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TitleSeismic velocity structure of the Queen Charlotte Terrace off western Canada in the region of the 2012 Haida Gwaii Mw 7.8 thrust earthquake
AuthorRiedel, M; Yelisetti, S; Papenberg, C; Rohr, K M M; Côté, M; Spence, G D; Hyndman, R DORCID logo; James, TORCID logo
SourceGeosphere vol. 17, issue 1, 2020 p. 23-38, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20200295
PublisherGeological Society of America
Mediapaper; on-line; digital
File formatpdf; html
ProvinceBritish Columbia; Western offshore region
NTS102O; 103B; 103C; 103F; 103G; 103J; 103K
AreaHaida Gwaii
Lat/Long WENS-134.1667 -130.5000 54.5000 51.1667
Subjectsgeophysics; structural geology; Science and Technology; Nature and Environment; submarine terraces; seismology; earthquakes; aftershocks; geophysical surveys; seismic refraction surveys; seismic reflection surveys; seismic velocities; seismic waves; p waves; s waves; plate tectonics; subduction zones; crustal structure; oceanic crust; deformation; downgoing slab; marine sediments; modelling; Queen Charlotte Terrace; 2012 Haida Gwaii Mw 7.8 Earthquake; Pacific plate
Illustrationslocation maps; geoscientific sketch maps; seismic reflection profiles; models; seismograms; tables; profiles
ProgramPublic Safety Geoscience Plate Boundary Earthquakes
Released2020 12 23
AbstractA well-recorded Mw 7.8 megathrust earthquake occurred on 27 October 2012 under the Queen Charlotte terrace off the west coast of Haida Gwaii, western Canada. In this study, we supplement limited earlier seismic refraction work on the offshore velocity structure off Haida Gwaii with data from ocean bottom seismometers (OBS) operating between 6 December 2012 and 5 January 2013. The OBS recorded a portion of the aftershock sequence, and an active-source seismic survey was conducted in January 2013 to acquire seismic refraction data in the region of the Haida Gwaii earthquake across the Queen Charlotte terrace. P-wave velocity analyses using first-arrival tomography showed relatively shallow (2.0-3.0 km below seafloor) high-velocity material with values up to 4.0 km/s beneath the terrace. At the one OBS station seaward of the deformation front on the abyssal plain, refraction velocities of ~4.5 km/s indicated the top of the oceanic plate at ~1-2 km below the seafloor. At several OBS stations, converted shear-wave velocities were determined within the sediment cover using reflected arrivals. The S-wave velocities ranged from 0.5 to 1.5 km/s, and the corresponding P/S velocity ratio was between 3.0 and 4.2. The new refraction data confirm earlier interpretations of high-velocity material in the shallow terrace that may indicate fractured oceanic crustal material lies significantly above the location where a subducted slab is thought to occur under the terrace. Transpressive deformation of the Pacific plate may explain these observations.
Summary(Plain Language Summary, not published)
A magnitude 7.8 megathrust earthquake occurred October 27, 2012, off Haida Gwaii, BC. Ocean bottom seismometers were deployed to record the aftershocks and to better understand the sediment and crustal structure of the region. An active source seismic experiment was also conducted to acquire velocity information up to 6 km in depth to better understand the tectonic structure. The experiment confirmed and extended findings from studies conducted more than 30 years ago. The presence of high-velocity material (~4.0 km/s) at shallow depth (2.0 - 3.0 km) beneath the Queen Charlotte Terrace was confirmed and may indicate the presence of highly fractured oceanic crustal material. Data from the abyssal plain showed refraction velocities up to 5.8 km/s indicating crustal material of the Pacific plate at a sub-seafloor depth of ~3.5 km. These observations suggest a complex deformation process in this area which pose challenges in understanding and interpreting the earthquake regime and hazard.

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