Spatio-temporal mapping and long-term evolution of debris flow activity after a high magnitude earthquake

A high-magnitude earthquake can trigger substantial amounts of co-seismic debris and unstable hillslopes in mountainous regions, which leads to a significant increase in the frequency and magnitude of post-earthquake debris flows. The long-term impacts of earthquakes on the debris flow activity have not been fully understood, primarily because of a lack of multi-temporal mapping inventories of debris flow activity. Therefore, it is crucial to analyze the spatio-temporal evolution of debris flow activities and their driving factors after the 2008 Wenchuan earthquake for disaster risk management and mitigation from a dynamic perspective. We used the fusion of remote sensing data to develop the first systematic multi-temporal inventory of debris flow activity to quantify changes in spatial and temporal activity of post-earthquake debris flows from 2008 to 2021. Subsequently, we examined the control of material sources, topographic and hydrological factors on the evolution of post-earthquake debris flow activities. A close relationship between the decline in debris flow activity and the reduced active material sources was observed in both time and space, so the supply capacity of hillslope deposits after the earthquake was the first-order indicator controlling post-earthquake debris flow activity. With the progressive depletion and stabilization of co-seismic landslide materials over time, topographic and hydrological factors occupied a dominant role in the occurrence of debris flow. In general, an oscillatory decay pattern of debris flow activity over time was observed after the 2008 Wenchuan earthquake. We observed that the period of post-earthquake debris flow activity varied considerably in different regions attributed to the magnitude of the earthquake, and the geological and climatic environments. Based on the debris flow channel disturbances, the dynamic evolution of landslide materials and hydrodynamics, we divided the debris flow active period after the Wenchuan earthquake into three phases: high activity period (lasting 2 - 5 years), medium activity period (lasting 3 - 10 years), and low activity period (lasting 9 - 18 years). Following the above three activity periods, the debris flow activity will evolve into a steady period. We also proposed a quantitative model for debris flow activeness in the main Wenchuan earthquake-affected area, which predicts that the activeness of debris flows will tend to pre-earthquake levels in 46 years following the earthquake.