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Abstract Glacial isostatic adjustment (GIA) is the main cause of deformation in intraplate North America. Here we use up to 3, 271 Global Positioning System station velocities to image this 3‐D deformation across the entire plate. We apply a new robust strain rate estimation algorithm (median estimation of local deformation), which does not require the assumption that part of the plate is unaffected by GIA, an assumption we show to be false. Our results show extension in the area underneath the Laurentide ice sheet, contrasted with a semiannular belt of horizontal contraction of up to ~4 × 10 −9 yr −1 around the former ice sheet. This contractional belt is kinematically linked to an ~1–2 mm yr −1 far‐field horizontal motion directed toward the ice sheet. Our results, together with a new robustly imaged vertical velocity field, are consistent with GIA as the main cause of deformation, although the contractional strain rates around the former ice sheet and the far‐field horizontal velocities are significantly higher than those predicted by the ICE6GC (VM5a) model. This finding suggests that our results, including the location of reversal in sign of horizontal motion relative to the ice sheet, will be useful to reevaluate the mantle viscosity structure used by GIA models. Besides the GIA‐attributed deformation, we find almost no other region with significant strain accumulation. A plate‐scale spatial correlation between strain rate and seismicity is absent, suggesting that GIA is a limited driver of contemporary seismogenesis and that intraplate seismicity must be attributable to factors other than secular strain accumulation.
Kreemer et al. (Tue,) studied this question.