Shear-Wave Splitting and Crustal Anisotropy in the Shillong-Mikir Plateau of Northeast India

Seismic anisotropy of crust beneath the Shillong-Mikir Plateau and the surrounding regions of northeast India have been investigated with the help of splitting analysis of S-wave of local earthquakes. We estimate a total 83 pairs of splitting parameters (Phi and delta t) from 67 local shallow focus earthquakes (depth <= 32 km) recorded by the 10 broadband seismological stations operated in the study region. The results show delay times ranging from 0.02 to 0.2 s, which correspond to anisotropy up to 4%, suggesting significant strength of anisotropy in the study region. Fast polarization direction (Phi) in the Shillong Plateau shows mostly NW-SE trend in the western part and NE-SW trend in the northern part. Phi s near Kopili fault (KF) follows NW-SE trend. Phi at most of the stations in the study region is consistent with the local stress orientation, suggesting that the anisotropy is mainly caused by preferentially aligned cracks responding to the stress field. On the other hand, anisotropy observed near the KF is due to aligned macroscopic fracture related to strike-slip movement in the fault zone.