Abstract: The generation and occurrence of alarge earthquake is the process of the accumulation and releasing rapidly of strain energy in source region and its vicinity. In the late stage of earthquake generation the accumulated strain energy in the source region and it svicinity reaches high level, which is situated in an instable critical state. The short-term and impending method of earthquake prediction must be based on features of the critical state. It is known that the most important feature of the critical state is triggering factor playing an important rote. The tidal force of solid tide is an important triggering factor. The author improv ed the local mean moo n time method which used only tidal force of the moon, and use the tidal force both the moon and the sun as the triggering factor, which is more scientific and reasonable. The research results show that several significant shocks happened in source region and its vicinity before a large earthquake are easily triggered by certain directions. The author predicted successfully Yiajang earthquake with MS 6.0 of Feb. 23, 2001 in Sichuan Province, which applies the above-mentioned phenomenon, combined other earthquake precursors.

Key words: Tidal force, Significant shock, Earthquake prediction, Critical state