precipitation of minerals such as quartz or the formation of talc, which is made possible by the intense degree of fracturing within the NFZ facilitating the infiltration of seawater, may reduce mantle velocities, as well as VP=VS ratios. The lack of seismicity observed along the interplate thrust zone in northern Cascadia may suggest that the megathrust fault is completely locked, consistent with prior studies.
Summary(Plain Language Summary, not published)
The Nootka fault zone (NFZ) is the plate boundary between the Explorer and Juan de Fuca plates. Three months of seafloor monitoring using 33 ocean bottom seismometers (OBS) off the west coast of Vancouver Island has allowed us to better understand the tectonic configuration and earthquake occurrence pattern within the NFZ. Our results show that NFZ is comprised of northern and southern primary bounding faults, and several secondary faults developed sub-perpendicular to the primary faults. Along the primary bounding and conjugate faults, earthquakes typically occur over the depth ranges of 15-20 km and 6-15 km, respectively. Four velocity-contrasting interfaces are detected and interpreted as the sedimentary basement, upper/lower crust boundary, oceanic Moho, and the base of the highly fractured and seawater/mineral enriched veins within mantle, respectively. No seismicity was observed along the interplate thrust zone in northern Cascadia, consistent with prior studies.
GEOSCAN ID314575

 
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