Study On The Crustal Heterogeneity And Seismogenic Mechanism Of The Strong Earthquake In Northeast China And North China

Earthquakes occur frequently and seriously in China,and China is the country with the largest number of strong earthquakes with magnitudes above 8.0 occurring in the interior of the plate in the world.The main reason is that the surrounding structure of the Chinese mainland is extremely complex.China is located in the world's two major seismic zones,the circum-Pacific seismic belt and the Eurasian seismic belt,due to the compression from the Pacific plate,the Philippine Sea Plate and the Indian plate,resulting in the development of intensive seismic fault zones in the Chinese mainland.Since the last century,nearly 800 strong earthquakes of magnitude 6.0 or higher have occurred in China,covering all provinces,autonomous regions and municipalities except Zhejiang and Guizhou provinces and the Hong Kong Special Administrative Region.China accounts for 7 percent of the world's land and 33 percent of the world's continental earthquakes,making it the country with the largest number of continental earthquakes in the world.Seismicity in China is characterized by the high frequency of occurrence,high magnitude,shallow focal depth,wide distribution area and uniqueness.Since the 20 th century,more than 550,000 people have lost their lives due to earthquake disasters in China,accounting for 53% of the total number of earthquake deaths in the world.The earthquake is one of the biggest natural disasters,which does great harm to human life and property.The effective prediction of short-term and imminent earthquakes is one of the most effective means to reduce disasters,but the prediction of earthquakes is still an unsolved problem in the scientific community,mainly because of the complex crustal structure,and the mechanism of earthquakes seismogenesis is not clear.The formation and occurrence of large earthquakes within continents are closely related to the complex crustal structure.Although previous studies on strong earthquakes in different regions have been done,all of them are focused on a single earthquake cluster or several earthquakes and lack comprehensive earthquake case analysis.In this study,26 strong earthquakes in North China and typical earthquakes in Northeast China have been compared and analyzed in detail.These earthquakes occurred in different tectonic settings,at different times,the different causes of differences.The characteristics of seismic velocity and Poisson's ratio structure in dozens of strong earthquake source areas are deeply analyzed.The same structural characteristics of different earthquakes are extracted,and the mechanical models of the formation and occurrence of strong earthquakes in the continental interior are established uniformly,which provides theoretical support for the risk assessment of earthquakes in the future.In this paper,the aim is to study the complex structure of lateral heterogeneity of crustal structure,and the formation and occurrence mechanism of strong earthquakes.We collect P-and S-wave travel time data in Northeast and North China.We selected suitable for different data volume and advanced traveltime tomography methods(including regional tomographic method of considering the complex discontinuity of the earth interior and double-difference traveltime tomography method which can be used in the intensive seismic zone)to obtain the high-resolution P-wave and S-wave velocity,and Poisson's ratio in north China,northeast China and Songyuan,Jilin.The relationship between strong earthquake formation and crustal heterogeneity in the study area is further discussed.The mechanism model of strong earthquake formation and occurrence in the continent is proposed.The main conclusions are as follows:(1)The Songliao Basin is unique and is the main active tectonic unit in Northeast China.The major strong earthquakes in the crust of Northeast China occurred around and inside the basin,which is closely related to the continuous activities of the Songliao Basin.The lithospheric structure shows that part of the lithosphere in the Songliao Basin has been delaminated and caused asthenosphere material upwelling,which is the main reason for the tectonic activities in this region.(2)Songyuan earthquake area mainly present low P-wave and S-wave velocity and low Poisson's ratio structure.,but in China's mainland,the large earthquakes,especially the earthquakes in north China(> 6.0),occurs mainly at the boundary between the high P-wave and S-wave velocity areas,and the high and low Poisson's ratio boundary area.So we infer that it had a lower risk of a larger earthquake in the Songyuan area.(3)The high-resolution P and S wave velocity and Poisson's ratio model shows that most of 26 strong earthquakes in North China seismic source location of the earth's crust structure has the same characteristic,which occurred at the boundary between the high and low seismic velocity and the high and low Poisson's ratio anomaly.In the lower crust to the uppermost mantle under the source zones of the large earthquakes,however,low-velocity and high-conductivity anomalies exist,which are considered to be associated with fluids.Both fluid and partial melt will cause a decrease in velocity and the increase of Poisson's ratio.We infer that the lower crust of North China is rich in fluid or partial melt.(4)The strong earthquakes in the continental interior mostly occur at the boundary of high-V and Low-V anomalies and tend to the side of the high-speed body.It is speculated that the high-speed area is usually brittle,and the stress is easy to concentrate on and causes earthquakes.Low velocities,on the other hand,are likely to be areas where the crust is fractured,fluid rich,or high temperature,and where tectonic movements tend to occur without seismic deformation.(5)Compared with the earthquakes in the subduction zone,a composite earthquake model is proposed,that is,strong continental earthquakes mostly occur in the upper crust with brittle areas in the region of high velocity,which is easy to concentrate stress.Because there is a large amount of mantle-derived melt or fluid injection,the fluids in the lower crust may cause the weakening of the seismogenic layer in the upper and middle crust and thus contribute to the rupture nucleation of the large crustal earthquakes.