Influence of Glacial Isostatic Adjustment on Intraplate Stress and Seismicity in Eastern North America in the Presence of Pre‐Existing Weak Zones

Abstract Eastern North America has hosted significant historical earthquakes, where seismicity clusters along tectonically inherited structures. Using the spherical finite‐element code CitcomSVE and fully 3D viscosity structure, we model the intraplate stress response to glacial isostatic adjustment (GIA) using ICE‐6G, both with and without low‐viscosity intraplate weak zones. We find that present‐day GIA‐induced stresses are generally small ( MPa across most of eastern North America), both at present day and during deglaciation, and can locally reach 3–4 MPa where weak zones are present. Associated rotations are limited to 1°, which are insignificant relative to the spread of observed stress data and far smaller than the continental‐wide clockwise rotations obtained from mantle‐flow models. However, GIA can still locally modify fault stability. In the New Madrid Seismic Zone, GIA promotes stability on the Reelfoot thrust fault while making NE‐SW strike‐slip faults less stable, suggesting a role in modulating present‐day seismicity patterns but not in triggering the 1811–1812 sequence. In the Western Quebec Seismic Zone, GIA increases Coulomb failure stress (CFS) on the Timiskaming Fault and nearby faults, but changes in CFS in the Charlevoix Seismic Zone are negligible at present day and only marginally higher during deglaciation. Overall, GIA perturbs CFS by only a few MPa, insufficient to independently drive fault failure under tectonic background stress (TBS) conditions derived from mantle flow models, which dominate regional‐to‐continental intraplate stress. However, alternate lithospheric viscosity structures and TBS states can greatly enhance GIA stresses and their impact on faulting in the crust.