Institute Of Geophysics,China Earthquake Adminstration

In this paper,static stress fields of three different dislocation models are calculated based on elastic homogeneous media,elastic layered media and viscoelastic layered media.The three dislocation models are positive fault type,thrust type and strike-slip type.Static stress field includes displacement field,stress field and Coulomb stress distribution in horizontal plane and section.The effects of the fine structure of the crust on the coseismic and post-seismic static stress fields are analyzed and discussed.The research content mainly includes the following aspects:Based on elastic uniform media,the formation mechanism of displacement field,principal stress,plane stress and Coulomb stress in the elastic uniform media is analyzed by simulating and calculating the changes of displacement field,principal stress,plane stress and Coulomb stress generated by the three dislocation models in the horizontal plane and section.The basic characteristics of the motion law and stress state of the material around the finite fault equal to the seismic and static stress field are summarized.The main results are as follows: the claim stress and principal compressive stress of the three dislocation models on the horizontal plane can be regarded as the magnetic field shape formed by the bar magnet;The horizontal plane stress shows a centrally symmetric shape with the epicenter as the origin.The variation of Coulomb stress in the strike-slip dislocation model presents an eight-petal pattern.In the 4 quadrant of the rectangular coordinate system with strike and dip as axes,the variation of Coulomb stress near and far is opposite.When the depth of moment center of finite fault is different,the static stress field does not simply shift with the depth of moment center.Based on various elastic stratified media with different depths and thicknesses,such as low speed or high speed layers,the effects of elastic stratified media on static stress field were analyzed by simulating and calculating the displacement field,principal stress,plane stress and coulomb stress changes generated by normal fault and strike-slip dislocation models on the horizontal plane and profile.In terms of the amplitude and horizontal displacement of the surface,compared with the uniform medium,the low-speed layer magnifies the amplitude and the high-speed layer weakens it.The increase of the thickness of the low-velocity layer amplifies it,and the increase of the thickness of the high-speed layer weakens it(except in a small area above the epicenter).The low speed layer downshift weakens it,and the high speed layer downshift amplifies it.For the vertical displacement of the normal fault earthquake on the surface,compared with the uniform medium,the low-velocity layer has a magnifying effect on the uplift area near the epicenter and the subsidence area far away from the epicenter.The high velocity layer amplifies the subsidence area near the epicenter and the uplift area far away from the epicenter.With the increase of the thickness of the low velocity layer and the high velocity layer,the amplifying effect is further enhanced.The downward movement of the low speed layer increases the amplification effect,while the downward movement of the high speed layer weakens the amplification effect.For the vertical displacement of strike-slip earthquake on the surface,compared with the uniform medium,the low speed layer(high speed layer)weakens(amplifies)the vertical displacement closer to the epicenter,and amplifies(weakens)the vertical displacement farther away from the epicenter.The downward movement of the low velocity layer(high velocity layer)magnifies(weakens)the vertical displacement of the surface.When the low speed and high speed layers overlap with the finite fault,the principal stress and plane stress on the depth of the moment center change greatly.In the section,the low velocity layer weakens the variation of plane stress and Coulomb stress at its depth,while the high velocity layer amplifies the variation of plane stress and Coulomb stress at its depth.Based on the viscoelastic layered media with three kinds of low speed layers and high speed layers,the effects of viscoelastic media on the spatial and temporal distribution of the postearthquake quasi-static stress field are presented by simulating the co-seismic and post-earthquake displacement,principal stress,plane stress and Coulomb stress variations in the normal fault and strike-slip dislocation models.The results show that the viscoelastic medium has little effect on the direction of the horizontal displacement,but has a magnifying effect on the magnitude of the horizontal displacement.It can amplify the uplift area near the epicenter and the subsidence area far away from the epicenter.The variation of face stress and Coulomb stress in viscoelastic media is magnified.The Coulomb stress at the epicenter of the co-earthquake is greatly unload,but the change of the Coulomb stress at the epicenter is recovered in a short time after the earthquake.That is to say,the change of the Coulomb stress at the epicenter is positive after removing the change of the Coulomb stress at the epicenter,which is of great significance for the study of aftershock stress trigger.Based on the viscoelastic stratified crust model,the temporal and spatial distributions of the coseismic and post-seismic static stress fields generated by the 2017 Jiuzhaigou earthquake(strikeslip earthquake)and 2015 Pishan earthquake(thrust earthquake)are calculated,and the rationality of the results presented in this paper is verified by both the actual earthquake examples and the simulation calculation.At the same time,combining with the Jiuzhaigou earthquake,using the temporal and spatial distribution of Coulomb stress variation in viscoelastic media,it is found that the main earthquake has a significant stress triggering effect on the aftershocks on a relatively short time scale.