Using GRACE Gravity Satellite To Study Temporal And Spatial Variation Of Regional Gravity Field Caused By Strong Earthquakes

With the continuous development of gravity satellite technology,the gravity satellite has been widely used in the research of regional gravity field inversion because of its many advantages such as all-days,global coverage,continuity,low cost,and no limitation by geographical terrain.In particular,the GRACE gravity satellite has played an important role in many disciplines such as geology,solid geophysics,oceanography,meteorology,space geodesy,and seismology because of its ability to accurately obtain regional time-variable gravity field.The occurrence of strong earthquakes leads to the change of regional gravity field,so the gravity satellite can be used to detect the time-dependent gravitational field changes,the deep material movement and mass migration caused by strong earthquakes,which has important theoretical significance and reference value for the understanding of the strong earthquake activity and its genesis mechanism.At present,the most powerful earthquakes in the world are mainly distributed on two major seismic belts: one is the “Mediterranean-Himalayan seismic belt” formed by the collision of land-land plates and the other is the “Pacific Rim Earthquake Belt” formed by the collision of ocean-land plates.However,there may be differences in the temporal and spatial characteristics of regional gravity field and seismogenic mechanism caused by these two types of strong earthquakes due to different regional tectonic backgrounds.In this paper we selected the Ms8.1 earthquake in Nepal on April 25,2015 and the Mw8.8 earthquake in Chile on February 27 th,2010 as two types of strong earthquakes.The GRACE long-term observational gravity data was used to deeply analyze the characteristics of the temporal and spatial variation of regional gravity field caused by different types of strong earthquakes.Then the seismogenic mechanism of two types of strong earthquakes was discussed from the perspective of geodynamics.The main results achieved in this paper are summarized as follows:(1)In this paper we expatiated on the principle and method of monitoring the seismicity using GRACE gravity satellite and deduced relevant theoretical formulas,and conducted a comprehensive comparative analysis of signal extraction problems and filtering techniques when using GRACE gravity satellite CSR_RL05 Level-2 data in inversion calculation.The impact of deduction of "hydrology" factors on the inversion results was particularly further discussed on the basis of past research.(2)The temporal and spatial changes of the regional gravity field before and after the Ms8.1 earthquake in Nepal were studied using the CSR_RL05 Level-2 data.The results showed that:With the epicentral area as the center,the change of gravity field in the pre-seismic area showed a distinctive four-quadrant space-time disparity distribution;the earthquake occured in the transition zone whose positive and negative gravity changes are 0 mGal;the post-earthquake regional gravity field showed "decrease-increase-decrease" variation which related to the afterslip and viscosity adjustment.From 2013 to 2016,the epicentral gravity experienced a change process of "decrease-increase-decrease-shock-decrease".(3)Based on the rectangular dislocation theory model,the results of the coseismic gravity change of the Nepal Ms8.1 earthquake were obtained and were compared with the actual observations of the GRACE gravity satellite.It was found that the theoretical results obtained by the model were an order of magnitude smaller than the actual observations,which further explained the reason why GRACE cannot detect the coseismic deformations of Nepal earthquake(Mw<8.0).The elastic mechanics rigid mold wedging model for the Ms8.1 earthquake triggered by land-land plate collisions was used to interpret reasonably the seismogenic mechanism.(4)The CSR_RL05 Level-2 data was used to study the temporal and spatial changes of the regional gravity field before and after the Mw8.8 earthquake in Chile.The results showed that the earthquake had a significant phase difference in the three stages of "pre-earthquake,co-seismic,and post-earthquake" : There was no significant change in the gravitational field before the earthquake;it showed 6.5 mGal positive and negative gravity differences changes and both the feature point and the gravity field showed a significant jump phenomenon in the coseismic.This feature was also consistent with the forward result of spherical dislocation,which further validated that GRACE could detect the earthquake whose magnitude is greater than 8.0 strong earthquakes.The seismogenic faults continued to show significant patterns of positive and negative gravity variations in post-earthquake.The time series of gravity changes between 2007 and 2010 in the epicentral region showed a change process of ”decrease-increase-decrease-increase-shock”.(5)The time series of the first-order and second-order gravity gradient changes of the coseismic region of the Mw8.8 earthquake in Chile were further studied.The results showed that gradient measurement method was more sensitive to the detailed signal.The northward gradient observation method could well suppress the north-south strip error to improve the observation accuracy.Gravity gradient measurement could reflect the seismic movements of crust,mantle material,regional density,and deep tectonic activity.The seismogenic mechanism of the Chile Mw8.8 earthquake triggered by the collision of the “ocean-continent” plate was interpreted reasonably from the ocean plate subduction theoretical mode.