Characteristics and Initiation Mechanism of the Liquefied Mudflow Caused by Jishishan Earthquake in Gansu Province and Its Enlightenment on Risk Assessment of Secondary Earthquake Disasters in the Upper Yellow River Basin

At 23:59:30 Beijing time on December 18, 2023, an Ms 6.2 shallow earthquake occurred in Jishishan County, Linxia Prefecture, Gansu Province. The earthquake triggered a coseismic giant mudflow in Zhongchuan Township, Minhe County, Qinghai Province, resulting in a catastrophic event. Research methods such as on-site investigation, drone photogrammetry, sampling, and experimental testing were adopted to analyze the development characteristics and causes of this mudflow hazard. The results indicate that: ① The mudflow has typical planar characteristics of valley-type debris flow, with clear boundaries between the formation area, the circulation area, and the accumulation area. The main source of the mudflow is the piedmont pluvial silty clay. ② Earthquake provides initial momentum, while melting snow, winter irrigation of farmland, and low temperature freezing raise the local groundwater level, which are the main triggering factors of the mudflow. The topographic slope provides subsequent kinetic energy, and the geological lithology provides condition for vibrational liquefaction, which are the inherent causes of the mudflow. ③ There are lateral and bottom scraping effects in the process of mudflow transportation, especially in the three flow direction changing areas, where lateral scraping and local accumulation are significant. Eventually, it quickly flows into Jintian Village and Caotan Village at the mouth of the gully. The results obtained can provide ideas and theoretical basis for the risk assessment of secondary disasters caused by earthquakes in the Guanting Basin and even the sedimentary basins in the upper reaches of the Yellow River.