2-D Sn wave attenuation tomography beneath the Eastern Himalaya

Abstract

SUMMARY The Arunachal and Bhutan Himalaya, which are tectonically distinct from other regions of the Himalaya, have a structure that is quite intricate. The eastern Himalayan segment is a component of the region where the Indian and Eurasian plates collided 50 Myr. The Indian Plate goes beneath the Eurasian Plate in the north, and in the eastern part of the region, the Indian Plate subducts under the Burmese Plate. Here, we studied the seismic attenuation of the uppermost mantle by measuring the quality factor of the Sn wave (SnQ) to understand the dynamics of the lithospheric mantle and the cause of the seismic anomalies found in this area. The upper mantle Q structure has significant lateral differences in Arunachal and the Bhutan Himalaya. Arunachal Himalaya’s central region is characterized by a very low $Q\, (\le 150)$. The successive low-high-low SnQ values in eastern Arunachal Himalaya near Siang region have been observed. The western Arunachal region, close to the Bhutan border, exhibits a contrast in Q values. We notice that low Q values (≤200) predominate in the central to eastern Bhutan Himalaya. The western part of Bhutan Himalaya exhibits relatively high $Q\, (\ge 200)$ values, mostly near Paro and Thimpu. Interestingly, a clear boundary between low and high Q has been observed near Kakthang thrust (KT) in the Bhutan Himalaya. We found significant lateral variation of frequency dependent parameter (η) across the study region. They range from 0.25 to 0.75, with low values (≤0.5) found mostly in the central Bhutan Himalaya and in a few isolated areas of the Arunachal Himalaya. Low Q and a relatively higher $\eta \, (\ge 0.5)$ might suggest that the scattering attenuation is the controlling mechanism for Sn wave attenuation in the upper mantle beneath Arunachal Himalaya. On the contrary, dominant low Q values across the central segment of the Bhutan Himalaya, along with a low to moderate body wave velocity and dominating low η values, subsequently corroborate that intrinsic attenuation is the dominant factor in the upper mantle of the central Bhutan Himalaya.