Seismic anisotropy and crustal deformation in the Satluj valley and adjoining region of northwest Himalaya revealed by the splitting analysis of Moho converted Ps phases

Abstract

• Significant crustal anisotropy has been detected in the Satluj valley, NW Himalaya. • Fast polarization directions are parallel or sub-parallel to surface geological features . • Strength of anisotropy suggests middle and lower crust as a primary source. • Extension tectonics in Tethyan Himalaya causes E-W or NW-SE anisotropy. • Structural anisotropy is predominant in Lesser and Higher Himalaya. Seismic anisotropy in the crust beneath the Satluj valley and the adjoining region of the northwest Himalaya has been studied with the help of shear wave splitting analysis of P -to- S or Ps converted phases originating at the crust-mantle boundary. A total of 144 splitting parameters (Φ, δ t ) have been computed from 130 teleseismic earthquakes recorded by 13 broadband seismological stations spanning from the Lesser Himalaya to Tethyan Himalaya passing across the Satluj valley region. The predominant NW-SE fast polarization directions (FPDs) in the Lesser and Higher Himalaya follow the strike of surface geological features suggesting structural anisotropy. The NW-SE oriented FPDs in the Tethyan Himalaya fairly coincide with the regional extensional strain. The presence of such extensional strain within the crust might cause Lattice Preferred Orientation (LPO) of anisotropic minerals resulting in observed anisotropy. The large strength of anisotropy (δ t : 0.15–0.80 s) suggests a primary contribution of anisotropy from the middle and lower crust. These observations support the assumption that the deep crust in the study region has undergone widespread and relatively uniform strain in response to crustal shortening and E-W extension.