Radial Anisotropy of the Upper Mantle Under Southeast Asia

Radial anisotropy of S-waves is observed as a difference between SV- and SH-wave velocities with vertical and horizontal polarization, respectively, which are inverted from Rayleigh and Love wave dispersion curves. In contrast to isotropic models, presently available distributions of S-wave velocities, accounting for the radial anisotropy, significantly contradict each other. One reason for such discrepancies is that, as a rule, different datasets (paths) for Rayleigh and Love waves are used to calculate the radial anisotropy coefficient. This leads to the fact that the inverted velocity patterns of SV- and SH-waves are smoothed over areas with different shapes and sizes. To exclude this effect, we offer an approach in which the initial data contain only Rayleigh and Love wave dispersion curves along the same paths in the same periods. Then, standard procedures of surface wave tomography and inversion of local surface wave velocities to S-wave velocity patterns are implemented. Using such an approach, we obtained the distribution of the radial anisotropy coefficient (\(\alpha = {{\left( {{{V}_{{SH}}} - {{V}_{{SV}}}} \right)} \mathord{\left/ {\vphantom {{\left( {{{V}_{{SH}}} - {{V}_{{SV}}}} \right)} {{{V}_{{{\text{av}}}}}}}} \right. \kern-0em} {{{V}_{{{\text{av}}}}}}}\), where \({{V}_{{{\text{av}}}}} = {{\left( {{{V}_{{{\text{SH}}}}} + {{V}_{{SV}}}} \right)} \mathord{\left/ {\vphantom {{\left( {{{V}_{{{\text{SH}}}}} + {{V}_{{SV}}}} \right)} 2}} \right. \kern-0em} 2}\)) in the upper mantle of Southeast Asia to a depth of 300 km within 70°–145° E and 20°–40° N. It has been shown that at depths of 50–70 km, maxima of the α‑coefficient are associated with areas with low SV-wave velocities. Moreover, at a depth of 50 km, the highest α values are confined to territories with the maximum horizontal displacement rates according to GPS data (relative to stable Eurasia). We also have found that the areas in which the radial anisotropy is truly negative (α < –1%), i.e., in which V_SV > V_SH, are confined to lithospheric plate boundaries.