Motions of three hundred and sixty Global Positioning System (GPS) sites in Canada and the United States yield a detailed image of the vertical and horizontal velocity fields within the nominally stable interior of the North American plate. By far the strongest signal is the effect of glacial isostatic adjustment (GIA) due to ice mass unloading during deglaciation. Vertical velocities show present‐day uplift (∼10 mm/yr) near Hudson Bay, the site of thickest ice at the last glacial maximum. The uplift rates generally decrease with distance from Hudson Bay and change to subsidence (1–2 mm/yr) south of the Great Lakes. The “hinge line” separating uplift from subsidence is consistent with data from water level gauges along the Great Lakes, showing uplift along the northern shores and subsidence along the southern ones. Horizontal motions show outward motion from Hudson Bay with complex local variations especially in the far field. Although the vertical motions are generally consistent with the predictions of GIA models, the horizontal data illustrate the need and opportunity to improve the models via more accurate descriptions of the ice load and laterally variable mantle viscosity.

[1] Global Positioning System (GPS) measurements to study regional deformation were initiated in northern Cascadia in the late 1980s and early 1990s. On the basis of a decade of GPS data, we derive a crustal velocity field for NW Washington-SW British Columbia. The permanent and campaign GPS velocities are defined with respect to North America in the ITRF2000 reference frame. Velocity uncertainties are estimated using a model of time series noise spectra. This new velocity field is the basis for interpretation of the tectonics of the northern Cascadia subduction system. GPS velocities are interpreted in terms of interseismic loading of the megathrust using different coupling models. Our data confirm that the upper part of the megathrust is nearly fully locked. An exponential model for the downdip transition zone gives slightly better agreement with the data compared to the common linear transition. The landward decrease of forearc strain loading is smaller than predicted by any of the current subduction interseismic models. This could be a consequence of a small (0-3 mm/yr) long-term motion of the southern Vancouver Island forearc, with respect to North America, or of a concentration of interseismic strain across the elastically weaker Cascadia volcanic arc. In northern Vancouver Island, our velocity field supports the existence of an independent Explorer microplate currently underthrusting underneath North America, at least up to Brooks Peninsula. Further north, GPS velocities indicate transient and/or permanent deformation of northernmost Vancouver Island related to the interaction with the Explorer microplate and possibly with the Queen Charlotte transform margin.

By using monthly mean water levels at 55 sites around the Great Lakes, a regional model of vertical crustal motion was computed for the region. In comparison with previous similar studies over the Great Lakes, 15 additional gauge sites, data from all seasons instead of the 4 summer months, and 8 additional years of data were used. All monthly water levels available between 1860 and 2000, as published by the U.S. National Ocean Survey and the Canadian Hydrographic Service, were used. For each lake basin, the vertical velocities of the gauge sites relative to each other were simultaneously computed, using the least-squares adjustment technique. Our algorithm solves for and removes a monthly bias common to all sites, as well as site-specifi c biases. It also properly weighs the input water levels, resulting in a realistic estimation of the uncertainties in tilting parameters. The relative velocities obtained for each lake were then combined to obtain relative velocities over the entire Great Lakes region. Finally, the gradient of the relative rates for the regional model was found to agree best with the ICE-3G global isostatic model of Tushingham and Peltier, whereas the ICE-4G gradients were too small around the Great Lakes.

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Relative sea-level projections are provided for 59 locations in Canada and 10 in the adjacent mainland United States (New England and Washington State) through the 21st century, relative to 1986-2005. The projections are based on the Representative Concentration Pathway (RCP) scenarios of the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR5). They include contributions from thermal expansion of the ocean (steric effect), land ice melting and discharge, and anthropogenic influences. The global mean sea-level projection for RCP8.5, the largest emissions scenario, at 2100 is 74 cm (5%-95% range is 54 to 98 cm). Global Positioning System (GPS) measurements of vertical land motion are incorporated into the relative sea-level projections. In the regions presented here, vertical land motion, largely arising from glacial isostatic adjustment, plays a prominent role in determining projected relative sea-level change. On the east coast, crustal subsidence, combined with dynamic oceanographic changes, generates relative sea-level projections that are similar to or larger than the global mean projections in large parts of Atlantic Canada and New England. On the west coast, most relative sea-level projections are smaller than the global means, although some sites in Washington State and southern British Columbia feature relative sea-level projections similar to the global values. The largest variation in projected relative sea-level rise occurs in the Arctic, owing to the very large spatial differences in present-day crustal uplift due to glacial isostatic adjustment. Here, projected relative sea-level at 2100 varies from around 1 m of sea-level fall (median values) where land is rising quickly on Hudson Bay, while it reaches about 70 cm of sea-level rise on the Beaufort coast where the land is subsiding. A scenario featuring partial collapse of a portion of the West Antarctic Ice Sheet provides an additional 65 cm of sea-level rise to RCP8.5, and may be appropriate to consider when tolerance to the risk of sea-level rise is low. The relative sea-level projections given here only provide a trajectory through this century, but the IPCC AR5 projects continued global sea-level rise in coming centuries. As understanding improves of the various components of sea-level rise, it will be necessary to update, on an occasional basis, the relative sea-level projections and re-evaluate the implications for infrastructure, habitat, and marine navigation.