Deep Earth Structure: Q of the Earth from Crust to Core

Abstract

We present a review of our knowledge pertaining to the nature and distribution of attenuation in the Earth, with an emphasis on the progress in the last 10 years. We discuss frequency dependence of the quality factor Q in the Earth, 1D profiles and lateral variations in the crust and mantle, and the attenuation structure in the inner core. Much progress has been made recently in characterizing the lateral variations in attenuation in the crust and their relations to tectonics. Characterizing the nature and distribution of attenuation deeper in the Earth is still hampered by the need to separate elastic and anelastic effects on seismic-wave amplitudes. Nevertheless, consistent and robust features are emerging in recent tomographic studies of attenuation in the upper 200–300 km of the mantle, which show that, in this depth range, lateral variations in attenuation are correlated with those in seismic velocities and with tectonics. This correlation fades out in the transition zone, where shear attenuation distribution bears more resemblance to velocity structure in the lowermost mantle. An unresolved issue is still a 20% discrepancy between the measurements of fundamental-mode Rayleigh waves obtained using a propagating wave versus a normal mode approach. Regional studies of attenuation confirm the relation of Q with tectonics in the upper mantle and very strong lateral variations in subduction zone areas. In the inner core, recent studies have confirmed the increase of Q with depth and have found an intriguing hemispheric variation in Q α at the top of the inner core. Much of the attenuation in the inner core appears to be due to scattering.