Eight on-bottom seismic refraction experiments are analyzed in an effort to resolve the structure of the emplacement zone of lavas and dikes at the fast-spreading East Pacific Rise. The results suggest that the volcanic section doubles in thickness within ~1 km of the axial summit caldera (ASC) due to the emplacement of lava that either overflows the ASC or is transported through lateral tube conduits, and the depth to the top of the sheeted dike complex increases from ~160 m within the ASC to ~340 m a kilometer away on the flank. ¿ American Geophysical Union 1992