Seismic attenuation near the East Pacific Rise and the origin of the low-velocity zone

Low shear wave velocities beneath mid-ocean ridges and in the low-velocity zone beneath oceanic plates commonly have been attributed to the presence of melt or dissolved water, but several recent studies have challenged that interpretation. The alternative is that the anelastic effects of increasing temperature may cause the observed drop in velocity along with a predicted increase in attenuation. We report the first measurements of surface wave attenuation within regional arrays of seismometers on the seafloor. Near the East Pacific Rise, there is much less attenuation than is predicted by models in which the velocity is controlled solely by the direct elastic and anelastic effects of changing temperature, suggesting that melt and water concentration do play an important role. There also is somewhat less attenuation than is found in global studies; we speculate that scattering from unresolved velocity heterogeneities contribute to the apparent attenuation in global studies.