Intraplate earthquakes and a seismically defined lateral transition in theupper mantle

We show that the lateral transition in the upper mantle between the cratoni c central/eastern and the tectonically active western regions of North Amer ica is narrow and deep-reaching by travel time analysis of Early Rise explo sion seismic data. A concentration of enigmatic intraplate earthquakes coin cides with the transition. Differential potential energy from density chang es across the transition may generate a high compressional stress level whi ch, locally, exceeds the shear strength of the lithosphere. The resultant d eformation is ductile in most parts of the lithosphere, but in the brittle upper crust it explains the enigmatic intraplate earthquakes around the lat eral transition in the upper mantle.