Discussion On The Relationship Between The Dense Area Of Medium-small Earthquakes And Historical Strong Earthquakes In The Northeastern Margin Of Tibetan Plateau

China is an ancient civilization with a long history.China long history and culture has recorded extremely rich historical data on earthquakes.These seismic historical materials have very high scientific research value.The older generation of historical seismologists in my country have analyzed and verified a large number of historical earthquake data and determined the basic parameters of many historical earthquakes.They also compiled the catalog of earthquakes we are using now.These earthquake catalogs have played an extremely important role in earthquake research.However,due to the influence of the long time and the different degree of detail of historical data,there are still many parameters in the existing earthquake catalog that are not clear about historical earthquakes.For many years,relevant scholars have been exploring how to more scientifically determine the range and magnitude of basic parameters such as the severely damaged area of historical earthquakes,to check whether there are obvious deviations or omissions from historical earthquakes and other related issues.With the development of modern network construction,more and more small earthquake activity data are available to people.At the same time,Wang Jian et al.proposed a grid point dense value method,This method combines seismic activity data with historical earthquake research,which makes up for the lack of historical earthquake research that only relies on scattered historical data and field geological phenomena.The northeastern of the Tibetan Plateau is the frontmost of the Tibet block compressing and expanding northeast.There are many active faults and strong structural deformation in this area.Active structural zones with different properties are formed due to their different structural positions and differences in mechanical mechanisms.There are frequent historical and modern strong earthquakes in this area.It is a frequent occurrence area of earthquake disasters in my country.There have been more than ten major earthquakes of magnitude 7 or above in history,and there were 5 earthquakes of magnitude 8 alone,including the Tianshuinan magnitude 8earthquake in 1654,the Pingluo magnitude 8 earthquake in 1739,and the Wudunan earthquake in 1879.the 1920 Haiyuan magnitude 8.5 earthquake and the 1927 Gulang magnitude 8 earthquake,etc.This article will use the grid point density method to study the northeastern margin of the Tibetan Plateau rich in historical and modern earthquakes.According to the different structural attributes,the northeastern margin of the Tibetan Plateau is divided into strike-slip structural areas,and the reverse strikeslip structural areas and thrust structural areas are separated to discuss the relationship between the historical strong earthquakes and the dense areas of modern small and medium earthquakes.Analyze and discuss the adaptability and application conditions of the grid point density method in the northeastern margin of the Tibetan Plateau.This thesis uses the grid point density method to analyze historical earthquakes in three areas with different structural attributes in the northeastern margin of the Tibetan Plateau(the southeast Gansu strike-slip structure area,the northeastern corner of the Tibetan Plateau reverse strike-slip structure area,and the reverse fault structure area in the middle and western part of the Qilian Mountains).Discussing and comprehensively analyzing the relationship between modern medium and small earthquake activity-intensive areas,the following points are mainly obtained:1.This paper uses the grid point density method to calculate the current small and medium earthquakes in the northeastern margin of the Tibetan Plateau.The seismic activity on the northeastern margin of the Tibetan Plateau is divided into several dense areas and compared with the severely damaged areas of historical earthquakes.Earthquakes all correspond to dense areas.It shows that this method is effective for studying seismic activity and its duration in historical strong earthquake regions.2.There have been 89 earthquakes of magnitude 6 or above in the study area.A comprehensive analysis of the corresponding relationship between historical earthquakes and small earthquake-intensive areas shows that the greater the magnitude of historical strong earthquakes,the longer the duration of their small earthquake-intensive activities.According to statistics,the magnitude(M)of historical strong earthquakes and the duration(T)of the dense area conform to the following law: T = 4.7 × 10^-7× M10.88.3.The study found that due to the long time span of some historical strong earthquakes,the stress has been adjusted,and there is no densely active area of small earthquakes around them,and it has become a small earthquake active area.4.It is found in the research that there are some small earthquake-intensive areas without historical earthquakes.Through comprehensive analysis of the collected data,it is concluded that the dense area is located in the mining area,which may be affected by mining activities,may also be the missing area of historical earthquakes,or be affected by the increase of fault activities.The risk of future major earthquakes is worthy of attention.5.The study found that it is difficult to identify the duration of early historical earthquakes by using the small earthquake intensive value method in areas where many historical earthquakes are concentrated.This is the limitation of this method.