D″ anisotropy beneath Southeast Asia

Earthquakes from the Banda Sea and Flores Sea region recorded at the Kyrgyz Network in the Tien Shan region show evidence for anisotropy in the lowermost mantle beneath South East Asia. Shear-wave splitting is measured for the ScS phase after a receiver-side correction is applied. Individual results show scatter in measured time lag and fast direction, but stacking these gives a stable estimate of 0.8 ± 0.04 s and 43 ± 4 degrees. This equates to a fast-shear wave with a polarisation deviation from horizontal (at the bounce point) of around 9 degrees. The simplest model to explain is a transversely isotropic medium with a slight dip to the east, and an inferred anisotropy of ∼0.25%. This is compatible with previous observations of VSH > VSV for this region. Scenarios which might explain the observed anisotropy include sheared melt pockets or aligned post-perovskite crystals associated with palaeosubduction of the Pacific plate and Indian plate.