Velocity structures of the upper 2 km of the oceanic crust were used to model seismic refraction data collected on the eastern flank of the East Pacific Rise at 12¿ N during the Rivera Ocean Seismic Experiment (ROSE). Data, generated with a Bolt 1500C air gun source, show multiple narrow (<1 km in range) peaks in the amplitude of the first refracted arrival. Record sections separated by as much as 150 km indicate similar amplitude versus range patterns, suggesting that the amplitude peaks are caused by consistent structural features and not by random crustal inhomogeneities. These amplitude highs do not consistently correlate with seafloor bathymetry and hence cannot be explained solely by topographic focussing of seismic energy. Through travel time analysis and ray tracing we find a crustal velocity model with two vertical zones of concave gradients which focus rays at 6 and 8 km range. Synthetic seismograms and power vesus range patterns computed for this model by the reflectivity algorithm are consistent with the two peak amplitude pattern observed on nearly all air gun record sections. These results suggest that the vertical gradient zone structure of the uppermost 2--3 km of the ocean crust is continuous to first order over the ROSE area.