The global attenuation structure of the upper mantle

A large data set of fundamental mode Rayleigh wave amplitudes is analyzed to derive a new global three‐dimensional model of shear wave attenuation in the upper mantle. The amplitude observations span a range of periods between 50 and 250 s and are derived from earthquakes with M W > 6.0 that occurred between 1993 and 2005. Four separate factors may influence an amplitude anomaly: intrinsic attenuation along the raypath, elastic focusing effects along the raypath, uncertainties in the strength of excitation, and uncertainties in the response at the station. In an earlier paper (Dalton and Ekström, 2006a), dependence of the retrieved attenuation structure on these terms was shown to be significant and an approach was developed to invert the amplitudes simultaneously for each term. The new three‐dimensional attenuation model QRFSI12, which is the subject of this paper, is derived using this method. The model contains large lateral variations in upper‐mantle attenuation, ±60% to ±100%, and exhibits strong agreement with surface tectonic features at depths shallower than 200 km. At greater depth, QRFSI12 is dominated by high attenuation in the southeastern Pacific and eastern Africa and low attenuation along many subduction zones in the western Pacific. Resolution tests confirm that the change in pattern of attenuation above and below 200‐km depth can be determined with confidence using the fundamental mode data set. The new model is highly correlated with global models of shear wave velocity, particularly in the uppermost mantle, suggesting that the same factors may control both seismic attenuation and velocity in this depth range. However, forcing the lateral perturbations in attenuation to match those found in global velocity models decreases the data variance reduction, which suggests that subtle differences between patterns of attenuation and velocity are robust.