The cratonward limit of large normal faults in the passive margin of the Late Proterozoic to early Paleozoic Iapetus Ocean is a fundamental boundary for the assessment of seismic hazards in populous eastern North America. Earthquakes at five localities in the Iapetan margin have been attributed to compressional reactivation of Iapetan normal faults. The northwesternmost locations of known Iapetan faults, both seismic and currently aseismic, lie along a line from Labrador to Alabama. This line coincides approximately with the northwestward transition from a more seismically active continental rim to a generally less active cratonic interior. Thus, the northwestern boundary of Iapetan faults separates two large regions of mostly different seismic hazard.