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TitleEarthquake activity in northern Cascadia subduction zone off Vancouver Island revealed by ocean bottom seismograph observations
AuthorObana, K; Scherwath, M; Yamamoto, Y; Kodaira, S; Wang, KORCID logo; Spence, G; Riedel, M; Kao, HORCID logo
SourceBulletin of the Seismological Society of America vol. 105, no. 1, 2015 p. 489-495,
Alt SeriesEarth Sciences Sector, Contribution Series 20140210
PublisherSeismological Society of America (SSA)
Mediapaper; on-line; digital
File formatpdf
ProvinceWestern offshore region
AreaCascadia subduction zone
Subjectstectonics; seismic data; earthquakes; earthquake studies; seismographs; plate margins; plate tectonics; fault zones; deformation; Nootka fault zone
ProgramPublic Safety Geoscience Western Canada Geohazards Project
Released2014 11 18
AbstractOffshore seismicity at the Cascadia margin is poorly constrained because nearly all previous recordings of earthquakes were made using land-based networks. We conducted earthquake monitoring off Vancouver Island in northern Cascadia using ocean bottom seismographs. Our results show that most of the offshore seismicity is concentrated along the Nootka fault zone. Otherwise seismicity is extremely low, with no earthquakes located along the shallow, seismogenic part of the megathrust. The lack of interplate seismicity may indicate complete healing and locking of the megathrust over three centuries after the great earthquake of 1700 and a somewhat lower degree of structure heterogeneity, such as subducting seamounts. Events along the Nootka fault zone occur over a 10¿15 km depth range. This wide distribution and the previously reported overall moment release rate suggest that a significant part of deformation of this fault zone is aseismic. Several earthquakes beneath the continental shelf may be related to faults dividing tectonic terrains within the overriding plate.
Summary(Plain Language Summary, not published)
One of the most important steps in assessing seismic hazard is to record and analyse regional seismicity. Our knowledge of seismicity of the Cascadia margin off SW BC is based almost entirely on recordings by our land-based seismic network. To better understand seismicity offshore where megathrust earthquakes occur, we conducted earthquake monitoring using ocean bottom seismometers. Our results indicate that most of the offshore earthquakes occur on the Nootka fault, and that the Cascadia megathrust is presently very quiet. The present seismic quiescience of the megathrust supports the notion that the fault is fully locked and accumulating energy for a future great earthquake.

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