Rayleigh wave attenuation across the Atlantic-Indian ridge.

Abstract

The regionalization method has been applied to the attenuation coefficients of the vertical components of Rayleigh waves for the Southern Atlantic-Indian Ocean to obtain the attenuation across the ridge region. The obtained ridge attenuation coefficients are similar to those for the North Atlantic ridge and much lower than the values for the East Pacific Rise. Inversion theory, in its stochastic form, shows that the low-Q zone under the ridge sems to be located between 50 km and 200 km depth, with the maximum Q-1 value (≅15×10-3) at a depth of about 140 km. Comparison of the ridge model in this study with models for the North Atlantic ridge and East Pacific Rise, obtained by others, indicates that the attenuation coefficients for the study region are similar to those for the North Atlantic ridge, and much weaker than those for the East Pacific Rise. Since the spreading rate in the southern part of the Atlantic-Indian Ocean is similar to that in the North Atlantic, and lower than the corresponding rate in the Pacific, it seems that there is a close relation between spreading rates and low-Q regions under the ridges, with the Q values being higher in the region where the spreading rates are lower.The above results suggest that the creep rate under the ridge of the Southern Atlantic-Indian Ocean may be similar to that for the Atlantic, and much lower than for the Pacific Ocean for regions of comparable ages.