Abstract: By calculating the azimuth, phase angle and change rate of tide force at the times of occurrences of the Wenchan M_S8.0 earthquake and 20 M_S≥5.0 aftershocks, and contrasting with their rupture types, we find that the angles between the direction of the horizontal tidal and compressive principal stress are within 33° for 6 earthquakes in 11 reverse earthquakes. All of the vertical phase angles of 8 strike-slip earthquakes are in the up peak period, and 2 normal earthquakes′ vertical phase angles are in the downward peak. Based on statistical phenomenon, we discussed the mechanism of tidal force triggering of the Wenchuan M_S8.0 Earthquake and its strong aftershocks .

Key words: Wenchuan earthquake, Strong aftershocks, Tidal force triggering