RECURRENT VOICES: SEAN MULLAN – PART 1 – Evolving perspectives on the Pacific-North American plate boundary near Haida Gwaii, BC
July 5, 2016
By Sean Mullan
On October 27th, 2012 a moment magnitude (Mw = 7.8) earthquake occurred off Moresby Island, Haida Gwaii, BC. This was the second largest Canadian earthquake ever instrumentally measured. The largest recorded Canadian quake (M-Surface = 8.1) also occurred along this margin in 1949 and was strike-slip in nature. The 2012 mainshock, resultant tsunami and aftershocks continue to stimulate geoscience research along the connected Queen Charlotte (BC) and Fairweather (Alaska) Faults. This system demarcates a boundary between the Pacific and North American Plates. Nearly three-quarters of its length is offshore, running 1,300 km north-westward and terminating in the Gulf of Alaska (Plafker et al., 1978; Carlson et al., 1988). The Queen Charlotte Fault portion passes west of the Haida Gwaii Islands (figure 1). The tsunamigenic 2012 earthquake has clearly demonstrated the reality of large thrust earthquakes along this mostly strike-slip fault. Fortunately, no lives were lost and structural damages were restricted to road-slumps and chimneys.
The region has a complex tectonic history and modern structure. The Pacific Plate grinds ~50 mm/year north-westward relative to the North American Plate and the Queen Charlotte Fault is classified as transcurrent (e.g. Hyndman, 2015) or transform (e.g. Smith et al., 2003). To the south of this fault is a triple junction where the Pacific, North American and Explorer Plates meet. North of Haida Gwaii, in the Alaskan Fairweather Fault region, the plate boundary is predominantly strike-slip. From ~6 Ma to present, oblique convergence has apparently resulted in a degree of underthrusting and subduction of the Pacific Plate beneath North America in the Haida Gwaii region (Hyndman, 2015). However, the underthrusting has not yet initiated Wadati-Benioff seismicity or arc volcanism (Hyndman, 2015). Internal deformation of both the Pacific and North American Plates along Haida Gwaii may be accommodating some of the compression (Rohr et al., 2000).
An improved understanding of regional tectonics in the Haida Gwaii area is vital to our understanding of seismic hazards. The 2012 Mw = 7.8 earthquake resulted from the rupture of approximately half the thrust boundary. If the entire margin had ruptured, a magnitude 8.0 quake could have created a sizable tsunami (Hyndman, 2015). At the time of the 2012 mainshock, Natural Resources Canada (NRCan) had a continuously recording GPS unit and four seismographs running in the onshore vicinity. In the weeks following the event, NRCan undertook field reconnaissance on Haida Gwaii and installed devices to measure aftershocks and ground motion (James et al., 2013). By early November 2012, a variety of instruments had been set-up in southern Haida Gwaii: three broadband seismographs; four short-period seismographs; and seven GPS units.
Dr. Michael Riedel led an offshore instrument deployment expedition for NRCan. During the operation, 14 ocean bottom seismometers were temporarily deployed from early December 2012 until early January 2013. Additional marine work included an airgun survey to study the aftershock region’s sub-surface structure. Dr. Riedel has recently taken up employment with the GEOMAR – Helmholtz Centre for Ocean Research in Kiel, Germany.
The Bulletin of the Seismological Society of America (BSSA) has published a special issue (Vol. 105, May 2015) focused on the 2012 Haida Gwaii events and a 2013 Craig, Alaska earthquake (MW = 7.5). A virtual special collection related to these incidents will grow with time and is accessible at this URL: http://bssa.geoscienceworld.org/content/105/2B.toc
PhD Candidate: Marine Sedimentology and Ocean Mapping,
University of Victoria
Carlson, P.R., Bruns, T.R., Plafker, G. (1988). Late Cenozoic offsets on the offshore connection between the Fairweather and Queen Charlotte Faults off Southeast Alaska. Marine Geology. 85. pp. 89-97.
Hyndman, R. D. (2015). Tectonics and structure of the Queen Charlotte Fault Zone, Haida Gwaii, and large thrust earthquakes. Bulletin of the Seismological Society of America. 105-2B. pp. 1058-1075.
James, T., Rogers, G., Cassidy, J., Dragert, H., Hyndman, R., Leonard, L., Nykolaishen, L., Riedel, M., Schmidt, M. and Wang, K. (2013). Field studies target 2012 Haida Gwaii earthquake. Eos Transactions. 94. pp. 197 – 198.
Plafker, G., Hudson, T., Bruns, T., Rubin, M. (1978). Late Quaternary offsets along the Fairweather fault and crustal plate interactions in southern Alaska. Canadian Journal of Earth Sciences. 15. pp. 805 – 816.
Rohr, K.M.M., Scheidhauer, M. and Trehu, A.M., (2000). Transpression between two warm mafic plates: The Queen Charlotte Fault revisited. Journal of Geophysical Research. 105. pp. 8147 – 8172.
Smith, A.J., Hyndman, R.D., Cassiday, J.F. and Wang, K. (2003). Structure, seismicity, and thermal regime of the Queen Charlotte Transform Margin. Journal of Geophysical Research. 108-B11. pp. 2539-2551.
*Part 1 of 4
**article originally released in the Geological Association of Canada (GAC) Marine Geoscience newsletter