Sn attenuation characteristics in the Bhutan Himalaya.

Seismic attenuation structure of the uppermost mantle is investigated using Sn waves in Bhutan Himalaya. Sn phase is the uppermost mantle-refracted
phase, which travels with a velocity of 4.3 - 4.7 km/s. Visual inspection of all the seismograms are conducted to examine the efficient, inefficient and blocked paths in the region. The inefficient, and blocked S n phases are mainly observed from the western side of our study region. Sn attenuation is determined using the two-station Methodology (TSM ). We have generated 460 station pairs from 1539 seismograms with magnitude ≥ 4 within an epicentral distance of 200 - 1650 km recorded at 38 seismic stations. Furthermore, a 2D Sn Q model is produced to understand the upper mantle rheology of the area. The central part shows a low Q (≤100) value while high Q dominates in northern and western parts of Bhutan Himalaya. The overall results correlate well with the tectonic setting beneath the study region. A comparison study is also made with the adjacent Arunachal Himalaya for better understanding.