We study the variability of the young oceanic crust as a function of age by analyzing 11 digitally recorded sonobuoys and three expanded spread profiles over 0.5-, 2.5-, and 4.5-m.y.-old oceanic crust. The data were collected during the Rivera Ocean Seismic Experiment at the East Pacific Rise between 11¿ and 13¿N. We analyze the data within one-dimensional travel time scheme using the tau-p method and travel time forward modeling. We perform the tau-p mapping by using a polynomial least squares technique that is fast and can be applied even to poor quality data as in this case. We do not see large changes in the velocity structure between 0.5- and 4.5-m.y.-old oceanic crust. The overall structure consists of a rapid increase in velocity from about 2.5--6 km/s within the uppermost 1 km of crust (layer 2A), a zone ≤1 km in thickness with little or no velocity gradient (layer 2B), a thin transition layer with a relatively rapid increase in velocity from about 6.2 to 7 km/s (layer 2C), and a thick nearly homogeneous basal layer with velocities around 7.2 km/s (layer 3). We find that the low velocities at the seafloor and large velocity gradients within the shallow crust can be explained by the hypothesis of large-scale cracks and their cracks and their behavior under confining pressure. For the variation of velocity V versus depth below seafloor z, this hypothesis yields V=(V0 2+Bz)1/2, a relationship that fits our solutions. ¿ American Geophysical Union 1988