Study On The Inversion Of Regional Tectonic Stress Field And The Division Of Seismic Belt

The division of seismic belts is one of the most important tasks in the study of medium and long-term earthquake prediction.In recent decades,much progress has been made in division of seismic belts.Various methods have achieved some success in explaining the occurrence of some strong earthquakes.But the division results are quite different and have encountered challenges of one kind or another.Looking through the various problems that exist in the seismic belt division,it is mainly because of regarding it as a geometric concept while not fully considering and applying the mechanical conditions of earthquakes.Based on previous researches and taking the characteristics of regional tectonic stress field as the starting point,this thesis puts forward three basic conditions for seismic belt: 1.Seismic belts should be some narrow and long strips with tectonic backgrounds of major earthquakes.2.The orientation of the tectonic stress field in different sections of the seismic belt should be similar.3.The generation and occurrence of major earthquakes on the seismic belt should influence and interrelate with each other.The North-South Seismic Belt and its adjacent zones are selected as the study areas which have high seismic activity.The CAP method is used to obtain ML?3.5 earthquakes focal mechanism solutions during January 2009 to August 2017.Besides that,we also supplement 259 mechanism solutions(M?4.5)between January 1976 and August 2017 form the GCMT.Through the regional stress tensor damping inversion method,we have obtained the distribution of the stress field by 1.0°×1.0° grid region of the study area.The results show that the maximum principal stress of the study area varies significantly from north to south and can be roughly divided into 3 segments.From 29°N to 34°N the maximum principal stress is nearly EW direction except that the NE segment of the Longmenshan fault zone is NW direction.The direction of the maximum principal stress in the north of 34°N gradually changes: nearly EW – NE,and in the south of 29°N gradually changes: nearly EW – NW/NE – nearly SN.The maximum principal stress directions in the North-South Seismic Belt and its adjacent zones have a clockwise rotational pattern from north to south.The stress field directions are basically the same inside each active block,and are distinct between different blocks.The stress direction transition bands correspond well with the block boundary.According to the analysis,it is difficult to apply the North-South Seismic Belt as a unified seismic belt to the medium and long-term earthquake prediction.As an application of the regional tectonic stress field,combining with seismicity data,it is analyzed that regarding the Longmenshan fault zone as a seismic belt caused the long-term prediction result of the Wenchuan M8.0 earthquake inconsistency with reality.Compared with the seismic active period classified according to the time distribution of major earthquake activity in large region,this thesis defines the seismic active period by the seismic belt,that is,the rupture processes of major earthquakes in succession in all sections of the seismic belt.This definition of seismic active period has a clear tectonic physics meaning.In the research and practice of medium and long-term earthquake prediction,it is more practical.At the end of this thesis,the preliminary idea of the research work in the future is made.Research on the characteristics of small-scale media can determine the seismic belt that is in an seismic active period and lacks major earthquake in the seismic gap whether having tectonic physics condition of major earthquake especially at the level M?7.The mathematical derivations of the method principle of the amplitude ratio of adjacent frequency spectrums for measuring the small-scale Q? and the calculation steps of the method have been given.Based on this,the corresponding software will be compiled and used to test the source area of the 2013 Lushan M7.0 earthquake.Furthermore,we can determine whether there are tectonic physics conditions of Ms?7 earthquake,in the south-west of Lushan,the south segment of Longmenshan fault zone,in which it still lacks M7.0 earthquake.