Upper mantle anisotropy in Sri Lanka using shear-wave splitting analysis: Geodynamical implications

Anisotropy, a term used by seismologists to describe the variation of seismic wave velocity in different directions, is an excellent tool to examine the deformation (past and present) in the upper mantle. Seismic anisotropy beneath Sri Lanka region is investigated using core-refracted SK(K)S phases at three stations, namely MALK, PALK and HALK. Shear wave splitting measurements were done for ~30 high-quality waveforms recorded in the region, using the rotational correlation (RC), the minimum energy (SC), and the eigenvalue (EV) methods. The shear wave splitting measurement results in Sri Lanka show the presence of two anisotropic layers in the upper mantle, viz., NW–SE fast polarization direction for MALK and HALK, and NE–SW for the PALK stations in the upper layer; NNE–SSW for MALK and HALK, and NW–SE for PALK in the lower layer. Overall, fast polarization direction for Sri Lanka region is found to be NE–SW. The delay time in the upper and lower layer is found to be between 0.3–0.5 s and 0.6–0.9 s, respectively. The observed average delay time (1.2 s) in the lower layer of upper mantle describes a ~134 km thick anisotropic layer with 4% anisotropy beneath Sri Lanka region. Comparison of absolute plate motion (APM) direction with fast polarization directions indicate that the simple asthenospheric flow (SAF) model prevails in this region. On the other hand, comparison of maximum horizontal stress (S _hmax) and the global positioning system (GPS) with the fast polarization direction indicates that there is partial contribution from lithospheric mantle. The anisotropy in Sri Lanka could be mainly governed by asthenospheric flow and partially due to lithospheric mantle.