Crustal And Upper Mantle Structure On The East Boundary Of Sichuan-yunnan Block From Receiver Function Study

Sichuan-Yunnan region is located in the southeast margin of Tibet plateau,adjacent to the Yangtze platform and the southeast Asian plate. Geological tectonicactivities are closely related to the strong collision between India plate and Eurasianplate since65Ma, and the strong tectonic activities caused by the collision are stillgoing on. Sichuan-Yunnan block is one of the main active blocks in this area. The eastboundary of this block is mainly controlled by south of Xianshuihe fault zone,Anninghe fault zone, Zemuhe fault zone and Xiaojiang fault zone from north to south.This region is located in middle and southern segments of south-north seismic belt,and seismic activity is very frequent. This area is with a strong earthquakebackground, strong earthquakes often occur here. What’s more, Anninghe fault zoneseismic gap and abnormally low b value in the middle of Anninghe fault zone and thenorth of Zemuhe fault zone indicate that in the next decade exists the medium-andlong-term major earthquake risk.Special tectonic environment, frequent seismic activities and strong earthquakeactive period maybe coming in the next decade or so lead to deep structure andcharacteristic of medium here has been hot topics in the study of geoscientists anddraw attention of many scholars. They use a variety of means to study this area andobtain electrical and velocity structure of this area. Thereinto, because receiverfunction research has an advantage of the velocity discontinuity study, the method thatusing teleseismic P wave extract receive function, and then invert to get S wavevelocity structure, crustal thickness and poisson’s ratio is one of the effective means toobtain the crust upper mantle S wave velocity structure, crustal thickness andpoisson’s ratio. A lot of studies on this region adopt this method and have acquiredgood results.However, because of the restriction of the observation condition, on the eastboundary of Sichuan-Yunnan block and its adjacent area, the distance between regional fixed stations is big and the distribution of these stations is uneven. Thesecauses lead to using receiver functions recorded by regional stations to study theproperties of the media of the fault zones and on either side of them can not get highlateral resolution.In this study, teleseismic waveform data from37temporary stations along themain fault zone (Anninghe, Zemuhe, Xiaojiang fault) of the eastern boundary of theSichuan-Yunnan block and14permanent stations in Sichuan regional seismic networkwere used to compute crustal thickness, velocity ratio (Vp/Vs) and s-wave velocitystructure by time domain iterative deconvolution, the H-к stack and waveforminversion method. Finally, we obtain the results of30stations.The results show that the crustal thickness becomes thinner gradually from west（~60km）to east，South China block（~35km）. Drawing the line on Anninghe-Daliangshan fault zone, the average crust thickness is about54km in the west, whileas the front junction between South China block and Sichuan-Yunnan block, theaverage crust thickness is42～48km in the east.Drawing the line on the junction of Xiaojiang fault zone and Zemuhe fault zone,it is obvious that the crust lithology and component are different between the southernpart and the nouthern part. The poisson’s ratio between0.20~0.27at north ofXianshuihe fault zone indicates that its crust matter mainly composed by the basicrock. Nevertheless, the poisson’s ratio at the nouthern part, that is Anninghe, Zemuheand Mabian-Yanjin fault zone, mostly lies between0.27~0.32. Anninghe-Zemuhefault zone has an ultrahigh Poisson’s ratio (>0.30), which could be considered as anevidence of local partial melt.Low-velocity layer are widespread in research area. But the low-velocity layerdepth and thickness are varietal. Near the Xiaojiang fault zone low-velocity layer isshallow in the North and deep in the South, while the east-west across the Xiaojiangfault zone there is no significant difference. Except for the widespread, low-velocitylayer thickness and depth near the Anninghe-Zemuhe fault zone are irregular. This suggests strong lateral heterogeneity. The low-velocity layer of the stations located atthe front junction between Sichuan-Yunnan block and the South China block and atthe position extending toward Sichuan basin appears in the10km or so.