On 18 December 2023 (CST), an Ms 6.2 earthquake struck Jishishan County, Gansu Province, northwestern China. The catastrophic Caotan loess landslide–mudflow, located approximately 14.8 km from the epicenter, was triggered simultaneously. Because the landslide caused extensive damage, affected a large area, and resulted in substantial casualties, we conducted a detailed investigation comprising field surveys, UAV surveys, borehole drilling, trench observations, and in situ tests. We analyzed the topography, geological setting, hydrological conditions, and geometry of the landslide. Strong seismic shaking initiated slope failure in the source area. The displaced mass rapidly transitioned into a mudflow and traveled down the Qijia Valley, producing an unusually long runout over gentle terrain. We discuss the probable initiation mechanism, the transition process from sliding to flow, and the movement characteristics using evidence from the field investigation. We identify the principal factors controlling the initiation and long-runout movement of the Caotan landslide. These findings provide insight for mitigating similar hazards in northwestern China and other loess-covered regions worldwide.
Abstract Background Paleofloods are critical geomorphic driving forces shaping the landform evolution of the Tibetan Plateau margin. Although paleoflood investigations have been conducted in the Yarlung Tsangpo River, including documented sedimentary records in the gorge and upstream reaches, studies on its secondary tributary, the Yigong River, remain insufficient. In particular, quantitative hydraulic simulations of prehistoric outburst floods in this basin are lacking, as existing research mainly focuses on the modern extreme flood events of 1902 and 2000, leaving Holocene paleoflood evolution poorly constrained. Methods Based on systematic field geological surveys, sedimentary records of paleofloods were identified, including valley fill deposits at the main channel edge, flood-induced giant bar deposits at the river confluence, and slack water deposits (SWDs) in the downstream main channel. Sedimentological and chronological analyses were conducted, and hydrological reconstruction was performed for different peak discharge scenarios. Results The giant bar deposits consist of massive coarse sand and gravel with scattered oversized boulders, while downstream sediments near Tongmai Bridge show an abrupt facies transition from coarse gravels to well-sorted sandy gravels. The SWDs are dominated by silty couplets and massive sand beds, representing distal flood sedimentary signatures. Integrated analyses constrain three Holocene outburst flood events at 14.8 ka BP, 8.8 ka BP, and 6.7 ka BP. Hydrological reconstruction yields a maximum peak discharge of 4.4 × 105 m³/s with an ancient dammed lake reservoir capacity of 3.5 × 1010 m³, and a minimum peak discharge of 2.3 × 105 m³/s with a reservoir capacity of 1.22 × 1010 m³. Conclusions This study confirms that the Yigong River is a high-frequency zone of Holocene outburst floods. The newly discovered paleoflood records fill a research gap on prehistoric extreme flood events in this sub-basin and provide crucial supplementary data for understanding catastrophic flood processes and geomorphic responses in the steep, tectonically active Yarlung Tsangpo River, offering valuable insights into regional paleohydrological evolution and extreme flood risk assessment.
The performance of landslide susceptibility maps relies heavily on the quality and resolution of the digital elevation model (DEM) utilized for deriving the causal factors of landslides. In many regions of the world, the unavailability of high-resolution DEMs necessitates the use of medium resolution (30 m) freely available DEMs. This study evaluates and compares the predictive performance, classification alignment, and spatial agreement of landslide susceptibility maps generated from three 30 m DEMs, ASTER, EDNA, and SRTM with a 1 m LiDAR DEM and introduces two integration approaches, an equal-weight linear combination and a Principal Component Analysis (PCA)-based method, to improve mapping accuracy where high-resolution data are unavailable. Using two machine learning algorithms, Random Forest and Extreme Gradient Boosting (XGBoost), we conducted our analysis in Watauga County, North Carolina. We incorporated seven DEM-derived factors alongside six non-DEM-based factors. We evaluated models based on predictive performance, spatial agreement, and classification alignment. The 1 m DEM–based models produced the highest AUC scores (95.01
Abstract Introduction The West North Pacific (WNP) exhibits the highest tropical cyclone activity globally, averaging 24 storms annually; a significant portion of these develop during the boreal summer and traverse the Pacific–Philippine Plate boundary. This work examines the possible relationships between the variation in TC activity (1977–2021) and the earthquake occurrence over the Pacific–Philippine Plate Boundary. TCs were analyzed in terms of their activity (accumulated cyclone energy) and frequency. The earthquakes were analyzed at several earthquake depth levels. Results A clear relationship between tropical cyclone (TC) activity and earthquake occurrence was observed along the Pacific–Philippine plate boundary, particularly during years of high TC activity. The frequency of all TCs over the western North Pacific showed a strong positive correlation with earthquake occurrence, reaching 0.73 overall and becoming stronger for shallower earthquakes, increasing from 0.75 for depths below 120 km to 0.79 for depths below 30 km. A similar depth-dependent pattern was found for TCs crossing the plate boundary, where the correlation increased from 0.35 for all earthquakes to 0.66 for earthquakes shallower than 30 km. Interestingly, even TCs that did not cross the plate boundary exhibited correlations exceeding 0.70, indicating a potential remote influence of tropical cyclones on regional seismic activity. Accumulated cyclone energy (ACE) provides an even stronger correlation with the occurrence of earthquakes, which is 37% higher at 30 km compared to TC frequency. Fisher’s exact and Chi-square tests confirmed that ACE and TC-frequency-based classification are highly comparable; however, ACE is more representative of TC intensity. Conclusion This research demonstrates the existence of a relationship between tropical cyclone activity and earthquake occurrences over the WNP. More precisely, there is evidence that tropical cyclones’ activity, especially its intensity estimated through accumulated cyclone energy, might affect the shallow seismicity over the Pacific–Philippine plate boundary. Moreover, even without crossing to plate boundary, TCs formed in the WNP can potentially influence seismicity.
Abstract Background Landslides are among the most widespread and destructive geological hazards in China, driven by complex mechanisms involving multiple interacting factors. Their impacts are particularly severe when intense rainfall and strong ground motion occur in combination. At the national scale, existing landslide hazard assessments often rely on empirical or machine learning approaches. While effective for regional-scale applications, these models are constrained by incomplete inventories and often lack sufficient physical interpretability, which restricts their use for physically interpretable, scenario-based slope-stability assessment. Results To address these limitations, we propose a scenario-based physically informed framework for national-scale landslide slope-stability assessment in China. This framework integrates the TRIGRS and Newmark models to simulate slope stability under 15 rainfall intensity–duration scenarios and 10 seismic ground motion levels. Failure-probability estimation is supported by incorporating the Rosenblueth point estimate method and empirical failure probability curves to translate model outputs into probabilistic metrics. Furthermore, a sequential rainfall-conditioned seismic scenario involving extreme rainfall followed by earthquake shaking is introduced to evaluate nonlinear amplification effects from multi-source interactions. The results indicate that comparatively high rainfall-induced failure probabilities occur in the southwestern mountainous regions and the Loess Plateau. In contrast, areas with comparatively high earthquake-induced failure probability expand significantly with increasing PGA, with comparatively unstable areas primarily located along the western Sichuan area, northern Shaanxi, and eastern Tibet, where tectonic activity is intense. Under the sequential rainfall–earthquake scenarios, the predicted unstable area is substantially larger than that under the corresponding single-trigger scenarios, indicating pronounced within-model scenario amplification. Conclusion These findings enhance the understanding of spatial distribution patterns of landslides under multi-source triggering conditions and provide a useful reference for national-scale scenario-based landslide assessment and hazard-informed planning.
