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TitleThe preservation potential of a sub-great megathrust earthquake, as inferred from 2 coseismic and tsunami effects of the 2012 Mw 7.8 Haida Gwaii event
AuthorLeonard, L J; Bednarski, J M
SourceBulletin of the Seismological Society of America vol. 105, no. 2B, 2015 p. 1280-1289,
Alt SeriesEarth Sciences Sector, Contribution Series 20140234
PublisherSeismological Society of America
Mediapaper; on-line; digital
File formatpdf; html
ProvinceBritish Columbia; Western offshore region
NTS103B; 103C; 103F; 103G
AreaHaida Gwaii; Graham Island; Moresby Island
Lat/Long WENS-134.0000 -131.0000 54.2500 52.0000
Subjectssedimentology; geophysics; earthquakes; tsunami; faults, thrust; mass wasting; landslides; subsidence
Illustrationssketch maps; photographs
ProgramWestern Canada Geohazards Project, Public Safety Geoscience
AbstractWe describe near-field coseismic and tsunami evidence collected following the 28 October 2012Mw 7.8 thrust earthquake that occurred offshore western Haida Gwaii, British Columbia, and discuss factors that influence its extent and preservation potential. Observations indicate minor geomorphic and sedimentological impacts on the rugged and unpopulated west coast of the islands, despite widespread coastal coseismic subsidence (?0:5 m), triggered landslides, and tsunami waves exceeding 3 m in runup (maximum 13 m) along ?230 km of coastline. Evidence left by the tsunami was minimal, likely because it occurred during a low tide that restricted its onshore reach, its flow depth, and its capacity to entrain and transport significant amounts of clastic sediment, sources of which are minimal or absent on the dominantly steep, rocky coastline.
It is unlikely that subaerial evidence of coseismic subsidence and tsunami inundation will be recognizably recorded in the coastal stratigraphy of western Haida Gwaii, due to the relatively small magnitude of subsidence, a lack of suitable coastal environments such as tidal marshes to record paleoelevation differences, sedimentation rates that are too low to bury a paleoseismic or paleotsunami record, and long-term relative sea level fall leading to erosion and bioturbation. A higher preservation potential is likely for tsunami deposits in coastal lakes, ponds, and bogs, as well as coseismic slope failure deposits offshore, in lakes and in sheltered fjords. Our findings imply that large tsunamigenic earthquakes are likely undersampled in the paleoseismic record of Haida Gwaii and of other plate margins with similar characteristics.
Summary(Plain Language Summary, not published)
We describe the effects of a M 7.8 earthquake off western Haida Gwaii, BC, 28 Oct. 2012, and discuss their preservation potential. Observations indicate relatively minor impacts on the rugged and unpopulated west coast, despite the occurrence of tsunami waves exceeding 3 m in runup (max. 13 m) along 230 km of coastline. Our findings imply that tsunami causing earthquakes that had magnitudes less than 8 may be vastly underrepresented here. The impact and preservation potential of the event was less because the tsunami occurred during low tide minimizing its reach and capacity to transport sediment. Sediment sources are minimal and only a few deposits were found at sites below the maximum runup elevation inferred by widespread flotsam. Preservation is also hampered by a long-term drop in sea-level, thus the best preservation potential of past earthquakes is likely in low-lying coastal lakes, or by earthquake induced slope failure deposits on the sea floor, in lakes or sheltered fjords.