Characteristics and lessons of liquefaction-triggered large-scale flow slide in loess deposit during Jishishan M6.2 earthquake in 2023

At 23:59 of December 18, 2023, a strong earthquake with a magnitude of M6.2 with a focal depth of 10 km struck Jishishan County in Gansu Province, China. The epicenter is located at N35.7° and E102.79°. A large-scale liquefaction-triggered flow slide in loess deposit on the secondary terrace with a gentle slope of 2°~3.5° of the Yellow River was induced by the earthquake, where is about 20 km away from the epicenter. The flow slide developed into a devastating mudflow to buried 51 houses in two villages of Jintian and Caotan, Qinghai Province and caused 20 people dead. A joint field investigation was carried out on the flow slide immediately after the earthquake. Based on the site reconnaissance, UAV survey, borehole exploration, electrical prospecting and analyses on both ground motion and the previous research achievements in liquefaction-triggered flow slides in loess deposit, the characteristics and mechanism of the large-scale flow slide are clarified preliminarily. The results show that the flow slide in loess deposit is triggered by the liquefaction in a large area of saturated loess layer below 11 m, the overburden soil mass slides into a gully with liquefied loess together along the liquefied layer in the direction of the gentle slope. And then, the mixture mass of soil and water continues moving down along the gully with a gradient of 1.5°~5°. During the flow sliding, stirring and rubbing each other continuously of the large amount of water gushes from the liquefied soil mass and the overburden soil has made its mixture to develop into a mud flow before the mixture arrives at a dam in the downstream of the gully. After meeting the dam, the mudflow overflows the banks at both sides, and at the same time, turns into the other neighbor gully at the left side for about a certain distance. Finally, the dam breaks and the mudflow moves down again along the gully, passes the two villages, and stops in a large area of farmland 317 m away from the villages. The lessons learned from the disaster are expounded for its risk assessment, mitigation and prevention.