TRANSITION FROM THE CASCADIA MEGATHRUST TO THE QUEEN CHARLOTTE FAULT IMAGED BY GPS

We use continuous and campaign GPS data acquired over the last 5 years to investigate the plate boundary transition from subduction to transform in northern Cascadia. From northern California up to central Vancouver Island, the Juan de Fuca plate subducts beneath North America. To the north, the strike-slip motion between the Pacific (PA) and the North America (NA) plates is accommodated along the Queen Charlotte (QC) Fault. The PA/NA motion is slightly oblique to the QC Fault direction, implying some convergence across the Queen Charlotte Islands margin. Between those two regions, the Explorer plate, a detached segment of the Juan de Fuca plate, is underthrusting beneath North America along northern Vancouver Island and the Queen Charlotte Sound. The Cascadia subduction zone and the QC Fault pose significant seismic hazard to the West Coast, with M~9 earthquakes along the former and M~8 events along the latter. In contrast, there is no record of large earthquake in the transition area. GPS velocities in southern and central Vancouver Island decrease landward from ~10 to ~5 mm/yr northeastward, indicating shortening of the forearc due to the interseismic coupling of the subduction fault. In contrast, GPS measurements indicate very low strain in northernmost Vancouver Island, with velocities 3-5 mm/yr mostly northward. This suggests little crustal deformation due to the Explorer plate underthrusting. GPS campaign results in the Queen Charlotte Islands show velocities of ~10 mm/yr, compatible with elastic strain accumulation along a locked QC Fault and with a slight margin-normal shortening component. The reduced motions both to the north and to the south of the central Queen Charlotte Islands could be consistent with stress accumulating more along the southern segment of the QC Fault, a region previously identified as a seismic gap.