Study On Seismic Gravity Variations Using GRACE Gravity Field Models

The whole process of seismic energy accumulation and fast release, slow recovery was accompanied by the mass migration and redistribution, which can cause the earth's gravity field to vary up to thousand meters in space and several seconds to decades in time. Therefore, gravity can been used as one of the most important seismic monitoring methods in earthquake research and prediction. However there are several disadvantages such as inhomogeneous, localized and limited temporal resolution for the present ground gravity measurement. It is hard to track the temporal and spatial gravity variations before earthquake. The ground gravity measurement cannot meet the amount of required information for the prediction and its development is limited by the environmental conditions and investment. It is speculated that China will be possibly to enter a new round of major earthquake active period in the next ten years, so in the future the earthquake situation in our country is very grim. Meanwhile, the earthquake disaster loss will increase the trend of the index with the rapid development of the social economy. Therefore, it becomes an urgent requirement to improve the observation ability of seismic information and the level of earthquake research.There are several advantages for satellite gravity observation such as global, quasi real-time and long-term near continuous observation. In this paper, we do researches on how to use the satellite gravity extraction of seismic gravity signal effectively and how to realize the GRACE application in seismic research. Results will provide unprecedented for a long time, large scale, high resolution reference for earthquake study and its application. The main work and results are as follows:We give a brief introduction on the background and significance of the seismic gravity study by analyzing the present situation and the insufficiency of ground gravity measurement. We point out the importance and necessity of satellite gravity and seismic studies and present the current research on domestic and international gravity satellite in the application of earthquake study. Then we propose the purpose and main research content of this dissertation.Extraction of tectonic gravity change is the premise of the seismic gravity study. The preliminary approach of the tectonic gravity signal extraction from GRACE are discussed. We introduced and compared the current major filtering methods which were used to deal with the north-to-south stripes in GRACE. Then we gave the systematical way to extract the tectonic gravity variation from GRACE after removing the hydrological factors and cycling variation. We analyzed and discussed the usages and differences of differential and cumulative gravity variations.Based on the method of earthquake dislocation theory and program, we obtain co-seismic gravity changes for fixed point on surface after the sea water correction. We put forward three methods to extract co-seismic gravity changes from GRACE spatial-temporal results:the differential method, the least square method, empirical orthogonal function method. Two feasible comparison of GRACE co-seismic change and the theoretical co-seismic gravity changes are proposed.In order to verify the correctness of extraction method of tectonic gravity, several giant earthquake cases on the word were studied.. Considering the magnitude, earthquake type, region and on human casualties and property losses of huge earthquakes, the 2004 Sumatra-Andaman Mw 9.1 earthquake,2011 Tohoku-Oki Mw 9.0 earthquake,2010 Chile-Maule Mw 8.8 earthquake,2012 Sumatra-Bengkulu Mw 8.6 earthquake and 2008 China-Wenchuan Mw 7.9 earthquake are chosen. We obtained the earthquake-related gravity changes before, during and after earthquakes, which provided data supports for the study of the focal mechanism and the inversion of seismic source parameters.After analyzing the characteristic of pre-seismic gravity changes, the evidence of anomaly variation before the Sumatra-Andaman Mw 9.1 earthquake, Tohoku-Oki Mw 9.0 earthquake and Sumatra-Bengkulu Mw 8.6 earthquake are given. The differential annual gravity changes between 2003 and 2004 in the source area of 2004 Sumatra-Andaman Mw 9.1 earthquake and its surrounding appeared obvious "positive -negative" anomaly on the two sides of fault. On the east side of Japan Island gravity increased abnormally three years before Mw 9.0 earthquake and decreased half year before earthquake. Gravity on the right side (near the inland) of the epicenter of 2012 Sumatra Mw8.6 earthquake began to increase anomaly the years before earthquake and increased year by year. The results shows that the satellite gravity does have the ability to detect the anomaly gravity variations before earthquakes, which have a certain reference value to the prediction and monitoring of earthquake as a new observation method.According to the current research results of earthquake samples, the close relationship between co-seismic gravity variation and its focal mechanism are put forward. The co-seismic gravity variations of thrust earthquakes increase in pressure zone and decrease in tension zone; and the co-seismic gravity changes of strike slip earthquakes decrease in pressure zone and increase in tension zone. This result reveals the dynamic relationship between the co-seismic gravity changes and the plate movement.The empirical orthogonal function method is an effective approach to study the spatial-temporal characteristics of the GRACE regional gravity field around the epicenter of earthquake, which can be effectively used to analyze the contribution of the seismic gravity variations to the regional gravity field quantitatively. The results show that the pretreatment of periodic signal will help to highlight the co-seismic gravity characteristics in time and space. Compared with the extraction of the temporal-spatial gravity effect of the strike slip earthquake, the method makes better effect on the thrust earthquakes.Finally, according to the long-term GRACE observation and the temporal-spatial distribution of earthquakes greater than 7.5, the relationship between earthquakes and gravity dynamic changes in China are studied. The results show that most earthquakes did not occur in the area where gravity changes vary with the biggest venue, but with the high gradient in tectonic background or on zero value line in the vicinity, which provide a certain prediction basis to determine the location of the earthquake by using GRACE. Earthquake time determination is based on the stress-strain relationship of rock.