The Upper Triassic Series of the Longmenshan region includes the Carnian Ma’antang Formation, the Norian Xiaotangzi Formation and the Rhaetian Xujiahe Formation. Field study firstly revealed existence of abundant soft-sediment deformations triggeried by earthquakes in several horizons of the Xiaotangzi and Xujiahe formations. Based on the locations of the identified liquefaction-induced soft-sediment deformations and their distances to the Wenchuan-Maoxian fault, it was estimated that the magnitude of the paleoseism occurred in the Norian Xiaotangzi Formation was about MS7.2. Actually large catastrophic earthquakes in Longmenshan had already occurred frequently during the Late Triassic, and the currently active seismic belt in Longmenshan is the continuation of the Mesozoic paleoseismic belt.