Effects of Partial Melting on Seismic Velocity and Attenuation: A New Insight from Experiments

The effects of partial melting on seismic velocity and attenuation have long been studied by focusing on the direct effects of melt, such as the poroelastic effect. The direct effects are generally very small for a very small melt fraction. Because geochemical studies have shown that the melt fraction during partial melting is very small (∼0.1%), it is difficult to explain upper-mantle low-velocity regions by the direct effects of melt. Recent experimental studies, by using a rock analog, have captured a significant enhancement of polycrystal anelasticity just before partial melting in the absence of melt. This newly recognized effect enables us to interpret seismological and geochemical observations consistently. The new anelasticity model significantly changes the interpretation of upper-mantle seismic structures. This review summarizes the recent progress in the understanding of polycrystal anelasticity, starting from a basic knowledge of linear anelasticity.