The ocean ebb and flow is induced by the tidal force due to the tractive force between the earth and the moon or the sun, which varies in a sinusoidal manner twice a day. At the same time, the force deforms the elastic rock plate of the earth surface, which could be a trigger of an earthquake. In the present statistical investigation of past earthquakes around the world, it was found that the occurrence of earthquakes has the highest event rate at the phase of the force minimum. In addition, there are some partial distributions of the events in the world with regard to the occurrence phase and the direction of the tidal force, which suggests a matched condition proper to the respective regions. In a simple model calculation of earthquakes, the tidal force is nearly the same with or somewhat lower than the increment per day of the shear stress of the rock plate at the critical state, being caused by the plate movement due to the mantle convection, which is sufficient for the triggering. The ratio between the tidal force and the stress increment is higher in a bigger earthquake, and the occurrence is limited in a narrower phase of the force variation, which provides a more intimate correlation. The direction of the tidal force at a time of an earthquake can be explained by the friction law between the adjacent rock plates.
