Abstract. Recovering from major earthquakes is a challenge, especially in mountainous environments where postearthquake hazards may cause substantial impacts for prolonged periods of time. Although such impacts were reported in the 1923 Great Kantō earthquake and the 1999 Chi-Chi earthquake, careless reconstruction in hazard-prone areas and consequently huge losses were witnessed following the 2008 Wenchuan earthquake in the Sichuan province of China, as several reconstructed settlements were severely damaged by mass movements and floods. In order to summarize experiences and identify problems in the reconstruction planning, a monitoring of one of the settlements, the town of Longchi, was carried out by image interpretation and field investigation. Seven inventories containing buildings, farmlands, roads and mitigation measures were made to study the dynamics of elements at risk and exposure over a period of 11 years. It was found that the total economic value of the newly reconstructed buildings was several times more than in the preearthquake situation in 2007, because of enormous governmental investment. Postseismic hazards were not sufficiently taken into consideration in the recovery planning before the catastrophic debris flow disaster in 2010. As a result, the direct economic loss from postseismic disasters was slightly more than the loss caused by the Wenchuan earthquake itself. The society showed an impact-adapt pattern, experiencing losses from disasters and then gaining resistance by abandoning buildings in hazard-prone areas and installing mitigation measures. The locations potentially exposed to postearthquake hazards were summarized, and a possible timetable for reconstruction was proposed. Problems might be encountered in hazard assessment, and possible solutions were discussed.

Abstract. Recovering from major earthquakes is a challenge, especially in mountainous environments where post-earthquake mass movements and floods may cause substantial impacts. We monitored the reconstruction of Longchi town in Sichuan, China, over a period of 11 years, following the 2008 Wenchuan earthquake. Seven inventories of buildings, land use, roads and mitigation measures were made by using remote sensing image interpretation and field surveys. Most of the buildings were rebuild by 2010 and reconstruction was completed by 2012. The total economic value of the new buildings in 2010 was much more than the pre-earthquake situation in 2007. Unfortunately, post-seismic hazards were not sufficiently taken into consideration in the recovery planning before the catastrophic debris flow disaster in 2010. As a result, the direct economic loss from post-seismic disasters was more than the loss caused by the earthquake itself. The society showed an impact – adapt pattern, taking losses from disasters and then gaining resistance.

Of the catastrophic earthquakes over the past few decades, the 2008 Wenchuan earthquake triggered the greatest number of landslides and deposited a large amount of loose material on steep terrains and deep gullies, which was highly conducive to the occurrence of post-earthquake debris flows. It is of great importance to clarify the evolution of debris flow activity for hazard evaluation, prediction, and prevention after a strong earthquake, especially in the face of large debris flow hazards. We established a long-time span database consisting of 1668 debris flow events before and after the earthquake, with information including the occurrence time, location, and scale (small, medium, and large). In order to analyze how the environmental background before and after the earthquake controlled the debris flow activity, we examined various controlling factors, including the material source, topography (relative relief and slope degree), rainfall, normalized vegetation index, and lithology. After completing the analysis of the spatial and temporal evolution of the debris flow events in the database, a 10 × 10 km grid was introduced to grade the controlling factors in ArcGIS. Based on the same grid, the density of debris flow events for each scale in different time periods was calculated and graded. We introduced the certainty factor to figure out the spatial–temporal relationships between debris flow activities at each scale and the controlling factors. The results can provide guidance on how to dynamically adjust our strategies for debris flow prevention after a strong earthquake. Lastly, Spearman rank correlation analysis was performed to clarify the variation in the magnitude of the influence of controlling factors on the debris flow activities of different scales with time. This can provide a reference for the dynamic evaluation of debris flow hazards in the Wenchuan earthquake-affected area.

No abstract