Abstract: This paper determines the focal mechanism solutions for the May 12, 2008 Wenchuan earthquakes (M_S8.0) occured in Sichuan province and part of its aftershocks up to Dec 10, 2008 using graphical interpretation of equal area projection and the first motion sign data of Pn and Pg waves from regional and distant stations. The focal mechanism solution of the Wenchuan M_S8.0 earthquakes is: Nodal Ⅰ: strike 5°, dip angle 48°, slip angle 39°; Nodal Ⅱ: strike 247°, dip angle 62°, slip angle 131°; P axis azimuth 309°, plunge 8°, T axis Azimuth 208°, plunge 54°, B axis Azimuth 44°, plunge 35°. Combining geology and spatial distribution of aftershocks, nodal II can be identified as the seismogenic fault. The faulting activities triggering the huge earthquake is reverse thrusting. The main fracture surface of S67°W is basically the same as the fault strike. The main compression stress P axis of N51°W is basically the same as the direction of regional tectonic stress field.
Aftershocks occurred in the southern and northern section of the Longmenshan fault zone have the predominant orientations, and are obviously different. Early aftershock faultings in the south are mainly characterized maily by reverse-dip slip, with some strike-slip. Later faultings in the north are characterized mainly by strike-slip, with some reverse-dip slip. Stress field in the south is controlled by the main earthquake stress field, while stress field in the north is controlled by both the main earthquake stress filed and regional stress field.

Key words: First motion sign data, Wenchuan earthquake, Focal mechanism