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 degrees-200 degrees 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 similar to 125 km and similar to 225 km beneath oceans and cratons, respectively, which may indicate the base of a compositional boundary layer. (C) 2009 Elsevier B.V. All rights reserved.