A 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.
Research Article|
December 23, 2020
Seismic velocity structure of the Queen Charlotte terrace off western Canada in the region of the 2012 Haida Gwaii Mw 7.8 thrust earthquake
M. Riedel;
M. Riedel
1
GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstrasse 1–3, 24148 Kiel, Germany
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S. Yelisetti;
S. Yelisetti
2
Department of Physics and Geosciences, Texas A&M University–Kingsville, Kingsville, Texas 78363-8202, USA
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C. Papenberg;
C. Papenberg
1
GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstrasse 1–3, 24148 Kiel, Germany
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K.M.M. Rohr;
K.M.M. Rohr
3
Natural Resources Canada, Geological Survey of Canada–Pacific, 9860 West Saanich Road, Sidney, British Columbia V8L 4B2, Canada
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M.M. Côté;
M.M. Côté
3
Natural Resources Canada, Geological Survey of Canada–Pacific, 9860 West Saanich Road, Sidney, British Columbia V8L 4B2, Canada
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G.D. Spence;
G.D. Spence
4
School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8P 5C2, Canada
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R.D. Hyndman;
R.D. Hyndman
3
Natural Resources Canada, Geological Survey of Canada–Pacific, 9860 West Saanich Road, Sidney, British Columbia V8L 4B2, Canada4
School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8P 5C2, Canada
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T. James
T. James
3
Natural Resources Canada, Geological Survey of Canada–Pacific, 9860 West Saanich Road, Sidney, British Columbia V8L 4B2, Canada
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Geosphere (2020)
Article history
received:
16 Mar 2020
rev-recd:
11 Aug 2020
accepted:
25 Nov 2020
first online:
24 Dec 2020
Citation
M. Riedel, S. Yelisetti, C. Papenberg, K.M.M. Rohr, M.M. Côté, G.D. Spence, R.D. Hyndman, T. James; Seismic velocity structure of the Queen Charlotte terrace off western Canada in the region of the 2012 Haida Gwaii Mw 7.8 thrust earthquake. Geosphere 2020; doi: https://doi.org/10.1130/GES02258.1
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