The Upper Triassic Series of the Longmenshan region includes the Carnian Ma’antang formation, the Norian Xiaotangzi formation and the Rhaetian Xujiahe formation. Through field study, it was the first time to identify rich soft-sediment deformations triggeried by earthquakes in multiple horizons of the Xiaotangzi and Xujiahe formations, including liquefaction-induced deformation (liquefied breccia, liquefied droplet, liquefied diapir, and liquefied homogeneous layer, etc.), plastic deformation (convolute deformation, soft pudding), and gravity related load, ball-and-pillow and pillow beds. The above soft-sediment deformations were responses to the Late Triassic tectonic movement in the Longmenshan region and they were records of seismic events associated with the breakup, collision and obduction strike-slip between the Songpan-Ganzi terrane and the Yangtze plate in the Indosinian period. Through combined analysis of paleoseismic events and sedimentary events, it was put forward that the tectonic movement process in the Longmenshan region during the Indosinian period was as follows: at the middle stage of the Late Triassic (the middle stage of the Indosinian tectonic period), breakup between the Songpan-Ganzi terrane and the Yangtze plate, the strike of the paleo-fault was near NS, the earthquake generated by the fault movement triggered a series of soft-sediment deformations in the neritic sediment of the Xiaotangzi formation; at the late stage of the Late Triassic (the late stage of the Indosinian tectonic period), the intracontinental subduction of the Songpan-Ganzi terrane and the Upper Yangtze plate occurred, and the earthquake induced lake sediment deformations in the Xujiahe formation; and at the last stage of the Late Triassic (the last stage of the Indosinian tectonic period), the Songpan-Ganzi terrane thrusted onto the Upper Yangtze plate through sinistral strike-slip obduction to form the Songpan-Ganzi Mountain and the Western Sichuan Foreland Basin, their boundary was the very Wenchuan-Maoxian fault now, and the epeirogeny and orogeny associated with paleoseism occurred in the Indosinian period. 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; the current records of liquefaction-induced deformations in the Xiaotangzi formation are far from the farthest liquefaction point away from the Wenchuan-Maoxian fault, so the actual magnitude of the paleoseism was far larger than Ms7.2 and there should have been stronger seismic events. Actually large catastrophic earthquakes (e.g., Wenchuan large earthquake of Ms8 on May 12, 2008) in Longmenshan had already frequently occurred during the Late Triassic, and the currently active seismic belt in Longmenshan is the continuation of the Mesozoic paleoseismic belt.