Abstract: The experiments of rock mechanics completed recently show that,in the triaxial stress state,all the increasing of maximum principal stress,the changing (increasing or decreasing) of intermediate principal stress and the decreasing of minimum principal stress can lead rocks to failure. It is suggested that earthquakes can be caused not only by strengthening of maximum principal stress,but also by decreasing of intermediate principal stress or minimum principal stress in the earth's crust. The action which can cause stress decreasing in the earth's crust was discussed,and the possibility of earthquakes caused by stress decreasing was searched by investigation of seismic activity. It is pointed that in seismic studies not only the earthquakes caused by stress strenthening and energy increasing,but also the earthquakes caused by stress decreasing and energy decreasing must be considered.