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 M_S7.2. 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.