The Longmenshan Fault (Chinese: 龙门山断层) is a thrust fault which runs along the base of the Longmen Mountains in Sichuan province in southwestern China. The strike of the fault plane is approximately NE. Motion on this fault is responsible for the uplift of the mountains relative to the lowlands of the Sichuan Basin to the east. Representing the eastern boundary of the Qinghai-Tibet Plateau, it is a border formation between the Bayan Kola block in the Plateau and the South China block in the Eurasian Plate. Both the 2008 Wenchuan earthquake and the 2013 Ya'an earthquake occurred along this fault.
"The late-Cenozoic deformations in this fault (that caused the 2008 Wenchuan earthquake) are concentrated in the Guanxian[disambiguation needed]-Jiangyou fracture (hill-front fracture), Yingxiu-Beichuan fracture (mid-fracture), Wenchuan-Mao County fracture (hill-back fracture), and their related folds. The recent Ms 8.0 earthquake occurred on the Yingxiu-Beichuan fracture, as a result of Longmenshan thrust pushing southeastward combined with clockwise shears.
Since Holocene (10,000), Yingxiu-Beichuan fracture has had evident activities. Its long-term geological slip rate is slower than 1 mm per year. GPS observations confirm the current structural deformation of the Longmenshan formation to be characterized by thrust and right-handed shears, but with a low deformation rate. Therefore, Longmenshan formation and its internal fractures constitute a special type that has low earthquake frequences but the potential to cause super strong earthquakes."
The American Geophysical Union publication Tectonics describes the 5 km high escarpment thus:
"In the Longmen Shan region, however, the topographic margin of the Tibetan Plateau is one of the world's most remarkable continental escarpments. Elevations rise from circa 600 m in the southern Sichuan Basin to peaks exceeding 6500 m over a horizontal distance of less than 50 km. Regional topographic gradients typically exceed 10% along this mountain front and rival any other margin of the plateau."
- Dong Jia et al., Dong; Wei, Guoqi; Chen, Zhuxin; Li, Benliang; Zeng, Qing; Yang, Guang (September 2006). "Longmen Shan fold-thrust be ilt and its relation to the western Sichuan Basin in central China: New insights from hydrocarbon exploration". AAPG Bulletin 90 (9): 1425–1447. doi:10.1306/03230605076.
- "汶川8.0级地震成因分析 (Cause Analysis of the M8.0 Wenchuan earthquake)" (in Chinese). China Earthquake Administration. 2008-05-30. Retrieved 2008-09-07.
- Chinese: 晚新生代的构造变形主要集中在灌县-江油断裂（前山断裂）、映秀-北川断裂（中央断裂）和汶川-茂县（后山断裂）及其相关褶皱之上，这次8级强震发生在映秀-北川断裂之上，是龙门山逆冲推覆体向东南方向推挤并伴随顺时针剪切共同作用的结果
- Chinese: 映秀-北川断裂全新世（10000年）以来具有明显的活动性，其长期地质滑动速率小于每年1毫米。GPS观测表明龙门山构造带的现今构造变形也是以逆冲和右旋剪切为特征，但变形速度不大。因而，龙门山构造带及其内部断裂属于地震活动频度低但具有发生超强地震的潜在危险的特殊断裂。
- E. Kirby, P. W. Reiners, M. A. Krol, K. X. Whipple, K. V. Hodges, K. A. Farley, W. Tang, and Z. Chen,, Eric (2002). "Late Cenozoic evolution of the eastern margin of the Tibetan Plateau". Tectonics 21 (1): 1001. Bibcode:2002Tecto..21a...1K. doi:10.1029/2000TC001246.
- Map of faults in SW China - MIT
- University of Arizona
- Simplified geologic map of the Longmen Shan region of the eastern margin of the Tibetan Plateau.
- China Virtual Museum of Earthquakes, map charting the Five Seismic Belts in China