Seismic anisotropy beneath the Indian continent from splitting of direct <i>S</i> waves

Abstract

In this study, we investigate the shear wave splitting beneath 39 broadband seismic stations located over various tectonic units of the Indian shield using direct S waves from deep to intermediate depth earthquakes. The delay times between fast and slow axes of anisotropy do not reveal anomalous values that reflect contributions from midmantle or the mantle transition zone. The fast polarization azimuths are generally consistent with the results from SKS splitting that indicate a predominance of plate motion related strain in the Indian continent. Splitting results at station HYB, which was hitherto considered isotropic, indicate significant anisotropy, with a NE oriented fast axis. Although a general trend of SH leading SV is indicated by most of the measurements, invoking radial anisotropy to explain this phenomenon appears farfetched since the near vertical incidence of the S waves in the usable distance range of 40°–80° makes them more sensitive to azimuthal anisotropy underneath the station. Forward modeling of the dependence of the splitting parameters on incoming polarization assuming horizontal axis of symmetry brings out two layers of anisotropy beneath the Indian shield with fast axis azimuths oriented N15°E in the bottom layer and N60°E in the top layer. While stronger anisotropy ( δt ≈ 1 s) in the bottom layer is related to asthenospheric flow, a weaker one ( δt ≈ 0.4 s) in the upper layer could represent anisotropy frozen in the lithosphere.