The global attenuation structure of the upper mantle

Abstract

[1] 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.

Global Imaging of the Earth's Deep Interior: Seismic Constraints on (An)isotropy, Density and Attenuation

“…Dalton et al (2008) compared their global Q μ model to those of Romanowicz (1995), Reid et al (2001), Warren & Shearer (2002), Selby & Woodhouse (2002) and Gung & Romanowicz (2004). When truncated at degree 8, the correlation of these models was found to be mostly below 0.4 throughout the upper mantle.…”

“…When truncated at degree 8, the correlation of these models was found to be mostly below 0.4 throughout the upper mantle. Furthermore, Dalton et al (2008) noted that global Q μ models differ most in the vicinity of mid-ocean ridges where characteristic low-velocity regions can lead to a strong focusing of wave energy that must be accounted for correctly. The strength of the lateral variations in global attenuation models varies by a factor of up to 4, which is significantly above the error estimates of around 10%, given, for instance, by Romanowicz (1995).…”

“…Instead of jointly inverting for all the necessary free parameters, both elastic focusing effects and source/receiver corrections are frequently ignored. While convenient, this approach was shown to produce incorrect estimates of attenuation structure in body and surface wave studies (Dalton et al, 2008;Tian et al, 2009). To isolate the signature of attenuation, several studies proposed to remove the focusing effect from observed amplitudes with the help of 3D elastic velocity models derived from travel time tomography.…”

“…They demonstrated that attenuation estimates depend significantly on each of the amplitude corrections, and that the neglect of elastic focusing translates into inaccurate Q structure. The phase delay maps were then used to remove the focusing effect from the amplitude data, which were then inverted for a global degree-12 Q μ model (Dalton et al, 2008). More than previous models, the work of Dalton et al (2008) reveals a clear correlation between shallow-mantle attenuation and surface tectonics.…”

“…The phase delay maps were then used to remove the focusing effect from the amplitude data, which were then inverted for a global degree-12 Q μ model (Dalton et al, 2008). More than previous models, the work of Dalton et al (2008) reveals a clear correlation between shallow-mantle attenuation and surface tectonics. In particular, high attenuation is found around 100 km depth beneath the circum-Pacific volcanic arc, the Lau basin, and along most mid-ocean ridges, including the East Pacific Rise, the Indian-Antarctic ridge and the Mid-Atlantic ridge.…”