Abstract
We present QADR17, a global model of Rayleigh-wave attenuation based on a massive surface wave data set (372 629 frequency-dependent attenuation curves in the period range 50-260 s). We correct for focusing-defocusing effects and geometrical spreading, and perform a stringent selection to only keep robust observations. Then, data with close epicentres recorded at the same station are clustered, as they sample the same Earth's structure. After this pre-selection, our data set consists of about 35 000 curves that constrain the Rayleigh-wave intrinsic attenuation in the upper mantle. The logarithms of the attenuation along the individual rays are then inverted to obtain global maps of the logarithm of the local attenuation. After a first inversion, outliers are rejected and a second inversion yields a variance reduction of about 45 per cent. Our attenuation maps present strong agreement with surface tectonics at periods lower than 200 s, with low attenuation under continents and high attenuation under oceans. Over oceans, attenuation decreases with increasing crustal ages, but at periods sensitive to the uppermost 150 km, mid-ocean ridges are not characterized by a very localized anomaly, in contrast to what is commonly observed for seismic velocity models. Attenuation is rather well correlated with hotspots, especially in the Pacific ocean, where a strong attenuating anomaly is observed in the long wavelength component of our signal at periods sampling the oceanic asthenosphere. We suggest that this anomaly results from the horizontal spreading of several thermal plumes within the asthenosphere. Strong velocity reductions associated with high attenuation anomalies of moderate amplitudes beneath the East Pacific Rise, the Red Sea and the eastern part of Asia may require additional mechanisms, such as partial melting.
Highlights
Most global tomographic studies of the upper mantle focus on the 3-D distribution of seismic velocities
These studies are generally based on surface waves, which provide global coverage of the upper mantle and have a strong sensitivity to the shear wave (S-wave) velocity
Recent global S-wave tomographic models are obtained from the inversion of massive data sets and show robust patterns, especially in the top of the upper mantle where they agree for horizontal wavelengths smaller than 1000 km (Meschede & Romanowicz 2015; Debayle et al 2016)
Summary
Most global tomographic studies of the upper mantle focus on the 3-D distribution of seismic velocities. Recent global S-wave tomographic models are obtained from the inversion of massive data sets and show robust patterns, especially in the top of the upper mantle where they agree for horizontal wavelengths smaller than 1000 km (Meschede & Romanowicz 2015; Debayle et al 2016). The Earth’s mantle does not behave as a perfectly elastic body and it is in principle possible to extract information on its anelastic properties from the decay of the amplitude of seismic waves. Such attenuation studies are difficult, because the wave amplitude is influenced by a number of mechanisms. Because of the difficulties of measurements and of the numerous mechanisms that need to be accounted for, less work has been done on attenuation than on
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