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TitleGPS observations of crustal deformation associated with the Mw 7.8 2012 Haida Gwaii earthquake
AuthorNykolaishen, L; Dragert, H; Wang, KORCID logo; James, T SORCID logo; Schmidt, M
SourceBSSA, Bulletin Seismological Society of America; Bulletin of the Seismological Society of America vol. 105, issue 2B, 2015 p. 1241-1252,
Alt SeriesEarth Sciences Sector, Contribution Series 20140570
PublisherSeismological Society of America (SSA)
Mediapaper; on-line; digital
File formatpdf
ProvinceBritish Columbia
NTS103C; 103G; 103B; 103F
AreaHaida Gwaii
Lat/Long WENS-134.0000 -130.0000 54.0000 52.0000
Subjectsstructural geology; tectonics; earthquake studies; faults, strike-slip; faults, thrust
Illustrationslocation maps; diagrams; tables; plots
ProgramPublic Safety Geoscience Western Canada Geohazards Project
Released2015 04 07
AbstractOn 28 October 2012, an Mw 7.8 earthquake occurred off the west coast of Haida Gwaii, Canada. While past large events at this margin reflect strike-slip motion between the Pacific and North America Plates (e.g., 1949 Ms 8.1), this earthquake involved low-angle thrust faulting with a slip direction almost perpendicular to the margin, a consequence of slip partitioning at this obliquely convergent margin. To refine regional source models, we investigated the co- and postseismic displacements using a network of Global Positioning System (GPS) sites. Coseismic movement at the campaign site closest to the coast and about 30 km from the epicenter is 115 cm to the south-southwest, with 30 cm of subsidence. The only continuously recording GPS station on Haida Gwaii at the time of the earthquake, about 80 km from the epicenter, provides a robust coseismic displacement estimate of 22 cm to the south-southwest. The coseismic results are consistent with a shallow-dipping thrust rupture underlying the Queen Charlotte Terrace, immediately seaward of the Queen Charlotte Fault. These results allowed us to update rupture models that were originally derived from seismic waveforms and tsunami data. Estimates of cumulative postseismic horizontal displacements over about one year from seven sites are up to 6 cm, with a systematic along-strike variation in azimuth from south-southwest to southeast from north to south in the study area. Besides viscoelastic stress relaxation, these postseismic trends may indicate afterslip on the deeper plate interface beneath northern Moresby Island and possibly aseismic slip along the Queen Charlotte or sub-parallel faults offshore of southern Moresby Island.
Summary(Plain Language Summary, not published)
Canada's second largest recorded earthquake occurred off the west coast of Haida Gwaii, BC on October 28, 2012. To study earthquake processes and hazard in this region, Natural Resources Canada installed a network of Global Positioning System (GPS) stations to determine how the Earth's surface moved during and after the earthquake. At the time of the earthquake, there was up to 115 cm of horizontal motion to the south-southwest and up to 30 cm of subsidence at these monitoring stations. This information was then used to update models of how the fault ruptured. Continued horizontal motion in the following year is up to 6 cm, possibly due to continued slip along a deep part of the ruptured fault surface and relaxation of the Earth's mantle. An additional process may be present in southern Haida Gwaii, such as slow creep along a crustal fault. Continued GPS monitoring may provide further insights into large earthquakes in the region.

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