Global seismological shear velocity and attenuation: A comparison with experimental observations

We present a comparison of seismologically observed shear velocity and attenuation on a global scale. These observations are also compared with laboratory measurements of the same quantities made on fine-grained olivine and extrapolated to upper-mantle conditions. The analysis is motivated by recent developments in global attenuation tomography and in laboratory measurements of velocity and attenuation at seismic frequencies and upper-mantle temperatures. The new attenuation model QRFSI12 is found to be strongly anti-correlated with global velocity models throughout the upper mantle, and individual tectonic regions are each characterized by a distinct range of attenuation and velocity values in the shallow upper mantle. Overall, lateral temperature variations can explain much of the observed variability in velocity and attenuation. The seismological velocity–attenuation relationship for oceanic regions agrees with the experimental observations at depths > 100 km and indicates lateral temperature variations of 150°–200 °C at 150 and 200 km beneath the seafloor. The seismic properties of cratonic regions deviate from the experimental trends at depths < 250 km, suggesting differences between oceanic and cratonic composition or water content at these depths. Globally, seismic properties shift into better agreement with the mineral-physics data at depths of ~ 125 km and ~ 225 km beneath oceans and cratons, respectively, which may indicate the base of a compositional boundary layer.