SHEAR WAVE VELOCITY, SEISMIC ATTENUATION, AND THERMAL STRUCTURE OF THE CONTINENTAL LITHOSPHERE

Theoretical models based on laboratory studies of dissipation of energy in the crystalline rocks typical for the Earth’s mantle suggest a temperature dependence of attenuation through the activation energy. We therefore compare global maps of the thermal structure of the continental lithosphere with the inverse attenuation of seismic shear waves Qs and seismic velocity Vs as determined from surface wave dispersion and amplitudes. Our study is based on recently available global databases.

We compare the values of Qs, Vs, and temperature T at the depths of 50, 100, and 150 km in the continental lithosphere. We find that qualitatively (by the sign of the anomaly) the maps of Qs closely correlate with lithospheric temperatures. The best correlation is observed for the depth of 100 km, where the resolution of the attenuation model is the highest. At this depth, the contour of zero attenuation anomaly approximately corresponds to the 1000^oC contour of lithospheric temperature, in agreement with laboratory data on a sharp change in seismic attenuation and shear velocities in upper mantle rocks at 900-1000^oC. The correlation between Vs and two other parameters (T and Qs), though present, is less distinct.

We find that most cratonic regions (the Siberian Craton, the East European Platform and the Baltic Shield, the North American Craton, West Africa, western Australia) show high lithospheric Vs, Qs and low T. In contrast, the South African craton has neither high Qs, nor low temperatures. Several prominent low Qs regions correlate with high lithospheric temperatures; this includes the Paleozoic West Siberian Basin, the Cenozoic-Paleozoic structures of the Western Europe, and western North America.

We calculate correlation coefficients between Vs, Qs and T and find that at any depth, for any pair of the parameters the correlation is less than 0.42. It implies that even if temperature variations in the lithosphere are the main cause of seismic velocity and attenuation variations, the relation between temperature and seismic properties is non-linear and the concept of the compositionally homogeneous lithospheric roots is not true.