Lateral Variation of Shear Velocity and Lg Attenuation Structure along the Hi-CLIMB Profile in Tibet

The Tibetan plateau, an uplifted topography situated to the north of the Himalayas, acts as a decisive region for interpretation of the mechanism of continent-continent collision between Indian and Eurasian plates. Sandwiched between the Tethyan Himalaya in the utmost south and Songpan-Gangze plate in the extreme north, the central Tibetan plateau is constructed of two major geological blocks named the Lhasa and Qiangtang terrane, which are detached by the Bangong-Nujiang suture belt. We have calculated Lg Q and shear velocity crustal structure across the ~900 km long Himalayan-Tibetan Continental Lithosphere during Mountain Building (Hi-CLIMB) profile using the decisive two-station method and joint inversion of receiver function and Rayleigh wave group velocity data. The vertical component of broadband seismograms of 37 regional earthquakes, well distributed in NE, SE, and NW backazimuths, recorded by 171 stations are refined to extricate the Lg amplitude spectra. The 1 Hz Lg Q (Q₀) estimated between 29716 pairs of two stations and out of these estimates 2228 number of high trait interstation Q₀ values are used as an input to the 1-D inversion done through singular value decomposition method with Tikhonov regularization to obtain a lateral variation of Lg Q along the profile. To reduce the inherent error in Q₀ measurements, the maximum azimuthal difference between the source and two stations is set to 5⁰ although a threshold of 15⁰ is allowed. The inversion results provide Lg Q₀ values ranging from 66 (parts of lesser Himalayas including Nepal) to 177 (Qiangtang terrane). Previous studies show that the Main Himalayan Thrust (MHT) underthrusting low-velocity sediments from the Ganges Basin manifest the ubiquity of water released from the underthrust sediments hence decreasing the fault strength and triggering large earthquakes. This instability in this part of the profile correlates well with extremely low values of Lg Q₀. Although inside the plateau Lg Q₀ values are seen to be increased, we observe consistent low Q₀ values across the profile which can be associated with a high Vp/Vs ratio, hence reflecting partial melt in the region. Based on our inversion, we estimated crustal thickness along the profile ranging from 45 km to 72 km (South to North) which is well correlated with the previous studies in this region as well as established a positive correlation with our measured Lg Q₀ values. The most striking result is the high Q₀ value in the Qiangtang terrane compared to the previous studies (Q₀<100). The observed difference may be due to the use of a small number of earthquake data, data used from NE and NW backazimuths, use of the high azimuthal difference between the source and two stations (15⁰), and due to the source/path effect, which is not completely removed in the two-station method. To reduce the attenuation-source tradeoffs and improve the resolution a joint tomography of single station and two stations method might be benevolent. The current study provides a new understanding of the region, improving our perception of the crustal formation of the region.