On April 11,2012,a large earthquake(MW8.6) and by far the largest strike-slip event ever recorded,struck the west coast of north Sumatra and was followed 2 h later by an MW 8.2 earthquake.A number of issues remain unclear such as the reason for the unusually large strike-slip earthquakes occurring in the intra-oceanic lithosphere,correlations between these two earthquakes,and the manner in which they influence seismicity.Therefore,this paper analyzes the nucleation background of the MW8.6 earthquake according to the region＇s geological environmentand geophysical observation results.Although the earthquake occurred in the Wharton Basin southwest of the Sunda Trench in the Indian Ocean,the strike-slip faults are well developed with complex deformation and frequent seismic activity.In the movement toward northwest of the Indian