The Crustal And Upper-mantle Structure Of Yunnan Region From Ambient Noise Tomography

Over the past few decades,the Yunnan region,which is located in the southeastern margin of the Qinghai-Tibet Plateau,has been a research hotspot for many scholars.The study area is the intersection area of the Qinghai-Tibet Plateau,the Indian plate and the stable South China plate.The main stresses in the historical periods are constantly changing,and the Yunnan area has strong geological tectonic movement and complex tectonic deformation.This paper summarizes the research results of crustal mantle structure in recent years in Yunnan,and then elaborates the theoretical principle and the key technology of realization of ambient noise tomography.Recent studies have shown that low velocity zones are widespread in the crust beneath Yunnan region.However,the relationships between the low-velocity zones and large faults,strong earthquakes distribution are less investigated by current investigations.By analyzing the three-component continuous waveform data recorded by 56 broadband seismic station(the Yunnan regional digital seismic network and Tengchong volcanic array)of two years(2014-2015),this paper study the crustal upper mantle velocity structure in Yunnan area on the basis of ambient noise tomography.On the one hand,this paper establishes the three-dimensional S-wave velocity structure model of Yunnan region,and deeply discusses the distribution of crustal low-velocity zones.The distribution of the low-velocity zones shows different features at different depths.At shallow depths,the shear wave velocity distribution is well correlated with near surface geological features.With depth increment,the low-velocity zones are gradually concentrated to the northern part of the study area,most likely reflecting variations in crustal thickness beneath the Yunnan region.The low-velocity zones are truncated at depth by several large faults in Yunnan.Especially,the Red River fault controls the distribution of the low-velocity zone.The spatial distribution of medium-strong earthquakes in Yunnan is uneven.It is interesting that most strong earthquakes(Ms ? 5)occurred in low-velocity zones or the transition zones between low and high velocity anomaly within the upper-to-middle crust.The crustal structure is composed of a brittle,seismically active upper-to-middle crust and a warm,aseismic lower crust.On the other hand,the radial anisotropy distribution of crust and upper mantle in Yunnan area is studied in this paper.The stress in the horizontal direction of the area makes the mineral microfractures mainly in the vertical distribution in the upper crust.As the depth increases,the increased stress causes fewer fissures.The lower crustal anisotropy may be caused by the presence of periodic thin interbeds.Compared to the upper crust,the lower crust is relatively soft and hot,and the deformation is more uniform.Previous studies have shown that the lower crus has the characteristics of low velocity,low resistivity,negative gravity anomaly and high heat flow,which implies that there may exist crustal material flow in the middle and lower crust of Yunnan.Influenced by the deep fault zones,crustal earthquakes and crustal movements,the anisotropy of the upper crust is more complicated than lower crust in Yunnan.The radial anisotropy of the Tengchong volcanic area indicates that the upper mantle thermal material rises through the fault to the lower crust,causing local melting and low velocity anomalies of the lower crust,showing negative radial anisotropy intensity inconsistent with the background.