Mudflows due to liquefaction triggered by earthquakes in terrains with gentle lower slopes: a case study: Mudflows in terrains with lower slopes

On December 18, 2023, at 11:59 p.m., a magnitude 6.2 earthquake occurred in Jishi County, Gansu Province (referred to as the 2023 M S 6.2 Jishishan earthquake), which led to a loess mudflow located 20 km away from the epicenter of the Yellow River terrace. Numerous houses were buried by thick layers of sediment, and 20 people died. In this work, the formation characteristics and processes of the loess mudflow triggered by the earthquake are studied via high-resolution remote sensing and unmanned aerial vehicle (UAV) imagery (~ 10 cm), field investigations, dynamic triaxial tests, ring shear tests, and Flo-2D simulations. Additionally, the characteristics of saturated loess liquefaction due to seismicity are analyzed. The results reveal that (1) the failure area of the loess mudflow caused by the earthquake was approximately 150,000 m 2 , with a maximum depth of 13 m. The geohazard chain impact area was 2.8 km long and covered an area of 0.43 km 2 . The average movement velocity of the loess mudflow was approximately 2 m/s, and the inundation area was approximately 1.9 × 10 5 m 2 with a deposit thickness of 0.5–3 m. (2) The loess mudflow involved saturated loess under a vibrating load, which suddenly increased the pore water pressure (PWP) and liquefied, ultimately causing catastrophic instability and failure. The formation process can be divided into the following stages: vibration-induced saturation loess liquefaction; the accumulation of water in the gullies promoted the failure mass flow; the collapse of the impounding dam accelerated the flow of the failure mass; and the open flat terrain led to an increase in the buried area under the failure mass. (3) High-amplitude and low-frequency dynamic stress caused the saturated loess to fail in the shortest possible time. The dynamic strength and antiliquefaction strength decreased with increasing vibration duration and were affected by the dynamic load and vibration frequency, with a marked decrease as the vibration frequency decreased and the dynamic stress ratio increased. (4) The pore water pressure (PWP) exceeded the normal stress, resulting in a negative effective stress. This caused the sliding zone to exhibit obvious liquefaction and flow sliding, leading to the formation of a liquefaction-type loess mudflow due to shear displacement. This phenomenon explains why saturated loess in the area frequently forms loess mudflows after failure.