Crustal thickness and Poisson’s ratio variations in the Siang Window and adjoining areas of the Eastern Himalayan Syntaxis

Abstract

We investigated the average crustal thickness and Poisson's ratio (σ) at 8 broadband seismological stations established in the Siang Window of the Eastern Himalayan Syntaxis. The H-k stacking method has been adopted on receiver functions of teleseismic earthquakes recorded by the seismological stations. The study reveals the variation of crustal thickness from ∼38 km in the Brahmaputra Valley (Pashighat) to ∼53 km at the northern boundary of the window. The estimated Poisson’s ratios show significant variation from 0.23 to 0.308 in the window. The average crustal Poisson’s ratios (σ) estimated at each station are divided into three categories, low (σ ≤ 0.25), intermediate (0.25 ≤ σ ≤ 0.27), and high (σ ≥ 0.27). The wide variations in σ can be attributed to the local geology. The extremely low σ at Along (0.23) can be explained by the presence of SiO2 rich quartzite in the vicinity, while the extremely high σ at Gelling (0.308) suggests the presence of partially serpentinized rocks in the ophiolitic mélange. The available data of Poisson’s ratios and Moho depths in and around the study region has been plotted to portray regional variations. The Siang Window is seismically active up to ∼40 km crustal depth.