Rayleigh Wave Group Velocity And Anisotropy Tomography In The Eurasia And Its Marginal Seas

The continent of Eurasia and its marginal seas are assembled by more than thirty plates and blocks with different scales,characteristics,and different ages.These plates and blocks in the Eurasia and its marginal seas can be divided intosix types: (1) large Precambrian cratonic plates or shields; (2) small Precambriancratonic plates or blocks; (3) Phanerozoic orogens; (4) continent-continent colli-sional orogens; (5) Neozoic marginal sea basins; (6) continental rift basins.Studies of seismic anisotropy in the crust and mantle are of wide applicat-ion and scientific implication in geodynamics, which could interpret much geol-ogical and geophysical problem. In the study, Rayleigh wave tomography was used to produce group velocity and azimuthal anisotropy of crust and upper mantle in the Eurasia and its marginal seas. Using a FTAN method and vertical component records of Rayleigh waves from 102 seismograph stations and nearly ten thousands earthquakes in the Eurasia and western Pacific marginal seas, wemeasured fundamental mode Rayleigh wave group velocity dispersion curves ina period range from 8s to 200s for 11213 paths. The study area was divided into a grid of 1o×1o, we take azimuthal anisotropy into consideration when we performed pure-path dispersion by inversion method. Dispersion data were then used to invert for 28 group velocity and azimuthal anisotropy spatial maps from 8s to 200s in the Eurasia and its marginal seas(10oE~150oE,10oS~80oN).High-resolution surface wave tomographic images indicate that: the velocitycharacteristics of the lithosphere and asthenosphere are quite similar for the sametype of plates or blocks. However, the velocity characteristics are different for different types of plates or blocks. For a period range from 30s to 60s, very low velocity distribution in the western region-centering around Qinghai-TibetPlateau; for a period range from 60s to 200s indicates: a giant low velocity anomaly belt 2500km to 4000km wide and 8000km long are shown in the easternpart of the Eastearn Asia and Western Pacific marginal seas. Compared with the sea, weak anisotropy and relatively complex direction of maximum velocityis observed in the Eurasia. The collision of Indian plate, Seberia plate and Ch-inamainland produced strong horizontal compression and a certain subduction,which is the most basic factor making the uplift of Tibetan plateau, tremendous thickness of crust in the latter region, as well as the formation of orientationarrangement of the olivine and substance migration in crust-mantle, hence app-eared obvious anisotropy. Anisotropic strength in the western part of the conti-nental China is greater obviously than in the eastern part of the continental China. In several regions with significant velocity anisotropy the direction of fastRayleigh velocity is consistent with the orientation of maximum crustal compres-sive stress, and also with the direction of present-day crust movement.