Study on regional gravity changes before the m_w9.0 japan earthquake detected by grace

Regional gravity changes before the MW9. 0 Japan earthquake are obtained from the observations of GRACE satellites. At the same time,time series of vertical displacements at GPS fiducial stations of TSKB,DAEJ,SUIY and HLAR are acquired in the ITRF 2000 reference frame. There are mainly two stages of the pre-earthquake regional gravity changes. The first is a process with slightly gravity change behaved as gravity rise and then decline during August 2002 to the end of 2007 or 2008,which can be fitted with quadratic polynomial. The second stage is showed as gravity increasing rapidly and then decreasing after the first process. Significant gravity decrease appeared in Heilongjiang and Ussuri and its surrounding region in 2008. The gravity increased significantly in vast region along the Pacific subduction zone in 2009 and 2010. And the areas of positive and negative gravity changes migrated towards the southwest. Comparing with 2009,the positive and negative gravity change areas are divided into west and east distribution along the plate boundary. Gravity decreased rapidly at locations of the eastern part from the beginning of 2010 until the occurring of the earthquake. While gravity changes at locations in the western part were still positive with high value,which began to decline after the earthquake. Far-field GPS vertical displacements showed an uplift beginning from the middle of 2006,and this trend turned in early 2009. Comparison with the gravity changes caused by variation of surface water contents shows that the regional gravity changes are mainly caused by changes of surface water contents. But gravity changes at locations P01 and P03 in the ocean side have the same characteristic. This suggests that regional gravity anomalies may be related to underground mantle migration and heat which caused regional climate changes. The relationship between these abnormal variations and seismogenesis needs to be studied in the future.