The Characteristics Of Crustal Shear-wave Splitting In The North-south Seismic Zone

Seismic anisotropy is one of the effective means to understand the internal structure,deformation characteristics and dynamic mechanism of the Earth.Seismic anisotropy has been studied in everywhere of the earth interior and mainly focus on crust and upper mantle.The mechanism of the seismic anisotropy is different.The existence of fractures in the crust(at least the upper crust)and the mineral lattice arranged in the upper mantle are the main sources of the anisotropy respectively.The complex deformation characteristics of the North-South seismic zone are related to the collision between the India plate and the Eurasian plate.The northern area is located in Qaidam-Qilian active block,the regional principal compressive stress is mainly in NE direction,and the faults are mainly in NW and NE orientations.The southern part is located in the eastern part of the collision zone,the regional principal compressive stress direction is complex and which is mainly in NNW direction,the faults are mainly in NW,NE and NS orientations.The relationships between the shear-wave splitting parameters and the regional principal compressive stress as well as the faults can be discussed by analysing crustal anisotropy,respectively.The state of regional crust-mantle coupling and deformation mechanism can be analyzed by comparing the crust with the upper mantle anisotropy.In this paper,the data are from temporary seismic stations in the North-South seismic zone set up under the project ChinaArray Phase I(2011-01—2014-06)and most of the project ChinaArray Phase II(2013-02—2015-12).We analyzed the data with the Systematic Analysis Method of shear-wave splitting(SAM method)in crust.The environment of regional stress and the characteristics of the tectonic revealed by shear-wave splitting parameters are discussed as well as the relationships between regional principal compressive stress directions and fracture distributions.The results show that the fast shear-wave polarization directions are from NE to NE and WNW in the north part of North-South seismic zone,the directions of the fast shear wave in the south are mainly characterized by the NNW in the interior of the Sichuan Yunnan rhombic block,the nearly NS on the western region and the NE in the southern region.The fast shear wave polarization directions are consistent with the regional principal compressive stress,which are gradually varied from NE to NNW from north to south.The NE and WNW or NW faults also have a great influence on the directions of principal compressive stress.The polarization directions of fast shear-waves at stations near the strike-slip faults are parallel to the fault strikes,and a part of these shear-wave polarization directions are nearly perpendicular to the fault strikes,which are similar with the regional principal compressive stress directions.Several stations exhibit complex features,reflecting the complexity of tectonic environment in the study area.The delay times of slow shear-waves in the south part are higher than the north part,reflecting that the southern section may have a greater degree of anisotropy in crust and the tectonic deformation is more intense.Compared with the upper mantle anisotropy of the North-South seismic zone,it is inferred that there may exist a complicated crust-mantle coupling phenomenon inside of the Sichuan-Yunnan rhombic block.The shear-wave splitting in the crust can not only reflect the regional stress characteristics,but also can reveal the regional tectonic information.The crust and upper mantle have different scales of anisotropic media in North-South seismic zone.In this paper,the seismic anisotropy characteristics of different scales and different depths are summarized by combining body wave and surface wave data in North-South seismic zone.Then we analyse the continuity and coupling of crust and mantle.The difference of the seismic anisotropy between the crust and mantle could indicate various deformation mechanism.Therefore,we propose that the seismic anisotropy in the crust and mantle of North-South seismic zone should be studied more quantitatively and synthetically both in theory and high density data process.