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TitleEarthquake and tsunami hazards in the inland sea of the San Juan Archipelago, Salish Sea of Washington State
 
AuthorGreene, H G; Barrie, J VORCID logo; Todd, B JORCID logo; Nishenko, S
SourceSeismological Society of America, Annual Meeting, Seattle, Washington, USA, abstracts; 2019 p. 1 Open Access logo Open Access
LinksOnline - En ligne (SSA 2019)
Image
Year2019
Alt SeriesNatural Resources Canada, Contribution Series 20180374
PublisherSeismological Society of America
MeetingSeismological Society of America, Annual Meeting, 2019; Seattle, WA; US; April 23-26, 2019
DocumentWeb site
Lang.English
Mediaon-line; digital
File formathtml; pdf (Adobe® Reader®)
ProvinceWestern offshore region
AreaWashington State; Salish Sea; San Juan Islands; United States of America
Lat/Long WENS-123.3333 -122.6667 48.7500 48.3333
Subjectsgeophysics; tectonics; structural geology; Science and Technology; Nature and Environment; Health and Safety; earthquakes; earthquake risk; earthquake damage; seismicity; seismic risk; tsunami; floods; landslides; mass wasting; geophysical surveys; seismic surveys, marine; seismic reflection surveys; bedrock geology; basement geology; structural features; faults; faults, strike-slip; fault zones; crustal structure; San Juan Archipelago; Cascadia Subduction Complex; Devils Mountain Fault Zone; Skipjack Island Fault Zone; cumulative effects
ProgramPublic Safety Geoscience Quantitative risk assessment project
Released2019 04 01
AbstractExtensive seismic-reflection profiling and bathymetric fault mapping in the inland sea of the San Juan Archipelago has enabled the mapping of local earthquake and tsunami hazards in the region. This inland sea comprises the central part of the Salish Sea and is part of the upper tectonic plate associated with the Cascadia subduction complex. The region is composed of subducted, accreted, and transported blocks of rocks that have been heavily glaciated, forming deep fiord channels, and sounds. The Archipelago is bounded by two E-W trending active strike-slip fault zones, the Devils Mountain Fault Zone in the south and the newly mapped Skipjack Island Fault zone in the north, both of which extend for over 55 km where mapped offshore but may have a total length of over 125 km if connected to onland faults. Similar fault zones to the south (e.g., Seattle, Tacoma fault zones) have produced Mw 6.5-7.5 earthquakes in the last 15 ka.
The Skipjack Island Fault Zone is especially significant as it crosses between the islands of the Archipelago and is near potentially unstable seafloor features. The close proximity of the fault zone to Orcas Island, composed of fractured Mesozoic basement rocks with a steep (locally near vertical) and high (732 m) NE facing side, could trigger rockfalls, which would produce impact tsunamis that could inundate the low-lying coastal areas of islands and the mainland of the US and Canada to the N and NE. In addition, the distal southern edge of the Fraser River Delta of Canada lies just north of the Skipjack Island Fault Zone. A moderate to large magnitude earthquake could mobilize mass wasting of deltaic sediment, thus producing a tsunami directed toward the islands of the San Juan Archipelago and low coastal areas of mainland US.
Summary(Plain Language Summary, not published)
Recent fault mapping in the inland sea of the San Juan Archipelago suggests that potential seismic geohazards are locally constrained. Recent seismicity in the region consists of small to moderate size earthquakes. However, historically, similar types of fault zones to the south associated with upper plate tectonics (e.g., Seattle, Tacoma fault zones) have produced Mw 6.5-7.5 earthquakes in the last 15 ka. Therefore, the potential for moderate to large earthquakes along the fault zones discussed here is considered high. A local fault zone has the potential of rockfalls, which would produce impact tsunamis that could inundate low-lying coastal areas of islands and the mainland of the US and Canada to the N and NE if shaken by an earthquake along the fault zone.
GEOSCAN ID313563

 
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