The Velocity Structure And Anisotropy Of Uppermost Mantle In The South Of Europe,East Asia And Middle Asia

Seismic tomography is a process using vast observed seismic data to rebuild the three-dimensional velocity structure.But,for the Uppermost mantle,the Pn and Sn wave are the most important seismic phase to study it's velocity structure.Using the data of Pn and Sn wave to study the structrue of the uppermost mantle is becoming a embranchment of seismic tomography.The plates and blocks in Eurasia and its surrounding marginal seas can be divided into six types:(1)large Precambrian cratonic plates or shields;(2)small Precambrian cratonic plates or blocks;(3)Phanerozoic orogens;(4)continent-continent collisional orogens;(5)Neozoic marginal sea basins;(6)continental rift basins.The north of Eurasia is belong to the first type,the most stable blocks with little change in velocity and few earthquake events.On the contrary,the tectonic characteristics in the south of Eurasia is more complex,the depth of Mohorovicic discontinuity change more from west to east.In this paper the author studies the theory and metod of Pn wave tomography.The images of P wave velocity lateral variation are rebuilded at the Moho-discontinuity beneath the south of Eurasia,and the anisotropy of P wave velocites are obtained on the basis of Pn travel time data in these area by tomography technique.The P wave velocity lateral variation structure at these Moho-discontinuity is similar to the variation of crust depth.Comparing with the tectonic structure,stable blocks with low temperature always have high velocity and active blocks with high temperature have low velocity.From the result of invertion,we also can see that not all the area of the south of Eurasia have strong anisotropy,the fast Pn wave directions are consonant with the direction of crustal deformation from GPS and the major principal stress.From this study,we can provide a reference to the study of geodynamics and geologic succession and earthquake source mechanism.It can also provide a scientific reference to solving tectonic problems geologic and resource survey in these area and provide a reference to building a model of high resoulation of lithosphere/ aesthenosphere and the velocity structure in the mantle and geodynamics.