Hypermobility of seismic earthflows associated with modern intensive farming

The expansion of modern agriculture has profoundly altered land use, topography, and hydrology in many regions, and such anthropogenic modifications may increase the susceptibility of landscapes to seismically induced landslides. To better understand how sustained irrigation practices influence the occurrence of seismic landslides, this study examines the highly mobile earthflows triggered by two major earthquakes: the 2018 Palu earthquake (Jono Oge, Petobo, Sibayala, and Lolu flows) near Palu City, Indonesia, and the 2023 Jishishan earthquake (Caotan earthflow) in Gansu Province, China. These seismic earthflows occurred on very gentle slopes (3°–5°) within intensively irrigated farmlands, where long-term irrigation has substantially elevated soil moisture content in shallow layers. Under strong seismic shaking, the saturated soils underwent liquefaction, leading to unconfined earthflows with exceptional hypermobility. The measured height-to-length (H/L) ratios—0.026 (Caotan), 0.013 (Jono Oge), 0.021 (Petobo), 0.027 (Sibayala), and 0.011 (Lolu)—are remarkably low and rarely achieved by other landslide types. Key triggering conditions include proximity to tectonically active areas, the presence of loose surface soils, persistent gentle slopes with ample space for deformation, and prolonged irrigation. Minor variations in the liquefaction process were also observed, likely due to local environmental factors. Overall, seismic earthflows on gently sloping, irrigated alluvial terrains represent a previously underrecognized but significant hazard, warranting greater attention and further investigation.