We show that the lateral transition in the upper mantle between the cratonic central/eastern and the tectonically active western regions of North America is narrow and deep-reaching by travel time analysis of Early Rise explosion seismic data. A concentration of enigmatic intraplate earthquakes coincides with the transition. Differential potential energy from density changes across the transition may generate a high compressional stress level which, locally, exceeds the shear strength of the lithosphere. The resultant deformation is ductile in most parts of the lithosphere, but in the brittle upper crust it explains the enigmatic intraplate earthquakes around the lateral transition in the upper mantle. ¿ 2000 American Geophysical Union