Layered Crustal Structure In Sichuan Basin And The Eastern Tibetan Plateau From Teleseismic Receiver Functions

H-κ stacking with receiver functions has been widely adopted in extracting crustal structure beneath a single station.Based on the assumption of a single-layer flat and homogeneous crust,this method is of high stability.And it can estimate the crustal thickness and average VP/VS ratio,instead of the layered crustal structure.To constrain sediment and basement crustal structure beneath basins,a sequential H-κ stacking has been proposed.But in application,for stations located above unconsolidated sediments and hard-rock sites which could be identified manually,the sequential H-κ stacking and single-layer H-κ stacking have been applied,respectively.This processing is relatively inefficient.To overcome the limitation,we extend the sequential H-κ stacking and apply it to the Sichuan basin and the eastern Tibetan Plateau,providing important evidence for the deep structure and evolution of the study area.The main results are given as following:1.Synthetic tests show that the improved sequential H-κ stacking can be used for both single-layer and two-layer crust models,and is effective to constrain the layered crustal structure.This method has two steps of stacking,which improves computational efficiency and is applied for large datasets efficiently by automatically determining the validity of the first H-κ stacking result.2.The seismic data recorded from the 49 regional permanent stations of the Sichuan basin for 7 years,as well as from temporary stations deployed in the Weiyuan area are used to investigate the fine crustal structures.The improved sequential H-κ stacking result shows that,some stations in the central and northern Sichuan basin ha ve a sediment thickness of 4.6-6.7 km,with average VP/VS ratios of 1.9-2.9.The Conrad discontinuity is obvious beneath some stations in southern Sichuan basin,with depths of 21-25 km.Thus,there is estimated the upper crust thickness accounts for～56%of the entire crust.With the boundary of the Longmenshan Fault,there is east-west lateral variation in the crustal thickness of the study area.The crust of Sichuan basin is about 4050 km,and the crust in west region has significantly thickened.Meanwhile,the relatively high VP/VS ratios appear in the southern segment of the Longmenshan Fault.In addition,doublet Moho structures appear beneath the Huayingshan fault zone locally.3.The layered crustal structure has been constrained by using seismic data recorded from the 55 permanent stations of the eastern Tibetan plateau for recent 10 years.The result shows that the crust thickens from east to west in the eastern Tibetan plateau,and the crust below the Lhasa block is the thickest,with a thickness of up to 84km.The mean VP/VS ratio is 1.75.Moreover,the Conrad discontinuity is also visible beneath some stations in the eastern margin of Tibetan plateau,with depths of 27-33km.Thus,there is estimated the upper crust thickness accounts for 54%of the entire crust.Combine with other geophysical studies,it is inferred that the joint thickening of the upper and lower crust is the crustal thickening mode in the eastern margin of Tibetan plateau.Meanwhile,the low-velocity layer exists in some stations beneath the eastern margin of Tibetan plateau,with depths of 26-31km.It is suggested that the crust flow is only exists beneath several local areas.In addition,doublet Moho structures appear beneath the Lhasa block and the Eastern Kunlun fault zone locally.