The global attenuation structure of the upper mantle

A large dataset 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 anomalies are considered to depend on four factors: intrinsic attenuation along the ray path, elastic focusing effects along the ray path, a source factor accounting for uncertainties in the strength of excitation, and a receiver factor accounting for uncertainties in the response at the station. The retrieved attenuation structure is shown to be dependent on corrections for focusing effects, source uncertainty, and receiver uncertainty and exhibits stronger agreement with lateral velocity variations than was true for earlier attenuation studies. Lateral variations in upper-mantle attenuation are large, ±60% - ±100%. The amplitude measurements are sufficiently sensitive to velocity structure that phase-velocity maps can be determined from those data alone. Comparison of the new attenuation model with global seismic-velocity models in the uppermost mantle shows a dependence of both quantities on continental temperature estimates and on tectonic region, with young oceanic regions characterized by the slowest velocity and highest attenuation, while the fastest velocity and lowest attenuation values are associated with continental shields and subsided platforms. Recent results from mineral physics allow temperature to be inferred from an observed relationship between velocity and attenuation. At 100 km, comparison of attenuation and velocity models suggests that lateral variations in temperature range from 250-450 K, depending on assumptions about mantle grain size. While oceanic regions agree well in both magnitude and trend with the predictions from mineral physics, fast-velocity and low-attenuation continental regions deviate from the predictions. Observations such as these may be valuable for constraining compositional variability in the upper mantle, or may instead be indicative of dry and depleted continental lithosphere.