GPS Observations of Crustal Deformation Associated with the 2012 Mw 7.8 Haida Gwaii Earthquake

On 28 October 2012, an M-w 7.8 earthquake occurred off the west coast of Haida Gwaii, British Columbia. Although past large events at this margin reflect strike-slip motion between the Pacific and North American plates (e.g., 1949 M-s 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 coseismic 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 subparallel faults offshore of southern Moresby Island.