Gravity Changes Of The Nepal M_S8.1 Earthquake Derived From GRACE Data

The M_S8.1 earthquake occurred in the Langtang area in eastern central Nepal on April 25,2015,the development and occurrence of earthquakes are accompanied by energy accumulation and release,materials migration and masses redistribution around the seismic center,which makes the gravity field change,the gravity changes caused by strong earthquakes can be observed by geodetic means such as GRACE gravity satellites.In this paper,the gravity change characteristics of the Nepal earthquake in and around the seismic center are successfully obtained derived from GRACE gravity satellite observation data,the gravity accuracy of GRACE satellite in the study area is discussed,according to the coseismic gravity changes calculated from PSGRN/PSCMP,the possibility of GRACE detecting the coseismic gravity changes of the earthquake is analyzed.The main results of this paper are as below:1.The gravity derived from GRACE satellite basically changes positive and negative with the boundary of the blocks in Nepal and its vicinity areas,the gravity of the Himalayan seismic belt located at the junction between the Indian plate and the Tibetan Plateau block has decreased continuously since 2012,the gravity of the Tibetan Plateau block has gradually increased since 2008,the positive gravity anomalies in the northern boundary zone of the Tibetan Plateau block gradually migrated and expanded eastwards in 2012,and the gravity of the South China block increased slightly obvious since 2011.The GRACE satellite gravity changes reflect the large scale structural dynamics characteristics in Nepal and its vicinity areas.2.The changes of gravity anomaly before the Nepal M_S8.1 earthquake were obvious,the gravity of the uplift area increased,while the gravity of the subduction area decreased,the earthquake just occurred around the zero line of the gravity change,the positive and negative peaks of gravity in the uplift area and subduction area both appeared in 2014,which both decreased in 2015,respectively,the dynamic change of gravity reflects mass migration before the earthquake and viscosity adjustment of crust and mantle after the earthquake in Nepal and its vicinity areas.3.The annual change of gravity is negative in the eastern Himalayas and northern India,which is positive in the central India plate and central Tibet Block,the distribution characteristic in the spac of which is positive and negative in the four quadrants,which is the results of long term joint effects of various factors such as the exploitation of groundwater,tectonism,glacial melting,and lake level rise.4.The coseismic displacements obtained from PSGRN/PSCMP and GPS observations have good consistency in the movement trend and magnitude,the coseismic displacements indicate that the focal mechanism of the Nepal M_S8.1 earthquake is thrusting,the theoretical change range of coseismic gravity from PSGRN/PSCMP is-0.14×10^-8ms^-20.18×10^-8ms^-2,while the gravity error in Nepal and its vicinity areas obtained from GRACE data is about1.10×10^-8ms^-2,which is bigger than the theoretically coseismic gravity change.It is rather difficult to extract the coseismic gravity change of the Nepal M_S8.1 earthquake from GRACE data.