Investigation of Lithospheric Structure in NE India Based on Love Wave Data

Abstract

We analyzed 228 earthquake data of 5≤Mw≤6.9 to estimate the Love wave group velocity tomographic image and investigate the lithosphere structure of NE-India. These events of 2001-2015 were recorded by 26 seismic stations of IMD, India, and IRIS. Multiple Filtering Technique is used to estimate fundamental mode Love wave group velocity dispersion curves between 4s and 70s for 846 paths. Then, we constructed Love wave group velocity maps at different periods from 6 s to 60 s through inversion over a 1°×1° grid indicating group velocity variations between ~2 km/s and 4.6 km/s in this part of the India-Eurasia and India-Burma collision zones. Tomographic maps at lower periods show good correlations with surface features. Group velocities at 6s to 16s are sensitive to the uppermost crust. They show high variation related to local geological features like sedimentary basins, basement rocks, Precambrian, and metamorphic rocks. Bengal-Basin and Indo-Burma Ranges have lower group velocities at periods ≤16s compared to those located at Shillong Plateau, Mikir Hills, and the Eastern Himalayan ranges. Low-velocity zone systematically shifts eastward towards the southern part of the Indo-Burma Range for periods from 16 to 38s. A prominent increase in group velocity from 38s is observed along a line trending in the NE direction through the Shillong Plateau, Mikir Hills, and Assam syntaxis. At periods >50s, low velocity is observed in the Tibetan plateau. Inversion of Love wave group velocity was carried out and a tomographic image of SH velocity variation was obtained for the study area. It shows a significant variation in the SH velocity for the crust and upper mantle region of the study area. Based on the estimated Love wave group velocity and SH velocity tomograms we came to the following conclusions. The sedimentary basins like the Bengal Basin, and Brahmaputra River Basin show up as low-velocity zones in both group and SH velocity tomograms. In the Bengal Basin, sedimentary layer thickness varies from 5km in the western part to 15km in the eastern part. Maximum thickness was observed in the SE part of the basin near the Indo-Burma Ranges. The Moho depth below the Bengal Basin varies between 28 km and 32km and 35km and 45km below NE India. The NE trending region showing high group and SH velocity values passing through the Shillong Plateau, Mikir Hills, and Assam syntaxis represent a zone where the Indian plate has buckled upward. This is caused by it being in a vice-like grip between the Eastern Himalayas towards its north and the Indo-Burma Ranges towards its east. The crust below the Tibet and Lasha block is much thicker (up to ~85 km) compared to other parts of the study area. A low-velocity zone is observed in the mid-to-lower crust beneath southern Tibet. This is caused by partial melting in this zone. Mostly the Love wave inversion result matches with previously observed Rayleigh wave inversion and discrepancies in some sections highlight the existence of radial anisotropy.