The great Wenchuan earthquake of May 12, 2008 occurs on the Longmen Shan fault zone which forms a prominent section of the seismically very active seismic belt called the North-South Trending Seismic Belt by Chinese seismologists. There have been 3 earthquakes with magnitudes from 6 to 6¹/₂ along the Longmen Shan fault zone during more then 2000 year documented history of the Sichuan province. Previous active faulting studies indicate slow (less than 3 mm/yr ) slip rate across the Longmen Shan fault zone. Why such a strong earthquake occurred in the Longmen Shan fault zone? What are the characteristics of the May 12 earthquake? What is the tectonic mechanism of the earthquake? On the basis of geological studies of the earthquake surface rupture zone and pre-earthquake GPS measurements in the region, we try to understand the questions mentioned above. The May 12 earthquake, 2008 is caused by displacement along the Yingxiu-Beichuan fault along which a more than 200 km long surface ruptures was formed. Another strand of the Longmen Shan fault zone, Guanxian-Jiangyou fault also ruptured as indicated by more then 60 km long surface ruptures. GPS measurements before the earthquake suggest that slip rate across the entire Longmen Shan fault zone does not excess about 2 mm/yr, and does not excess 1 mm/yr across individual fault strand. These data agree with seismogeological studies and historical seismicity of the Sichuan province. Using maximum coseismic displacement obtained from seismogeological studies and inversions from seismic waves, the recurrence intervals of great earthquakes, such as the 2008 Wenchuan earthquake, can be estimated to be 2000 ~6000 years. The Longmen Shan fault zone has a high dipping angle (more than 50°~60°) near surface and low-angle at depth (15~20 km). This kind of listric shape favors significant strain or energy accumulation to form great earthquake. The May 12 Wenchuan earthquake, 2008 is characterized by slow strain accumulation, long recurrence interval, and significant damage power. It is a new type of earthquake event that deserves further studies.