Mapping of stress and structure controlled upper crustal anisotropy in Kumaon-Gharwal Himalaya

Abstract

Shear wave splitting analysis of local earthquakes provides valuable insights into the structure and stress controlled upper crustal anisotropy signatures. We examine these signatures at the Kumaon-Garhwal Himalaya and provides an excellent example of anisotropic crustal structure in the ongoing continent-continent collision settings between Indian and the Eurasian plates. A total of 256 local earthquakes were selected for the analysis (1.0 ≤ M ≤ 5.4) between January 2017 and February 2021, recorded at a dense network of 51 broadband stations. The result indicates the observed crustal anisotropy is parallel to stress-aligned micro-cracks far from the major Himalayan fault zones and structure parallel near the fault zones. The dominant fast polarization directions (FPD) in the Inner Lesser Himalaya (ILH) and Higher Himalaya, are consistent with the maximum compressive horizontal stress directions (SH_max), essentially influenced by the local stress field. Whereas in the Outer Lesser Himalaya (OLH) and Sub Himalaya, fast directions are sub-parallel to the structural trends, suggesting that the anisotropy is associated with the shear fabric from recent deformation episodes related to structural induced anisotropy. The computed normalized delay times show a mean value of 1 ms/km, within the OLH and Sub-Himalaya, while in ILH and Higher Himalaya, this value increases up to 5.3 ms/km. The associated crack densities are 0.0038 and 0.0207, with shear wave velocity anisotropy of 0.38% and 2.07% respectively. Significant scatter within the depth range of 10–15 km is observed in normalized delay times, suggesting the source of anisotropy within the upper crust.