Shear wave velocity, seismic attenuation, and thermal structure of the continental upper mantle

Abstract

SUMMARY Seismic velocity and attenuation anomalies in the mantle are commonly interpreted in terms of temperature variations on the basis of laboratory studies of elastic and anelastic properties of rocks. In order to evaluate the relative contributions of thermal and non-thermal effects on anomalies of attenuation of seismic shear waves, Q −1 , and seismic velocity, Vs, we compare global maps of the thermal structure of the continental upper mantle with global Q −1 and Vs maps as determined from Rayleigh waves at periods between 40 and 150 s. We limit the comparison to three continental mantle depths (50, 100 and 150 km), where model resolution is relatively high. The available data set does not indicate that, at a global scale, seismic anomalies in the upper mantle are controlled solely by temperature variations. Continental maps have correlation coefficients of <0.56 between Vs and T and of <0.47 between Qs and T at any depth. Such low correlation coefficients can partially be attributed to modelling artefacts; however, they also suggest that not all of the Vs and Qs anomalies in the continental upper mantle can be explained by T variations. Global maps show that, by the sign of the anomaly, Vs and Qs usually inversely correlate with lithospheric temperatures: most cratonic regions show high Vs and Qs and low T, while most active regions have seismic and thermal anomalies of the opposite sign. The strongest inverse correlation is found at a depth of 100 km, where the attenuation model is best resolved. Significantly, at this depth, the contours of near-zero Qs anomalies approximately correspond to the 1000 ◦ C isotherm, in agreement with laboratory measurements that show a pronounced increase in seismic attenuation in upper mantle rocks at 1000‐1100 ◦ C. East‐west profiles of Vs, Qs and T where continental data coverage is best (50 ◦ N latitude for North America and 60 ◦ N latitude for Eurasia) further demonstrate that temperature plays a dominant, but