Study On The Shear Velocity Structure And Anisotropy Of Eastern Part Of Ordos Block

The Eastern part of Ordos block also called Fen-Wei graben is located at interaction region of Indian Ocean Plate,Pacific Plate and Eurasian Plate,which is a boundary and decouple strip between geotecture of eastern China and western China,as well as the transition band of defferential motion between Ordos block,North China and South China.In addition,the Eastern part of Ordos block is also the boundary of active block and the biggest continental rift in China during the Cenozoic Era.The Eastern part of Ordos block with dense populaton and abundant resources is one of the most dominant origins of Chinese civilization,as well as an earthquake prone area.According to historical records,two 8 magnitude earthquakes,and six(7.0-7.9)magnitude earthquakes occurred in this area.The geological structure of Fen-Wei graben is complicated with lots of faults developed.In addition,the genetic mechanisms and depths of earthquakes beneath different fault basins may be different,especially the shallow depths of earthquakes beneath Datong volcano.Many researchers have intensive study in structure of crust and uppermost mantle beneath the Eastern part of Ordos block by various geophysical techniques,and have achieved a lot of significant outcomes.Therefore,a certain understanding is obtained for geological structure background,deep tectonic environment,generation mechanism of destructive earthquakes in the Eastern part of Ordos block.Because of the limitation of station distribution,data quality and the shortage of traditional methods,the structure of crust and uppermost mantle beneath study area is not clear enough.Taking some examples,it is difficult for body-wave tomography methods to obtain delicate velocity structure of the shallow curst.Surface-wave tomography is able to obtain higher resolution shallow structure,and then the shear wave velocities can be calculated by inverting surface-wave phase and group velocities.However,phase and group velocities at periods larger than 8 s are insensitive to shear wave vleocities of shallowest layers,especially for the sedimentary layers.In addition,the nonuniqueness of the individual inversion for phase or group velocities is serious due to insufficient constraints to shear velocity models.And traditional linear inversion methods can not quantify the effectiveness of the inverted results.The relatively sparse distribution of stations also limited the resolution of the computed velocity structure.Additionally,there are few studies for azimuthal anisotropy of the study area.The three-component continuous seismic data(from June,2014 to June,2016)recorded from 34 temporary broadband stations deployed by China University of Geosciences(Wuhan)and the three-component continuous seismic data(from January,2013 to December,2014)recorded from 72 permanent stations provided by China Digital Seismic Network are collected to obtain the accurate 3-D shear velocity structure and azimuthal anisotropy in the Eastern part of Ordos block.Then these stations are named EOBSArray.Rigorous quality controls are made for three-component continuous seismic data.The pure-path phase,group velocity dispersion curves,H/V and azimuthal anisotropy of phase velocities at different periods are obtained by analyzing and comparing the techniques of surface-wave tomography or measuring horizontal-tovertical ratios(H/V)based on different theories and sources in detail.A Bayesian MonteCarlo approach is applied to inverting pure-path Rayleigh-wave(phase-or group-)velocities and H/V simultaneously for 3-D shear velocities of the Eastern part of Ordos block.The main research contents and results of this thesis can be summarized as follows:(1)Systematical quality control is applied for the recently deployed the Eastern part of Ordos block Seismic Array(EOBSArray),which consists of GPS timing or response function correction,location information inspection and orientation measurement of sensor.The misorientation of each station is measured based on ambient noise crosscorrelations and earthquakes,respectively.The orientation measurements from ambient noise cross-correlations are highly consistent with the results from earthquakes.Moreover,orientation measurements from long-term cross-correlations of ambient noise are more stable than that from teleseismic earthquakes.Orientation measurements of sensors on land from relatively long period(20-50 s)ambient noise cross-correlations are more stable than those from short-period(10-20 s).Orientation measurements from ambient noise cross-correlations using only one month of noise data are still reliable and stable.Part of response functions belong to EOBSArray exist errors.More than 90% of EOBSA stations' s misorientation angles are less than 6o.