A Research Of Tidal Triggering Effect For The Great Earthquakes

The mean magnitude value of tidal stress caused by the sun and moon tidal force in the interior earth is only 103Pa. The actual tidal stress in the earth crust is generally greater than the average tidal stress magnitude. Assuming that an order of magnitude was increased, the rate of tidal stress change (Semidiurnal tide as the example) would reach to 104Pa/6h, while the rate of tectonic stress change would be 102Pa/6h. Generally believed that the accumulating rate of tidal stress is two orders of magnitude bigger than that of tectonic stress (Sun w.K.,1992). Therefore, when the tectonic stress in the focal region has reached a critical threshold for fault rupturing, it is possible for tidal stress to accelerate the fault rupturing.Tide has been concerned by domestic and foreign scholars as a trigger factor. Study of the relationship between solid tide and earthquake mainly come from three aspects: the relationship between lunisolar location relative to earth and earthquakes; the relationship between tidal force and earthquakes; the relationship between tidal stress and earthquakes. In resent years, scholars focus on research tidal stress which is related with seismogenic structure.This paper has focused on global earthquakes with Mw≥7.0, hypocenter depth less than or equal to 70km, in the Harvard Centroid Moment Tensor (CMT) catalog during the period from 1976 to 2009. We have obtained fault plane solutions of 233 earthquakes by choosing the real fault plane of each earthquake through the focal mechanism solution, and then divided the earthquakes according to fault types. We have discussed the relationship between the direction of tidal maximum shear stress on the real fault plane and slip direction. The diurnal-lunar tide phases of tidal normal stresses, tidal shear stresses along the slip direction and Coulomb failure stresses on the fault plane in the origin time have been calculated. We also have obtained Coulomb failure stresses on the fault plane in the origin time. Then we have picked earthquakes which were triggered by the Coulomb criterion and compared the results.The results show that tidal triggering effect is not for each earthquake and is not concentrated in a particular type of fault. The key of whether tide can trigger earthquake is the correlation between the direction of tidal maximum shear stress and slip direction, and it has some correlation with fault types. For reverse fault earthquakes,the triggering effect is most obvious when the direction of tidal maximum shear stress is the same with the slip direction and it most weak when tidal maximum shear stress goes in the opposite. For the oblique-slip fault earthquake, the triggering effect is most obvious when the direction of tidal maximum shear stress is the same with the slip direction(angle of -30°～60°) or surrounding it(angle of 30°～60°,-30°～-60°).As for the strike-slip fault earthquake, the triggering effect is most obvious when the direction of tidal maximum shear stress is the same with or perpendicular to the slip direction. This phenomenon is due to the direction of tidal maximum shear stress perpendicular to fault strike for the vast majority triggered earthquakes in origin time and regardless of type of fault. The reliability of this conclusion will be further demonstrated in future studies.We have researched the diurnal-lunar tide phases of tidal normal stresses, tidal shear stresses along the slip direction and Coulomb failure stresses in the origin time by Schuster's test. The results show that no obvious relationship can be found. Only reverse fault earthquake's diurnal tide phases of tidal shear stresses and oblique-slip fault earthquake's lunar tide phases of tidal shear stresses have correlation with earthquakes.