This paper focuses on the relationship between seismic sequences and dynamic changes of tidal force and additional tidal stress when geological structure stress reaches the critical value of rock failure. Some earthquakes above Ms7.0 and their seismic sequences in China mainland since 1970 were selected to analyze the relationship between dynamic changes of horizontal tidal force and occurrence time of large earthquakes. The result shows that the tidal force has a predominant azimuth range at the time of foreshocks, main shocks and aftershocks in certain spatiotemporal range. The azimuth range of horizontal tidal force at the time of triggered earthquakes is only 35% of total azimuth ranges in the chosen period. For some complex structure area, there might be more than one predominant range of azimuth, which is related to seismic structure probably. According to focal mechanism of 9 great earthquakes in China from 1966 to 1976, tidal normal stress, shear stress and their change rates in slip orientation were calculated. The results show the tidal shear stress is nearly at the maximal value at the moment of earthquake occurrence.

By analyzing the digital tilt tidal observations at Yongsheng station and strain tidal observations at Houma station, this paper suggests that the solid tidal force might trigger tiny abrupt deformation when the seismogenic system enters into an instable critical state. The step and abrupt changes correlate to a certain extent to the direction of tidal force and the orientation and size of tidal shear stress along the fault strike. In the paper, we study for the first time the relationship between deformation anomaly and additional tidal stress in order to approach the physical mechanism of anomalies appeared immediately before earthquakes and the distinguishing method.