Seismic heterogeneity in the upper crust eruption site on the East Pacific Rise , 9 ø 50 ' N near the 1991

We report the results of a small-aperture uppercrustal tomography experiment conducted on the East Pacific Rise near 9ø50'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 x 16 km 2 area. Twoand three-dimensional inversions indicate that the average upper crustal velocities, both on and off axis, increase by -0.2 km/s north of 9ø52'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 9ø52'N being on average at least 300øC warmer than to the north. Off axis, layer 2A may be thicker south of 9ø52'N if a higher proportion of eruptions flow off axis.