An M5.1 earthquake occurred in Badong County,only 66 km from the Three Gorges Dam,on December 16,2013. The continuous gravity observation data obtained at Yichang seismostation nearest to the epicenter( 96 km) were analyzed,and it was found that the continuous gravity observation data obtained in this rainy season did not exhibit a characteristic of seasonal change in gravity identical to that in the past years,and thereafter the M5.1 Badong earthquake occurred. Numerical simulation revealed that the water storage and discharge of the Three Gorges reservoir generated seasonal change in gravity,and the changes in atmospheric pressure and gravity load were not the main sources of the seasonal change of continuous gravity observation data whether in respect of magnitude or phase and did not have obvious breaking change on annual variation before the earthquake. Through analysis of the seasonal change data observed on the same site including cavern temperature,rainfall data and global terrestrial water model( CPC) simulated water load,it was thought that,in the observation room with cavern temperature change of only-0.11°C / a at Yichang seismostation,the seasonal change of continuous gravity observation result mainly originated from the seasonal change in rainfall. In the case that the changes in rainfall and its water load did not have evident breaking change on annual variation law before the earthquake,if the M5.1 Badong earthquake was the cause of the breaking change on annual variation law in Yichang this time,then it was believed through analysis of crust expansion ratio that similar anomaly should occur at a crust expansion and compression intersection,no more than 100 km away from the epicenter.
