Gravity Changes Of The 2011 Tohoku-Oki Earthquake Based On GRACE Data

GRACE gravity satellites play a key role in the fields of seismology and the other related fields due to its ability to detect the regional time-varying gravity field with high precision.According to previous studies GRACE gravity satellites can detect the co-seismic gravity changes caused by strong earthquakes.However,there is no more discussion on the processing results of the different versions of GRACE data(RL05 and RL06)on the basis of different filtering methods(Study on seismic gravity changes using RL06 data for the first time).So this paper mainly uses two versions of GRACE gravity satellite data and two combined filtering methods to extract the signal of co-seismic gravity changes in the focal area and adjacent area caused by the 2011 Tohoku-Oki earthquake.And th e characteristics of gravity changes trend at the extreme point of the co-seismic gravity changes are studied.At the same time,it is compared with the theoretical results of the forward calculation of dislocation model to verify that GRACE gravity satellites can detect the gravity changes caused by earthquake and select more appropriate data and filtering methods.The main research work and achievements of this paper are as follows:(1)We summarize and analyze the GRACE satellite programme and the different versions of the GRACE data,the theory and method of extracting gravity changes' signal caused by earthquakes are also studied.And we summarize and compare the three filtering methods which can deal with the banding error and the high frequency noise in the spatial distribution of gravity field.Then the time series fitting method for extracting seismic gravity changes is selected to study.As well as,the signal of gravity changes caused by land water reserve is extracted by GLDAS monthly water reserve model data,and we also deducte it from the GRACE gravity changes to obtain the signal of gravity changes mainly caused by earthquakes.Moreover we study and analyze the theory and method of calculating the co-seismic gravity changes in two dislocation models.Since the earthquake occurred in the ocean,the influence of seawater on the extraction of the co-seismic gravity changes is considered emphatically.The dislocation model—"the layered viscoelastic plane model" for the forward calculation of the co-seismic gravity changes and its supporting calculation programs—PSGRN/PSCMP are applied.(2)We use the combined filtering methods of "P3M6+Gaussian smooth filtering" and"P3M6+Fan filtering" to process the RL05 and RL06 version data of GRACE Level-2 in different time ranges respectively,the signal of co-seismic gravity changes is extracted and the spatial distribution map of gravity changes is drawn on the basis of deducting the influence of land water storage.At the same time,two extreme points of gravity changes are selected as the characteristic points in the region of positive and negative gravity changes,and the time series analysis of monthly gravity changes is carried out.The detailed trend of the gravity changes with time is analyzed before and after earthquake,and the results under different data and different filtering conditions are obtained.(3)By using the calculation program(PSGRN/PSCMP)of "the layered viscoelastic plane model",the results of forward evolution of the theory and the changes of seismic gravity are obtained.After the correlation correction processing,we compare and analyze the results with different versions of the data,and different combined filtering methods.Compared with the results of different versions of the GRACE data and combined filtering methods,it can be seen that there is a slight difference in the order of magnitude of gravity changes,but it is still consistent in the overall spatial distribution and change order,which we can verify that the GRACE gravity satellite can indeed detect the real co-seismic gravity changes.And in the selection of different versions of GRACE data and combined filtering methods,after comparing the data processing results,it is considered that combined filtering of Fan filtering can better weaken the influence of banding error than combination filtering of Gaussian filtering.And the gravity changes' amplitude of RL06 data(-5.97?1.72,uGal)by using combined filtering of Fan filtering is closer to the results of dislocation model(-7.10?0.69 ?Gal)than the results of RL05 data,and it is considered that RL06 data and the combined filtering methods of "P3M6 decorrelation filtering+Fan filtering" with appropriate filter parameters are more suitable for the study of seismic gravity changes,it can weaken the error of high frequency noise and the banding error more effectively,and retain the real signal of gravity changes.