Study On The Variation Characteristics Of Gravity Tide Time Before Lushan Ms7.0 Earthquake

Continuous gravity monitoring is to obtain the gravity time change of fixed station through continuous observation on continuously operating station,which includes tidal and non-tidal components,but generally based on tide observation.Earth tide is a kind of periodic deformation phenomenon caused by the gravitational effect of the sun,moon and other celestial bodies on the earth.Geodetic observations such as gravity,tilt,strain and longitude and latitude of the earth’s surface will show corresponding periodic small tidal changes,which can be detected by relevant measuring instruments.The tidal deformation of the earth is closely related to the physical characteristics of its medium,which is an important basis for the transition of the earth’s internal structure and seismic activity.During the earthquake incubation process,deformation or material migration in the source region may occur,which may cause small disturbances in the solid tide signal.Among the continuous gravity observations recorded by the station instrument,the gravity variation caused by the earth solid tide caused by the near-celestial bodies such as the sun and moon is the main part of the instrument observation,while the non-tidal gravity variation caused by other tectonic factors,crustal movement,atmospheric load and land water is smaller than the signal energy of tidal components.Continuous gravity observation in China started from 1958 to 2009.With the improvement of instrument observation accuracy and the continuous strengthening of environmental monitoring level,a digital and networked observation network was basically formed,which provided data basis for continuous gravity observation and detection of earthquake precursor information.The relationship between the temporal and spatial variation characteristics of tides before earthquakes and earthquakes has always been an important problem for gravity workers.After the Wenchuan earthquake in 2008,a number of new observation stations were set up in succession in China,which provided more sufficient data for the study of the 7.0-magnitude Lushan earthquake and provided an opportunity for the study of continuous gravity observation to extract information related to earthquake precursors.Therefore,by carefully processing and calculating the tidal time change signal before the Lushan earthquake,this paper analyzes and extracts the relevant seismogenic information,which can provide a reference for earthquake prediction and mechanism research using continuous gravity measurement.The main research work and results of this paper are as follows :(1)three kinds of earth tide preprocessing software are analyzed and compared,and Tsoft software is selected for data preprocessing;by comparing three harmonic analysis methods,Venedikov harmonic analysis method is selected to calculate the tidal factors and phase lag of the main tidal waves.(2)data refinement at stations.(3)in this paper,the M2 wave is selected from the calculation accuracy of tidal factors,the calculation of monthly tidal factor variance and the statistical comparison of percentage difference.(4)this paper corrected the pressure of gravity data before the earthquake,and found that the pressure had no significant impact on the trend change of M2 wave tidal factor with time;Study of M2 wave before the earthquake trend change over time and space distribution,the results show that the forth before the earthquake the M2 tidal wave factor change is divided into the overall trend of rise and decline in the overall trend,the trend of rising and falling on the space distribution is four quadrant,forth the earthquake in four quadrant center site,may be caused by the existing "locked-in shear" before the earthquake area disturbance caused by the deformation of media or density.(5)analysis of the data of half a year after the earthquake shows that the M2 wave tidal factor shows an upward trend before the earthquake and a downward trend after the earthquake;And before the earthquake shows a downward trend,post-earthquake will show an upward trend,from before the earthquake to the post-earthquake changes can reflect the earthquake breeding and post-earthquake recovery process.The spatial variation of tidal parameters of M2 wave gravity wave after the earthquake shows that during the recovery process after the lushan earthquake,it also generally presents a four-quadrant distribution of rising and falling trend,and lushan earthquake is located near the center of four-quadrant symmetry.