Research On The Regional Time-varying Gravity Field And Bouguer Gravity Anomaly By Using GRACE Gravity Satellite

Satellite gravity measurement technology,with its wide range,regular updates,no difficult geographical area restrictions,far from weather,terrain,traffic and other environmental factors,attracts more and more attention of geography workers.Since entering the new century,the launch of gravity satellites like CHAMP,GRACE,GOCE make this new space geodetic method gradually play a vital role in the study of solid geophysics,geology,marine mapping,meteorology,and geodetic geology.Large earthquakes have occurred frequently in this new century,which not only causes very huge losses to people's lives and economic property,but also provides an opportunity for the scholars to carry out seismic research.Satellite gravity measurement techniques also facilitate the use of this technique to understand crustal stress changes,gravity anomalies,seismic gravitational fields and their changes,fault movement,and seismic activity at the time of the earthquake.With the development of satellite gravity techniques,it is more convenient for us to understand crustal stress changes,gravity anomalies,seismic gravitational fields,fault movements,and seismic activities when earthquakes happen.From the large geological scale we can see both Wenchuan Ms8.0 earthquake on May 12 th,2008 and Lushan Ms7.0 earthquake on April 20 th,2013 occur in Longmenshan fault zone.The Longmenshan fault zone with several internal fractures sets in the transition area between Qinghai-Tibet Plateau and Sichuan Basin.This area has the characteristics of special geological environment,complex terrain structure,frequent occurrence of large earthquakes and strong tectonic activities.As we all know,the Nepalese region located in the middle of the Himalayan orogenic belt happened Ms8.1 Nepal earthquake on April 25 th,2015,while the subduction from the Indian plate to the Eurasian plate is the main dynamic cause in the Himalayan region.So far we can also see the complex geological background in the Nepalese region.Therefore,this paper mainly studies multi-seismic regions and the time-varying gravity field and the Bouguer gravity anomaly change in the regions through satellite gravity technology.We choose the Longmenshan fault zone and the Nepalese region to study and explore their relationship with regional tectonic movement and faults' activity.The main research work and results achieved are as follows:1.This paper introduced GRACE satellite gravity technology,and discussed the theoretical methods and strategies of satellite gravity data processing.Mainly deducing the calculation method of time-varying gravity field and Bouguer gravity anomaly,which involved the process of Earth gravity field,the principle of filtering,Legendre function,time-varying gravity field and Bouguer gravity anomaly.2.Using the C ++ language to compose the standard forward lexicon of the Legendre function.And writing a procedure to calculate and correct the time-varying gravity field and Bouguer gravity anomaly based on the monthly GRACE gravity field model.3.Based on the GRACE monthly model of The Longmenshan fault zone in 2008 and 2013,according to the procedures the change of time-varying gravity field and the gravity anomaly monthly and yearly situations were calculated combining with regional crustal movement characteristics and seismicity.The results proved that the time-varying gravity field in the Longmenshan fault zone showed a tendency of "negative-positive-negative" corresponding to the earthquake.Considering the mantle convection theory,the author speculated the reason: before the two earthquakes regional epicenter shell and mantle material increased with a faster growth rate,while during the two earthquakes it had a slower growth rate,and after the two earthquakes the materials obtained a faster growth rate again,which might be the confirmation of post-earthquake field change reply theory.4.According to ten years' Bouguer gravity anomaly changes in the Longmenshan fault zone,the author concluded: In the decade from 2006 to 2015,the western part of the fault zone near the Qinghai-Tibet Plateau mainly showed obvious negative anomaly,while the eastern part near the Sichuan Basin showed positive anomaly,and the Longmenshan fault zone basically set in the transition range.The author speculated that the phenomena was caused by the QinghaiTibet Plateau material flowing to the southeast.5.Based on the monthly gravity field in the Nepalese region in 2015,according to the procedures the change of time-varying gravity field and the gravity anomaly monthly and yearly situations were calculated combining with its regional crustal movement and history seismicity.Through the analysis the author revealed that the region where the crust-mantle material increased faster is shifted from the northwest to the southeast.6.Through analyzing the monthly changes of Bouguer gravity anomalies in the Nepalese region in 2015,the author found the northern part of the Nepalese region was in a great negative anomaly,which showed that the strong squeezing stress caused by the collision of the Indian Ocean plate against the Eurasian plate made the Himalayan region's long-term subduction.However,the southern part of Nepal including Kathmandu showed positive anomaly,which indicated that the Nepal earthquake triggered a pull force to the plate.The author thought it was related to the north-south displacement of the internal faults in Nepal.However,From the results of the ten years' Bouguer gravity anomaly situation,we can see northwest part of Nepal showed positive anomaly,which indicated that the Himalayan region was geologically in a long-term stretch and uplift.