We report the results of a small-aperture uppercrustal tomography experiment conducted on the East Pacific Rise near 9 ¿50'N, the site of a volcanic eruption in 1991. The experiment geometry comprised 3 ocean bottom seismometers and 491 airgun shots fired in a 16¿16 km2 area. Two- and three-dimensional inversions indicate that the average upper crustal velocities, both on and off axis, increase by ~0.2 km/s north of 9 ¿52'N. This location coincides with the northern limit of the 1991 eruption and of vigorous hydrothermalism. Our data can be explained by a 100 m southward increase in layer 2A thickness On axis, previous work indicates that layer 2A thickness does not vary in this region. We infer that the sheeted dikes above the axial magma chamber are thermally segmented with the dike layer south of 9 ¿52'N being on average at least 300 ¿C warmer than to the north. Off axis, layer 2A may be thicker south of 9 ¿52'N if a higher proportion of eruptions flow off axis. ¿ 2000 American Geophysical Union