With the movement of Indian plate northward in the swaying way, a pair of rotational shear belts formed in front of Pamir arc and Asam arc, which are two projecting points of Indian plate. And M≥7 strong earthquakes occur alternately from one belt to another in period of every 90 years.
After a swaying movement, due to the elastic reaction of Himalayan arc, a moderate strong earthquake zone causally formed in the middle of Qinghai-Xizang plateau adjacent to 90°E, at the both sides of the vertical line to Himalayan arc.