Significant seismic anisotropy beneath southern Tibet inferred from splitting of direct S-waves Arun Singh a,⇑ , Tuna Eken b , Debasis D. Mohanty a , Dipankar Saikia c , Chandrani Singh a , M. Ravi Kumar d a Department of Geology and Geophysics, Indian Institute of Technology Kharagpur, India b Department of Geophysical Engineering, The Faculty of Mines, Istanbul Technical University, Turkey c Indian National Centre for Ocean Information Services, Hyderabad, India d National Geophysical Research Institute, Council of Scientific and Industrial Research, Hyderabad, India article info Article history: Received 18 July 2015 Received in revised form 3 November 2015 Accepted 5 November 2015 Available online 14 November 2015 Keywords: Seismic anisotropy Tibet Himalaya Shear waves abstract This study presents a total of 12008 shear wave splitting measurements obtained using the reference- station technique applied to direct S-waves from 106 earthquakes recorded at 143 seismic stations of the Hi-CLIMB seismic network. The results reveal significant anisotropy in regions of southern Tibet where null or negligible anisotropy has been hitherto reported from SK(K)S measurements. While the individual fast polarization direction (FPD) at each station are found to be consistent, the splitting time delays (TDs) exhibit deviations particularly at stations located south of the Indus–Tsangpo Suture Zone. The fast polarization directions (FPDs) are oriented (a) NE–SW to E–W to the south of the Indus–Tsangpo Suture Zone (b) NE–SW to ENE–SSW between Bangong–Nujiang Suture Zone and the Indus–Tsangpo Suture Zone (ITSZ) and (c) E–W to the extreme north of the profile. The splitting time delays (dt) vary between 0.45 and 1.3 s south of the ITSZ (<30°N latitude), while they range from 0.9 to 1.4 s north of it. The overall trends are similar to SKS/SKKS results. However, the differences may be due to the not so near vertical paths of direct S waves which may sample the anisotropy in a different way in compar- ison to SKS waves, or insufficient number of SKS observations. The significant anisotropy (0.8 s) observed beneath Himalaya reveals a complex deformation pattern in the region and can be best explained by the combined effects of deformation related to shear at the base of the lithosphere and sub- duction related flows with possible contributions from the crust. Additional measurements obtained using direct S-waves provide new constraints in regions with complex anisotropy. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Measurements of seismic anisotropy from the splitting of core refracted phases like SK(K)S are often utilized to decipher the mantle deformation patterns in different tectonic environments including rift zones, mountain belts, mid oceanic ridges and conti- nental shield regions (Savage, 1999). The prominence of these phases stems from the fact that they are devoid of contamination from effects of source-side anisotropy and also provide high lateral resolution in comparison to surface waves. The other phases frequently used to obtain the splitting parameters are the direct S, ScS, PKS and PKKS. PKS and PKKS are free of source side splitting effects, but harder to interpret because of interference with SKP and SKKP, respectively. The use of direct S-waves is limited when the strong anisotropy that likely exists beneath the region near the source and/or the region along the propagation path outside the receiver-side can contaminate the S-wave signal. S-waves are important for obtaining splitting parameters, they not only supplement the SK(K)S observations but also fill the azimuthal gaps and sample a wider range of incidence angles. The S waveforms from deep focus (>200 km) earthquakes in the teleseismic distance range are generally used to characterize the receiver-side anisotropy assuming that source side anisotropy is minimal at these depths. Previous estimates of delay times and the fast polarization directions (FPDs) from Tibetan region using direct S waves from deep focus earthquakes are considered to be accurate if they are consistent with those obtained from the SKS splitting (McNamara et al., 1994; Huang et al., 2011). The consider- ations are that the direct S-waves from earthquakes at depths >200 km actually encounter regions where a weak LPO (lattice pre- ferred orientation) is developed due to change in the deformation mechanism of mantle flow (Karato and Wu, 1993). Anisotropy in the mantle is dominated by the lattice preferred orientation http://dx.doi.org/10.1016/j.pepi.2015.11.001 0031-9201/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: arun@gg.iitkgp.ernet.in (A. Singh). Physics of the Earth and Planetary Interiors 250 (2016) 1–11 Contents lists available at ScienceDirect Physics of the Earth and Planetary Interiors journal homepage: www.elsevier.com/locate/pepi