Field documentation of coseismic landslides triggered by the 18 December 2023 Ms 6.2 Jishishan, Gansu earthquake in Northwestern China

On December 18, 2023, an Ms 6.2 earthquake struck Jishishan, Bao’an, Dongxiang, and Salar Nationality Autonomous County, Linxia Hui Autonomous Prefecture, Gansu Province, China. This moderate-to-strong earthquake triggered more severe co-seismic landslides than expected, including the catastrophic Zhongchuan Town mudflow, which resulted in the loss of > 20 lives. By conducting comprehensive field surveys immediately after the earthquake, we present a detailed field panorama of the earthquake-triggered landslides and provide a field-based understanding of delineating the spatial distribution patterns and failure mechanisms of the landslides. The majority of the co-seismic landslides are small-to-medium scale shallow ones and concentrated in the loess area in Zhongchuan, Guanting towns of Qinghai Province and Dahejia, Shiyuan, Liugou towns in Gansu Province. Loess falls are the most common co-seismic landslide type and are usually distributed on steep cut-slopes of roads and loess terraces where human activities are intense. The second-most common loess slides tend to occur on valley slopes covered by thick loose loess. Landslides in the bedrock area are less frequent, and the majority are complex geo-disasters involving rock falls and debris avalanches that originated in the upper part of the slopes and tend to cluster at the northern end of Jishishan Mountain. Generally, the distribution and occurrence mechanism of the co-seismic landslides are closely related to lithology, seismic response, terrain, hydrogeology, and human activities in the areas where they originated. Measures need to be taken on the irrigation method and the discontinuities and suspended loose debris to avoid further post-seismic failures.