Crustal structure, deformation from GPS, and seismicity related to oblique convergence along the Queen Charlotte Margin, British Columbia




Bustin, Amanda M. M.

Journal Title

Journal ISSN

Volume Title



Tectonic processes and seismic hazard along the west coast of British Columbia result from oblique convergence between the continental North America plate and the oceanic Pacific and Juan de Fuca plates. This dissertation integrates seismic and geodetic techniques to examine the tectonic interaction along these plate boundaries. The Queen Charlotte Fault zone is the transpressive boundary between the North America and Pacific plates along the northwestern margin of British Columbia. Two models have been suggested for the accommodation of the -20 mm/yr of convergence along the fault boundary: (1) underthrusting; (2) internal crustal deformation. Strong evidence supporting an underthrusting model is provided in this dissertation by a teleseismic receiver function analysis that defines the underthrusting slab. Forward and inverse modelling techniques were applied to receiver function data calculated at two permanent and six temporary seismic stations within the Islands. The modelling reveals a --10 km thick low-velocity zone dipping eastward at 28° interpreted to be underthrusting oceanic crust. The oceanic crust. which may be anisotropic, is located beneath a thin (28 km) westward shallowing (10°) continental margin. The majority of seismicity along the Queen Charlotte Fault zone plots within the modelled underthrusting crust, suggesting that these earthquakes are occurring on faults that extend down into the slab or they might be intraslab events. None of the earthquakes within the Queen Charlotte Basin have occurred deep enough to be intraslab earthquakes. The Wadati-Benioff seismicity may be inhibited beneath the basin by the hot young oceanic crust or by the short distance of underthrusting. GPS measurements have been recorded within the Queen Charlotte Islands during 8 years of campaign surveys. The crustal velocity field derived from the GPS data indicates northward margin-oblique motion of 10-15 mm/yr. Comparisons of the observed velocities with elastic dislocation modelling showed that the majority of the transpressive Pacific/North America motion is accommodated along the locked Queen Charlotte Fault zone and thrust fault with the remaining plate motion taken up by north-northeast migration of the margin at rates of --6 mm/yr. The migration is also supported by the stress pattern derived from focal mechanisms. However. the GPS determined deformation rates are much greater than the rates estimated in the Queen Charlotte Basin from the seismicity catalogue. The extra shortening could be accommodated by large characteristic thrust earthquakes within the basin. The deformation rates estimated from the seismicity along the Queen Charlotte Fault zone are consistent with the relative Pacific/North America plate motion: however. the seismic moment released by thrust earthquakes along the fault is insufficient to account for the component of convergence. This suggests that large thrust earthquakes may occur along the plate boundary. An additional part of this dissertation involved constraining the rupture parameters of the Nisqually earthquake. a M 6.8 event which occurred on February 28. 2001 within the subducting Juan de Fuca slab at the Cascadia subduction zone. The fault parameters of the event were estimated from moment tensor solutions, and by comparing the surface displacements from GPS and InSAR data. with predictions from elastic deformation models. The results are in agreement with the earthquake mechanism from waveform analyses. but provide substantially stronger constraints.



structural geology, earthquakes, Queen Charlotte Islands, British Columbia