Variations in the Lg Q Seismic Attenuation in Tarim Basin, Tibet and Bengal Basin

Roy, Subhadeep (2019) Variations in the Lg Q Seismic Attenuation in Tarim Basin, Tibet and Bengal Basin. Masters thesis, Indian Insitute of Science Education and Research Kolkata.

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Abstract

Several studies have been made concerning the structural and physical characteristics of the upper mantle under the Tibet region. The Indian crust colliding with the Tarim basin at about 80⁰E reaches the Bangong Nujiang Suture (BNS) belt around 88⁰E. Currently, the collision between the Indian and Eurasian plates to the southwest results in the upliftment of the Tibetan plateau. The areas of active tectonics are known to have Lg Q values relatively higher than the areas of passive tectonics or where there is a presence of craton. At the Bengal basin, previous research based on the depth of Moho discontinuity shows us that the crust is more likely to be continental in nature in the western and middle parts of the basin and is significantly thinned in the eastern and south-eastern parts. The variation of Lg attenuation has been studied for the central and southern Tibet, Bengal basin and the area around the Tarim basin. By using the standard Two-Station method, we have studied all available good quality events keeping epicentral distance within 20⁰, maximum depth of 100 km,Mb greater than 4.5 and Lg group wave velocity between 2.5 km/s to 3.6 km/s, we calculate the interstation Q₀ (1 Hz Lg Q) and η values, generate contour plots and perform Q attenuation tomography to study its variations. Based on the overall results obtained from these calculations, the Q₀ values are observed to be lower in the Tarim basin and Bengal basin and relatively higher near the central and southern Tibet areas. The underthrusting of the Indian lithosphere and presence of fluid-like materials in the upper crust might be one of the causes of a relatively higher Lg attenuation being seen in these regions. The trend seen in the Bengal basin is that the Lg Q values increase as we move from northeast to southwest. Further, studies based on the Lg Q attenuation tomography tells us about the possible crustal type in the Bengal basin. The obtained results and estimates are found to be consistent with other studies done in those areas.

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