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

We report the results of a small-aperture upper-crustal tomography experime nt conducted on the East Pacific Rise near 9 degrees 50'N, the site of a vo lcanic eruption in 1991. The experiment geometry comprised 3 ocean bottom s eismometers and 491 airgun shots bred in a 16 x 16 km(2) area. Two- and thr ee-dimensional inversions indicate that the average upper crustal velocitie s, both on and off axis, increase by similar to 0.2 km/s north of 9 degrees 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 sout hward 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 degrees 52'N being on average at least 300 degrees C warme r than to the north. Off axis, layer 2A may be thicker south of 9 degrees 5 2'N if a higher proportion of eruptions flow off axis.