Research On The Crustal Structure And Seismogenic Environment Of Strong Earthquake In Shanxi Province Based On Background Noise

Shanxi Region features high earthquake frequency,shallow seismic source and heavy disaster,which is a famous strong earthquake zone in China and foreign countries.M?5 earthquake has occurred 85 times since there were the historical records,of which M6.0-6.9 earthquake occurred 11 times,M7.0-7.9 earthquakes 5 times and M8 earthquake once.This paper studies the M?5 earthquakes in Shanxi Province and the earthquake of magnitude 0 or above reported in Shanxi Province from 1981 to 2014.It is concluded that the M?5 earthquakes mainly occur between the depths of crust 10-20km;the M?5 medium and small earthquakes occur mainly between the depths of the crust 5-25 km,and the M?6 earthquakes mainly concentrate in five basins of the Shanxi fault depression.Therefore,it is of great significance to study the relationship between the development,occurrence of the earthquake in Shanxi and the crustal structure,and the seismogenic environment sign in the crust.This paper uses the favorable conditions of the existing 155 national seismic stations for their recorded continuous waveform in a complete year,and the empirical green's function between the stations is extracted to obtain nearly ten thousand Rayleigh wave dispersion curves,and the 16-period group velocity imaging figure of the Rayleigh wave group velocity of the period 6s-45 s is obtained through inversion of Rayleigh wave group velocity,and the crustal structure drawing of the 16-layer different depth planes and the Rayleigh wave velocity characteristics of the Shanxi crust is obtained.On the basis of the above work,the perspective imaging figure of Shanxi crustal structure is given comprehensively and systematically.Based on the study of the Rayleigh wave group velocity in different periods,it is concluded that:(1)The image with the depth of the crust 11 km as shallow reflects the shallow crustal structure and the geological structure,and the middle Shanxi fault depression zone is a low velocity area compared with the uplift area of the East and West Sides.The images of different wave velocity regions are consistent with the geological structures such as fault zone and uplift zone,convex area and fault zone.(2)Taking the depth of the crust 11 km as deep,the crust is divided into three units of the western Ordos block(that is,the original Lvliangshan uplift area),Shanxi fault depression and Taihang mountain uplift area.(3)The extension of Lishi fault and Jinhuo fault in Shanxi Region is shallow and that of boundary fault of the Shanxi fault depression zone is deep.Therefore,the Lvliangshan uplift area is classified as the crustal division evidence of the Ordos block area,and the Shanxi fault depression zone is the viewing point of the eastern boundary of the Ordos block area.(4)The Shanxi crust is divided into two northern and southern wave velocity zones with a bound of N38 t,specifically in the depth of 13 km in the crust,showing that the northern velocity is high,the southern velocity is low,and in the depth of the 16 km in the crust,it shows that the northern velocity is low,and the southern velocity is high.(5)There is a low velocity body anomaly band near the EW direction at N38°in the middle of Shanxi Province,with a maximum width of about 120 km and a maximum length of about 300 km.The low velocity body anomaly belt is a tectonic belt near the EW direction,passing through the Lvliangshan uplift zone,Shanxi Fault depression zone,Taihang mountain uplift zone and the Taihang mountains piedmont fault,straight to the low velocity body of the North China Plain fault depression zone.It is found from comprehensive research that the seismic structure of the M?6 earthquake in the fault depression zone of Shanxi is not only determined by the deep fault,but also more importantly,the vicinity of high and low velocity transition zones of the medium in the crust is the seismic source position,that is vicinity of the media "soft" and "hard" change zones,the seismic source body is the accumulation seismogenic energy of the "soft"," hard" media of the crust,which is a sign of the structure of the seismogenic environment in Shanxi Region.Based on the calculation of 6 seismogenic bodies of the M?5 earthquakes in Shanxi Region,it is found that the volume of the seismogenic body is positively correlated with the maximum magnitude of the seismogeny,and for the seismogenic body of the same order of magnitudes,the maximum magnitude of the seismogenic body with lowvelocity properties is higher than that of the seismogenic body with high-velocity properties,which provides a new index for the future study of potential earthquake hazard areas and the prediction of more than 5 magnitudes earthquake areas.In this paper,the 16 characteristic imaging figures of Rayleigh wave group velocity in different periods of Shanxi crust can provide the basic data for further study of seismology and geology involved crustal structure in Shanxi Region.The conclusion of this study has theoretical and practical significance to study the source nature of Shanxi Region as well as the seismogenic structure and the seismogenic environment.The results of this paper are of great scientific significance to understand the seismogenic environment in this Region and the dynamic mechanism of North China Craton.