Global maps of Rayleigh wave attenuation for periods between 40 and 150 seconds

Studies of seismic attenuation must account for the large amplitude deviations caused by elastic focusing of energy. In a new approach, we jointly invert phase and amplitude measurements of 19,000 minor are Rayleigh waves between periods of 40 and 150 seconds. The simultaneous inversion ensures that attenuation and phase velocity are mutually consistent because the phase and focusing term of amplitude are modelled using a common elastic model. At the shortest periods the maps show a good correlation between attenuation and phase velocity, suggesting a common cause in the uppermost mantle, most probably thermal in origin. This correlation is lost at longer periods. The main signal beyond periods of 100 seconds is a strongly attenuating circum Pacific zone and a pronounced ring of high attenuation around Africa. This feature seems reliable in our attenuation maps but not correlated to an equivalent structure in phase velocity. We thus favour scattering of wave energy on large size structures as a possible cause.