Crustal Thicknesses And Poisson’s Ratios Of The Southeastern Margin Of The Tibetan Plateau

The southeastern margin of the Tibetan Plateau,located to the east of the collision zone of Indian plate and Eurasian plate,which forms the south section of the NorthSouth seismic belt of mainland China,is one of the regions with frequent seismic activities and severe earthquake hazards in China.There are many deep and large-scale faults in this region,including Red River Fault and Xiaojiang Fault,both of which are key areas to understand deep structures and dynamic mechanism in the southeastern margin of the Tibetan Plateau that attract many geophysicists and seismologists to develop multiple research of different aspects.Accurate crustal structure of this region would give important references to research topics like crustal flow,mantle material movement,etc.Receiver function is an effective way to achieve crustal and upper mantle structure,and it is widely used in seismological research.Receiver functions,presented as time series with the impacts of earthquake source,travel path,and other factors removed,can show the relative response of Earth structure under the stations.They mainly contain the information of P-to-S converted waves and their multiples generated from the velocity discontinuities under the receivers.H-κ stacking of receiver functions is widely used to obtain crustal thickness and / ratios under seismic stations.This method stacks multiple receiver functions obtained from multiple earthquake events,and searches for the optimal solution for and where the stacking amplitude reaches maximum.Chuxiong basin and Simao basin are two major sedimentary basins located at the southeastern margin of the Tibetan Plateau,with widely distributed low-velocity sedimentary layers.Previous studies have shown that the existence of a low-velocity sedimentary layer in the upper crust will strongly affect the results obtained by the conventional H-κ stacking due to the strong near surface reverberations caused by the discontinuities related to the low-velocity sedimentary layer.To eliminate this effect,we firstly process the receiver functions with a resonance removal filter,and then apply an improved time-corrected H-κ stacking to obtain the crustal thicknesses and / ratios beneath the stations.This study uses data of 1268 teleseismic events with magnitude larger than M5.0 and epicentral distance of 30° to 90° recorded by 133 seismic stations from Chin ArrayHimalaya Phase I and Western-Sichuan dense array.By deploying the water level deconvolution technique and a selection procedure based on signal-to-noise ratio(SNR),we obtained 16,712 high quality receiver functions.We use the improved H-κ stacking method to obtain the crustal structure beneath the stations in the study area.The results are further compared with those obtained by the conventional H-κ stacking method.In this study,we obtained the sub-sediment crustal thicknesses and / ratios beneath the stations in the study area.The Poisson’s ratios were further calculated based on the relationship between / ratio and Poisson’s ratio.Based on the general distribution of the sub-sediment crustal thickness and Poisson’s ratio and the variation of crustal thickness along two profiles which go across the Red River Fault and Xiaojiang Fault,we analyzed the crustal structure,material movement and other problems in the southeastern margin of the Tibetan Plateau.The results show that crustal thickness varies greatly in the study area,ranges from 30 km to 60 km.The northern part of the study area has a thicker crust,and the sub-sediment crustal thickness in the southern part is somewhat smaller.The Red River Fault draws an apparent boundary on crustal thickness.To the east of the northwester part of the Red River Fault,crustal thickness becomes lower from northwest to southeast,indicating that it is the boundary of the southeastern escape of the Tibetan Plateau;the crust is thicker within the Sichuan-Yunnan rhombic block than in the adjacent area;the strong variation of crustal thickness on the different side of Xiaojiang Fault indicates that it blocks the southeastern escape of the Tibetan Plateau.Poisson’s ratios are generally high in the north and low in the south,the feature of which is positively correlated with the variation of crustal thickness.This phenomenon indicates lower crust thickening in the study area.Higher Poisson’s ratio around the Red River Fault may be related to partial melt at the bottom of the lower crust.The relatively low Poisson’s ratio in the southeastern part of the study area infers that there are more felsic components in this region than the average continental crust.After applying the resonance removal filter and the time-corrected H-κ stacking(i.e.improved method),sub-sediment crustal thickness results beneath most of the stations are 2-4 km thinner than crustal thickness obtained from conventional method,indicating that the improved method can effectively remove the effect of low-velocity sedimentary layers.