Seismological constraints on attenuation in the Earth: A review

Abstract

Resolving anelastic attenuation, its spatial and depth variations, can provide important constraints on the composition and dynamics of the earth. However, measuring Q is difficult because of interfering non-linear elastic effects which contribute to the amplitudes of seismic waves. We review recent progress in the determination of global Q-structure in the earth using seismic data. The 1D profile of Qμ appears to be reasonably well constrained in the upper-mantle and transition zone, less so in the lower mantle and D. Some discrepancies remain, in the inner core, between body wave and free oscillation estimates of average Qμ and its depth variation and it is not yet clear whether most of the bulk attenuation is in the core or in the mantle, although recent normal mode studies favor the latter. Large lateral variations in Q are now well established in the crust and the uppermost mantle, where they correlate with past and present tectonic activity. Only few studies to date have proposed 3D tomographic images of the deeper parts of the mantle. While significant lateral variations are well documented, and appear to correlate with those of elastic heterogeneities, large uncertainties remain in the amplitudes of the lateral variations and their details. While there may be some indication for lateral variations in D, it is not clear yet whether such variations are required at all in the bulk of the lower mantle. An interesting recent finding has been that of anisotropy in attenuation in the inner core. More work is needed to document and understand the latter, constrain Q (1D and 3D) in the lower mantle, and resolve lateral variations in the upper mantle more quantitatively. A particular challenge for the future is to be able to accurately quantify the perturbing effects due to elastic structure in global measurements of surface wave and body wave amplitudes.