Crustal Attenuation In The Southern Tibetan Plateau By Lg-wave Q Value Tomography Through The Reverse Two-Station Method

India-Eurasia collision zone is the largest continental collision system on the earth.It is a large-scale natural experiment of continental lithosphere contraction and deformation and also the largest example of extensional tectonic activity for the continental collision zone.Therefore,it has attracted wide attention of geologists and geophysicists all over the world,and more and more geological and geophysical data have been obtained.There have been many researches on the geodynamic model of the Tibetan Plateau,emphasizing the evolution history of the collision zone and the current geodynamic state and proposing many reasons and hypotheses for the tectonic extension of the Tibetan Plateau.However,these theories have not been generally accepted.More evidences are needed to support the active tectonics and dynamic evolution model of the Tibetan Plateau.At present,a variety of research methods,such as seismic velocity structure,deep seismic reflection profiling,magnetotelluric study and so on,have been employed.The results indicate that there is a high-temperature molten layer in the crust of Tibetan Plateau,which is an important factor for energy attenuation of seismic waves.Lateral inhomogeneity of seismic wave attenuation is an effective geophysical approach to infer the crustal dynamic evolution of the Tibetan Plateau.This paper studies the attenuation characteristics of seismic waves in the southern part of the Tibetan Plateau in an attempt to provide constraints on the distribution of melting layers in the crust and bases for the dynamic evolution model of the front end of plate collision.Based on 757 seismic waveform data recorded by 27 broadband stations of Tibet Autonomous Region seismic network from 2017 to 2019,this paper presents a research on Q value tomography of the southern Tibetan Plateau using 1 Hz Lg-wave and Reverse Two-Station Method for the first time.Compared with the previous research methods,the Reverse Two-Station Method has strict requirements on the distribution of stations and events and has a low data utilization ratio,but it has fewer free parameters and smaller calculation error,which make it the most stable and accurate calculation method at present.In this research,1981 Lg-waves were captured by the3.5—2.4 km/s Lg wave velocity window defined in this paper,and the Q value of 13543routes were calculated.After testing the checkerboard recovery of 1°×1°grid and 0.5°×0.5°grid respectively,we got the Lg-wave Q₀ value tomography of southern Tibetan Plateau with resolution of 0.5°×0.5°.The inversion result shows a strong attenuation of the southern Tibetan Plateau crust whose Q value is generally low,especially in the Nagqu-Lhasa-Shigatse area and the Nagqu-Nyingchi area of middle east Tibet.The Q anomaly distribution corresponds to the negative anomaly of P-wave velocity,the geothermal distribution and two rift valleys in the east.Combining geophysical evidence,it is inferred that there may be two fluid melting channels in the southern part of the Tibetan Plateau.The main channel is between Yadong-Gulu rift and Sangri-Cona rift.The secondary channel is separated from the main channel and extends westward along the Yarlung Zangbo Suture Zone.Based on GPS data of different blocks,it is considered that there are many evolution models of the dynamic process in the south of Tibetan Plateau.The west of Yadong-Gulu rift is more in line with the theory of shortening and thickening while the east of that is inclined to the"water pump"theory.