Evidence of Lower Crustal Low‐Velocity Layer in the Eastern Himalaya

ABSTRACT To evaluate seismic risk and comprehend the tectonic activity in the northeast Himalayan region, it is essential to have comprehensive knowledge of the crustal parameters and shear‐wave velocity contrast across the Moho ( δβ M ). Estimations of δβ M were carried out using seismological data recorded at 8 BBS located in the northeast corner of the Indian Plate in the Eastern Himalayan Syntaxis. The estimation of δβ M utilises P‐to‐s converted wave amplitude data that takes place at the crust–mantle boundary. The values of δβ M range from 1.72 to 1.88 km/s across the area under investigation. The Moho depth, V p / V s , and Poisson's ratios ( σ ) values have been taken from the published results below these stations. The crustal thickness ( H ) in the study region varies from ~38 km in the Brahmaputra Valley (Pashighat) to ~53 km at the northern boundary (Gelling) of the window. The average crustal σ varies from 0.23 to 0.38 throughout the study region. The thinner crust beneath the Tidding–Tuting Suture compared to the Indus Tsangpo Suture Zone of northwest Himalaya is caused due to the differences in convergence rate, higher exhumation rate, and mechanisms to accommodate collision and rotational tectonics. The scaling relation between δβ M with H , σ , and V p / V s is positive, which indicates the thick crust associated with a high δβ M , meaning that the presence of fluid at the lower crust in the study region. The scaling relation, variation in δβ M , σ , and V p / V s ratio can be explained by the presence of a low‐velocity zone in the lower crust. Previous studies suggest that the Siang Window is seismically active up to ~40 km crustal depth. It is suggested from present research that the accumulation of strain in the crust–mantle transition zone, which is rich in metamorphic dehydration fluids, dominated by water but possibly containing other volatiles, might be sufficient to generate seismicity in the lower crust. The sharp velocity contrast in the crust–mantle boundary may suggest strong deformation coupling between crust and the uppermost mantle. The identification of such a velocity contrast could allow tracking of temperature–pressure conditions responsible for the genesis of the crust–mantle boundary and their evolution.