The 1982 MAGMA seismic refraction experiment yielded a large set of accurate P wave travel times corrected for bathymetry and anisotropy which sample the structure of the East Pacific Rise (EPR) at 12¿50'N. The arrivals were recorded using ocean bottom seismographs at three sites: on axis, and at 7 km and 16 km east of the axis. We invert 2320 travel times for the two-dimensional crustal seismic velocity structure across the EPR axis, assuming that the velocity structure is invariant along strike. The travel times provide strong evidence for compressional velocity anisotropy in the upper crust corresponding to ~10% faster velocities for propagation parallel to the axis than perpendicular to the axis; the travel times used for the tomography are corrected for the effects of this azimuthal anisotropy. Our preferred model contains only the structure clearly required by the data (structure which is stable under excessive smoothing) and achieves a variance reduction of 81% relative to the laterally homogeneous starting model. We resolve a substantial zone beneath the rise axis in which the velocity is reduced by 0.4 to 0.7 km/s; this low-velocity zone (LVZ) is about 7 km wide and extends from a depth of about 1.5--2.0 km down to Moho at a depth of 5.5 km. The LVZ is slightly asymmetric, extending 1 km further to the east than to the west of the axis.

In the shallow (<1.0 km depth) crust, a pattern of velocity variations is imaged in which velocities are high at the spreading axis, decrease between 3 km and 7 km east of the axis, and then increase again between 10 and 15 km east of the axis. We investigate the resolution of the inversion using an impulse response method; the LVZ and off-axis upper crustal variations are well resolved. In addition, the travel time data indicate that an axial high-velocity anomaly less than 2 km wide exists in the upper crust but is not resolved by the inversion. The small velocity reductions of the LVZ are consistent with hot rock containing only a small quantity of melt. These results, combined with the multichannel seismic reflection lines and expanded spread profiles from the northern EPR, suggest that the zone of high melt fraction under the spreading center is confined to a narrow, thin lens capping a broad zone of hot plutonic rocks. The upper crustal velocity reduction within 1 km of the axis reflects near-axial thickening of the extrusive layer and the later reduction probably reflects porosity increases due to near-axial tectonism; the upper crustal velocity increase beyond 15 km off axis is attributed to porosity decreases associated with hydrothermal alteration. ¿ American Geophysical Union 1992