Research On 3D Gravity Inversion In The Source Area

The density distribution of material inside the earth is one of the important contents of studying the internal structure of the earth.The three-dimensional density structure of the lithosphere plays an important role in studying the deep earth's structure,crustal movement and mantle plume and other dynamic processes.The anomalous distribution of crustal depth and density is an important clue to the study of crustal tectonic environment and its dynamic evolution,and it can also be used as an important basis for the exploration of oil and gas and mineral resources.The study of the underground three-dimensional density structure or the evolution of the density disturbance before the earthquake is one of the foundations and frontier hotspots in earthquake prediction research,and it can provide an important reference for the study of the mechanism of earthquake occurrence.The methods for studying the density structure inside the earth are mainly divided into two categories: one is the indirect method,which uses seismic data to obtain the velocity structure inside the earth,and the velocity structure can be converted into a density structure through the relationship between density and velocity.As for the relationship between velocity and density,there is only an empirical formula;the second is the direct method,which uses gravity anomaly data to directly invert the density distribution inside the earth,and can obtain the lateral unevenness of the density distribution in the study area.The former often provides inversion constraints for the latter.The research on the inversion of the density structure of the focal region mainly includes the study of the static density structure and the evolution process of the density disturbance before the earthquake.The former explores the structural conditions of the epicenter,and the latter explores the occurrence and development of the epicenter.Through the gravity survey of the source region,high-precision Bouguer gravity anomalies can be obtained;through repeated measurement of the gravity in the source region(flow gravity),the dynamic changes of the gravity field related to the source evolution can be obtained,both of which are the density structure of the source region and its The basic signal of change inversion.The main research contents and results of this paper are as follows:1.Firstly,the gravity three-dimensional inversion method based on the block growth mode was analyzed,and the effectiveness and stability of the inversion algorithm based on the block growth mode were verified through simulation trial calculation,and then the practicality was implemented through two inversion research examples.It is proved that the elimination of the regional gravity field trend makes this method have a certain advantage in the local inversion of the source area,and provides a better way to study the seismic tectonic environment and its changes.2.Using the three-dimensional gravity inversion method of the block growth model,using the finer Bouguer gravity anomaly data in the Sanhe-Pinggu earthquake area(m Gallevel)to invert and study the three-dimensional density structure in the shallow part(0~12km)of the area Distribution characteristics.The Bouguer gravity anomaly in the Sanhe-Pinggu earthquake area shows that the M8.0 magnitude earthquake is located at the location of the transitional low value between the local high gravity of TongxianYanjiao,the local high gravity of Sanhe-Mafang and the low gravity of Dachang;the three-dimensional density of shallow layers in the study area The structural results show that the density of the two sides of the northeast Xiadian fault is obviously different,which may indicate that the seismogenic fault of the Sanhe-Pinggu M8.0 earthquake is the Xiadian fault,and the cutting depth of the fault is at least 10 km,which means that the earthquake is significantly wrong.The dynamic depth is nearly 10 kilometers.3.Try to apply the three-dimensional gravity inversion method of the block growth model to the inversion of dynamic gravity changes before a typical earthquake.Using the two typical gravity dynamic data before the Jiuzhaigou M7.0 earthquake(April 2014 to April 2016 and April 2014 to April 2017),the corresponding three-dimensional density change inversion(?Gal level)was performed.The inversion results show that there is a northwest-nearly north-south positive density change zone on the middle side of the earthquake,which may be related to the interaction between the east(North China block)and the west(Qinghai-Tibet block).There is a local negative density change area(through the deep part)near the epicenter,which shows the state of material expansion and expansion.The earthquake site is located at the northern end of the Huya fault and the top edge of the local expansion area,which may be related to deep thermal action.