Abstract: The mechanism concerning the relation between tidal force triggering of significant shocks and the large earthquakes is roughly discussed in this paper. Results show that the mechanism is complicated. The tidal force may increase the shear stress on the fault plane and is favorable for earthquake happening if the direction of horizontal tidal force near the fault strike of the earthquake when the earthquake occurs; the tidal force may reduce the compressional stress on the fault plane and reduce the friction force on the fault plane and is also favorable for earthquake occurrence if the direction if horizontal tidal force is near the direction of principal extensional stress T axis of the earthquake. There are two same phenomena in the three examples discussed here. The first is that directions of horizontal tidal forces when significant shocks occur are close each other, and the second is that rupture plane strikes of significant shocks before a large earthquake are close to rupture plane strike of the large earthquake.

Key words: Tidal force, Significant shock, Triggering function, Earthquakes case