Comparing the earthquake phenomena with the characteristics of static force breaking and fatigue breaking, the author considers that the fracture and faulting in rock causing earthquake are fatigue breaking. Based on some results about studying fatigue fracture law of compressed vessel and chemical machines at the end of 1970s, the author has calculated conscientiously, regarding the earth as a spherical vessel with thin wall. The results show that fatigue breaking in 35°N zone of the earth is the most severe, regardless of the action of centrifugal force of the earth rotation or the gravitation of any celestial body in the solar system. Thus, the cause of intense seismicity in 35°N zone is revealed.