Constraining Seismic Anisotropy Beneath The Mainland Of China Using Shear Wave Splitting Data From China National Seismic Network

Studying mantle anisotropy plays a key role in understanding the deformation of the crust and the flow pattern in the mantle.Among the different methods for studying mantle anisotropy,shear-wave splitting measurements,determining time delay ?t between fast and slow waves and polarization direction ? of fast wave,has been routinely applied to different tectonic regions.However,combining results in different studies is difficult because of different criteria applied for data selection and determination of good splitting and Null.In my thesis,I use earthquakes from North America,Central Africa and Vanuatu-Fiji-Tonga-Kermadec Islands recorded by China National Seismic Network from 2015 to 2019 and measure shear wave splitting parameters of SKS and SKKS to study the anisotropic characteristics beneath the Mainland of China.My results show that the fast direction(phi)changes from near W-E in the western China(northwest China and Qinghai-Tibet)to near N-S in the central China(Ordos and Sichuan Basin),and then turns near W-E in the eastern China(North and South China),The delay times of slow wave(delta t)in the eastern China are larger than those in central and western China.Together with previous studies on the crustal anisotropy and crustal thickness of Mainland of China,our results indicate that the lithosphere and asthenosphere contribute most observed SKS and SKKS splitting.We also determine the coupling between crust and mantle beneath Mainland of China based on whether ? is consistent with the direction of GPS velocity.We find that the crust-mantle coupling regions are mainly at the northeastern margin of Qinghai-Tibet block,the southern margin of Ordos,and North and South China.The decoupling regions are mainly located at the eastern Qinghai-Tibet block,the Tianshan region,the western margin of Ordos,and Yunnan region.In addition,we also measure the shear wave splitting parameters of SKS and SKKS phases using the earthquakes in Fiji-Tonga,Vanuatu Islands,Kermadec Islands,Solomon Islands,and New Guinea recorded by the USArray from 2008 to 2018 and study the lower mantle anisotropy of the northeastern boundary the mid-Pacific LLSVP.We find several samples with different splitting measurements of SKS and SKKS sampling the edge of the mid-Pacific LLSVP,which may reflect the strong change of the anisotropic characteristics at the boundary of the LLVSP.