(2)The surface wave Empirical Green's Functions with period range from 8 to 40 s are obtained from ambient noise correlations.The beamforming technique is applied for analyzing the azimuth distributions of noise sources generating surface waves and the accurate distribution of noise sources generating body waves,respectively.The majority of the noise sources from the second microseisms are located at deep ocean for EOBSArray.The locations of noise sources are varied with season.Moreover,the noise sources are located in southern(Indian Ocean and southern Pacific Ocean)and northern oceans(northern Atlantic Ocean)during summer and winter,respectively.There are the same sources to generate the Rayleigh waves and Love waves.(3)The horizontal-to-vertical ratios(H/V)of Rayleigh waves.H/V are measured from ambient noise cross-correlations(10-40 s)and earthquakes(10-90 s)for EOBSArray,respectively.No obvious systematic error between the two methods is observed with the comparison of the two methods,so the H/V measurements from ambient noise cross-correlations are combined with that from earthquakes to determine the H/V as the final results.The conclusions are drawn as follows: The distributions of H/V are correlated well with the surficial geological structure and topography.The relatively high H/V is observed at Ordos block,the basins within Fen-Wei graben,Bouhai Bay basin and South North China basin.The H/V measured at western Ordos block(Tianhuan depression)is highest,which is related to the huge thick sediments.The relatively low H/V is observed at central and sourthern Lvliang mountain,Taihang mountain.The lowest H/V is observed at Taihang mountain.(4)Azimuthal anisotropy of Rayleigh wave phase velocities.Ambient noise tomography and tele-seismic tomography are performed for obtaining the pure-path Rayleigh wave phase velocities with period range 8-70 s,group velicities with period range 8-40 s and the azimuthal anisotropy of phase velocities with period range 8-40 s,respectively.The conclusions are drawn as follows: The azimuthal anisotropy fast axis directions with period range 10-30 s have little change,which indicate that the deformation of crust and mantle beneath eastern China is generally continuous in vertical direction;The fast axis directions are close to the directions of lithosphere extension and velocity field by GPS,which indicates that direction east is the main flow direction of materials accounting for eastward extrusion of materials by thickened crust of western China;There is a significant difference between the azimuthal anisotropy of 10 s and 20 s for Datong volcano and Wutai mountain,which indicates that the crust is not continuous and exists decoupling structure;The direction EW is the main fast axis direction of Orods block,which is close to the direction of tension stress during Late Mesozoic.The azimuthal anisotropy observed at Ordos block is possibly the “fossil anisotropy”.(5)Shear velocity model and geological implications.The shear velocity structure of crust and uppermost mantle is obtained by the joint inversion of pure-path Rayleigh wave phase,group velocities and H/V based on a Bayesian Monte-Carlo approach.The resolution of shallow structure is improved and the high resolution velocity structure of crystalling crust and sediments are obtained by joint inversion.The conclusions are drawn as follows: The sediments with smallest thickess are located at Taihang mountain and Lvliang mountain,and the sediments with largest thickness are located at western edge of Ordos block which belong to Foredeep of intracontinental Foreland basin;The sediments thickness of Ordos block is decreased generally from west to east accounting for the reduced rate of subsidence from load center to marginal uplift and the Shanxi block uplift;The thickness of sediments beneath western Ordos block is generally larger than that beneath Fen-Wei graben located at eastern Ordos block,which is related to that the thickest sediments of Ordos block located at intracontinental Foreland basin are formed in Late Jurassic-Early Cretaceous Epoch and the basins within Fen-Wei graben are formed in Cenozoic Era;In the Fen-Wei graben,the thickness of sediments beneath Datong basin and Taiyuan basin is larger than 2 km,which is larger than the thickness of sediemnts beneath Linfen basin accounting for that the main stress of Linfen basin is shear stress and the main stress of Datong basin and Taiyuan basin is tensile stress which makes the rate of subsidence faster and contributes to the basin developmet;The crustal thickness is generally decreased from west to east in study area,for example,the crustal thickness of most parts of Ordos block ranges from 40 km to 45 km and the crustal thickness of Bouhai Bay basin and South North China basin is about 30 km,which indicates that the deformation by subduction of Pacific plate grow weaker gradually from east to west;The crustal thickness beneath the northern part of Fen-Wei graben is deeper than that of the southern part,which is related to that the graben is formed by tension stress gradually from south to north;The distinct lower velocity layer is observed within the crust beneath Datong volcano where the lithosphere thickness is the thinnest in the study area,which is related to that the thermal materials such as magma upwell and remained in the crust.