Abstract: The triggering of the seasonal changes in the earth rotation rate and thetidal-Generating force from the sun and the moon for the stron gearthquakes in North China has been analyzed in this paper in Hejlan,Haicheng and Tangshan from 1966 to 1976.The research results show that the o-rigin time of the strong earthquakes is related to the porlar time of the seasonal changes in the rotation rate,the time of the spring tide and the time of the maxlmum horlzontal tidal force half一day period. It suggests that earth rotation notrandom, and the tidal forces,which are not random can control the strong earthquakes in North China.

Key words: Strong earthquakes in North China, Earth rotation, Tidal-generating force, Triggering