A new study by researchers at the University of Tsukuba is shedding light on how centuries-old construction in Japan’s mountains may still be influencing landslide risks today.
The team examined ridge-top landscapes in Hiroshima Prefecture, comparing 39 castle sites with 39 unmodified ridges. Using digital elevation models with a 1-m resolution, they measured two features—slope gradient and curvature—to distinguish natural from altered ground.
Castle sites stood out. Their slopes varied more sharply than those of natural ridges, reflecting the cutting, levelling, and terracing carried out during construction. Those differences were not just structural. Sites with the greatest variation in slope—and more concave, valley-like shapes—also showed more signs of shallow landslides.
At Kagamiyama Castle, for example, LiDAR mapping revealed scattered traces of slope failure across areas that had been heavily modified. These features are often subtle and hard to detect on the ground but appear clearly in high-resolution elevation data.
LiDAR, which uses laser pulses from the air to map the Earth’s surface, has become a standard tool in Japan for surveying forested and mountainous regions. It has exposed a wide range of landforms shaped by human activity, many of them dating back centuries. What has been missing is a consistent way to measure how far those landscapes depart from their natural state.
The Tsukuba study offers one approach. By focusing on measurable terrain properties rather than visual interpretation alone, the researchers were able to classify ridge sections with greater consistency and compare them across sites.
The results suggest that past construction may still influence how slopes behave. Uneven surfaces and small depressions can affect how water collects and drains, which in turn can weaken soil and increase the chance of shallow slides, especially during heavy rain.
That has practical implications. Detailed topographic records are scarce for periods before the mid-20th century, making it difficult to factor older land changes into modern hazard assessments. Methods that can identify those changes from current data could help fill that gap.
Japan experiences frequent landslides due to its steep terrain and seasonal rainfall. While large-scale risks are well studied, smaller, localized factors—such as historic earthworks—are less often considered. This research points to one way of bringing those factors into the analysis.
The authors note that the same method could be applied elsewhere, particularly in regions where past construction reshaped hillsides but written or mapped records are limited.
For now, the findings add another layer to the understanding of how landscapes evolve. In this case, the imprint of medieval construction remains visible—not only in the form of ruins, but in the way the ground itself responds to natural forces.
Header Image Credit : Masami Inomata, Hiroshima Institute of Technology
Sources : University of Tsukuba – 10.1080/02723646.2026.2643590
