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Abstract
P-wave reflections generated by earthquakes and recorded by the Transportable Array across the eastern US image subsurface structures ranging from coastal plain sediments to layering within the asthenosphere. Reflections from the Moho increase in two-way travel time from 10-11 s in the east to 14-18 s, becoming more complex beneath the highest elevations of the Appalachian Highlands, and then decrease somewhat before leveling off or deepening further to the west. A new criterion for identifying the base of the lithosphere (LAB) based on a downward increase in horizontal reflectivity determined objectively using waveform coherence suggests a westward increase in LAB depth from 32-38 s (115-140 km) along the east coast to 40 s (150 km) beneath the continental interior. This transition in reflection density is interpreted as layering caused by horizontal strain in the lowermost lithosphere and drag-induced flow in the asthenosphere.