Based on the rupture model of Mainling M6.9 earthquake in Tibet on November 18, 2017, the spatial distribution of static Coulomb failure stress change at different depths are calculated respectively according to two different receiving fault selection schemes. The one scheme is that we set the parameters of receiving fault at different position to be consistent with the main shock; The other scheme is on the assumption that fault's orientation is randomly distributed under the ground, and we select the receiving fault which is most prone to slide under the influence of coseismic stress field produced by main shock. Therefore, the geometrical orientation of receiving fault will vary with space. According to the above two results of static Coulomb failure stress change, we discussed the static Coulomb stress influence produced by the main shock to short-term aftershocks and the Medog M6.3 earthquake in Tibet on April 24, 2019, respectively. The result shows that: 1)When the parameters of receiving fault are same with the main shock, the proportion of aftershocks in the positive zone of static Coulomb failure stress change is small at each depth. The focal mechanisms of aftershocks in the positive zone of static coulomb fracture stress are deemed similar to the main shock. We thought that they are motivated by the continuous rupture of the main shock. 2)Most of the aftershocks are in the negative zone of static Coulomb failure stress change at each depth. We inferred that this phenomenon which may be on account of the focal mechanisms of these aftershocks is quite different with the main shock. From the result of receiving fault to choose the most prone to slide under the coseismic stress field produced by main shock, we can clearly see that all the aftershocks are within the zone of static Coulomb failure stress change greater than the trigger threshold of 0.01MPa at different depths. It indicates that all the aftershocks are likely to be triggered. It was speculated that the aftershocks in the negative zone of static Coulomb failure stress change occurred in the crushed zone caused by violent rupture of the main shock. There are countless cracks in the crushed zone, and the orientation of these cracks is abundant. Perhaps, because most aftershocks occurred on these various cracks, their focal mechanisms are quite different from the main shock. The value of elastic constants will be reduced significantly in the crushed zone. All the results in this paper also indicate that considering the elastic constants difference between in and out of the source region is beneficial to accurately estimate the static Coulomb stress influence between earthquakes in the source region. 3)Different institutes and authors used different data and methods to get several different focal mechanisms of the Medog earthquake. According to these results, we calculated a central focal mechanism solution, which has a minimum difference with these focal mechanisms. On the basis of this central focal mechanism solution, the static Coulomb stress influence of the Mainling earthquake to the Medog earthquake is calculated quantitatively. Result indicates that the magnitude of static Coulomb failure stress change generated by the Mainling earthquake is quite small on both two nodal planes of the central focal mechanism solution of the Medog earthquake, this means that the Medog earthquake is independent of the Mainling earthquake.

Located among the South China block, Tibetan plateau, Alxa block and Yinshan orogenic belt, the Ordos block is famous for its significant kinematic features with stable tectonics of its interior but frequent large earthquakes surrounding it. After the destruction of the North China Craton, the integrity, rotation movement and kinematic relations with its margins are hotly debated. With the accumulation of active tectonics data, and paleomagnetic and GPS observations, some kinematic models have emerged to describe rotation movement of the Ordos block since the 1970's, including clockwise rotation, anticlockwise rotation, clockwise-anticlockwise-alternate rotation, and sub-block rotation, etc. All of these models are not enough to reflect the whole movement of the Ordos block, because the data used are limited to local areas.
    In this study, based on denser geophysical observations, such as GPS and SKS splitting data, we analyzed present-day crustal and mantle deformation characteristics in the Ordos block and its surrounding areas. GPS baselines, strain rates, and strain time series are calculated to describe the intrablock deformation and kinematic relationship between Ordos block and its margins. SKS observations are used to study the kinematic relationship between crust and deeper mantle and their dynamic mechanisms, combined with the absolute plate motion(APM)and kinematic vorticity parameters. Our results show that the Ordos block behaves rigidly and rotates anticlockwise relative to the stable Eurasia plate(Euler pole: (50.942±1.935)°N, (115.692±0.303)°E, (0.195±0.006)°/Ma). The block interior sees a weak deformation of~5 nano/a and a velocity difference of smaller than 2mm/a, which can be totally covered by the uncertainties of GPS data. Therefore, the Ordos block is moving as a whole without clear differential movement under the effective range of resolution of the available GPS datasets. Its western and eastern margins are characterized by two strong right-lateral shearing belts, where 0.2°~0.4°/Ma of rotation is measured by the GPS baseline pairs. However, its northern and southern margins are weakly deformed with left-lateral shearing, where only 0.1°/Ma of rotation is measured. Kinematics in the northeastern Tibetan plateau and western margin of the Ordos block can be described with vertical coherence model with strong coupling between the crust and deeper mantle induced by the strong extrusion of the Tibetan plateau. The consistency between SKS fast wave direction and absolute plate motion suggests the existence of mantle flow along the Qinling orogenic belt, which may extend to the interior of the Ordos block. SKS fast wave directions are consistent with the direction of the asthenosphere flow in Shanxi Rift and Taihang Mountains, indicating that the crustal deformation of these areas is controlled by subduction of the Pacific plate to North China. The week anisotropy on SKS in the interior of Ordos block is from fossil anisotropy in the craton interior. After comparing with the absolute plate motion direction and deformation model, we deem that anisotropy in the interior of Ordos block comes from anisotropy of fossils frozen in the lithosphere. In conclusion, the Ordos block is rotating anticlockwise relative to its margins, which may comes from positive movement of its margins driven by lithospheric extrusion or mantle flow beneath, and its self-rotation is slight. This study can provide useful information for discussion of kinematics between the Ordos block and its surrounding tectonic units.