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TitleShear wave velocity structure beneath Haida Gwaii, Canada, in the vicinity of the 2012 Mw 7.7 Earthquake
AuthorCassidy, J FORCID logo; Gosselin, J; Dosso, S E
SourceSeismological Research Letters 85, no. 2, 2014 p. 437
LinksOnline - En ligne (HTML)
LinksOnline - En ligne (PDF, 3.37 MB, page 437)
Alt SeriesEarth Sciences Sector, Contribution Series 20140005
PublisherSeismological Society of America
MeetingSeismological Society of America Annual Meeting; Anchorage, AK; US; April 30 - May 2, 2014
Mediaon-line; digital; paper
File formathtm
ProvinceWestern offshore region; British Columbia
NTS102O; 103B; 103C; 103F; 103G; 103J; 103K
AreaHaida Gwaii; Queen Charlotte
Lat/Long WENS-134.5000 -130.0000 54.5000 51.0000
Subjectsgeophysics; tectonics; earthquake studies; earthquakes; earthquake magnitudes; earthquake risk; earthquake mechanisms; earthquake foci; tectonic setting; aftershocks; landslides
ProgramPublic Safety Geoscience Western Canada Geohazards Project
AbstractIn this study we constrain the S-wave velocity structure beneath Moresby Island, Haida Gwaii, in the vicinity of the 2012 Mw 7.7 earthquake (Canada's second largest instrumentally recorded earthquake). We utilise data from two existing Canadian National Seismograph Network stations (DIB and MOBC) and take advantage of four additional 3-component seismic stations deployed to monitor aftershock activity of the 2012 earthquake. It has long been known that there is an element of convergence between the Pacific and North American plates along the primarily strike-slip margin in the vicinity of Haida Gwaii. The M 7.7 earthquake of 2012 was not associated with the strike-slip Queen Charlotte Fault, but rather with a blind thrust fault just to the west of Moresby Island. It was the largest thrust event ever recorded in this region. Previous studies have provided evidence for a subducting slab beneath the northern part of Moresby Island. Our research focusses on new data from seismic stations on central and southern Moresby Island and we look for evidence of subduction along this margin. In this study we utilise receiver function analysis (time-domain deconvolution) combined with a Bayesian inversion technique to better constrain uncertainties and the likelihood, and nature of interfaces. At each station across Moresby Island, two key features are identified:
1) A continental Moho at 15-20 km depth,
2) A significant low S-wave velocity zone at depths ranging from 25 km to 40 km, generally increasing eastward beneath Haida Gwaii.
These results are consistent with a subducting oceanic plate beneath Haida Gwaii and represent the first evidence that the oceanic plate extends beneath all (or the majority) of Moresby Island. This new model will have applications to earthquake hazard evaluation across Haida Gwaii and further south along this margin.
Summary(Plain Language Summary, not published)
The October 27, 2012 magnitude 7.7 earthquake resulted in up to 4 m of movement along a previously unknown fault just off the west coast of Moresby Island, Haida Gwaii. Following that earthquake, 6 new seismic stations were deployed on Moresby Island to help monitor aftershocks activity. We utilise seismic data from those new stations to map, for the first time, the detailed earth structure beneath Haida Gwaii. Using these new data we image the base of the continental crust at 15-20 km below Moresby Island. More importantly, we image a significant low-velocity zone that is dipping to the east beneath Moresby Island (at depths of 25-40 km). We interpret this as the Pacific Plate pushing beneath Haida Gwaii. This is critical new information for assessing ground shaking from future earthquakes.

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