Early-warning systems for natural disasters are important tools for disaster risk reduction and for achieving sustainable development and livelihoods. In 2005, the Japanese government initiated a new nationwide early-warning system for landslides disasters. The main methodology of the system is to set a criterion for occurrences of debris flows and slope failures based on several rainfall indices (60-min cumulative rainfall and soil-water index) in each 5-km grid mesh covering all of Japan. Because many of the records of mass movements are lacking in scientific precision on timing and location, the system applies Radial Basis Function Network methods to set the criterion based primarily on rainfall data recorded as not triggering disasters. Since the end of March 2007, under torrential rainfall conditions, early-warning information has been disseminated as part of weather news using TV, radio, and the Internet. Because of the increasing worldwide recognition of the importance of early-warning systems for natural disaster reduction, the aim of this article is to introduce the new Japanese early-warning system to the international landslide community. In this article, the method, the system, and the result of its application to landslide disasters in 2009 are presented.

Many shallow landslides were triggered by this M w 6.6 (M j 6.7) earthquake. The basement complex in the affected area (sedimentary rocks) is covered with thick pyroclastic fall deposits derived from the Tarumae Volcano, etc., and the strong seismic shocks triggered shallow landsliding of them. Shallow landslides moving along valley type topography traveled greater distances than those moving along planar slope topography. Some shallow landslides occurred on relatively gentle slopes (< 30°). The earthquake also induced several large-scale deep-seated landslides, including one that has formed a landslide dam in the Hidaka-horonai River. Landslides were densely distributed over hilly regions (elevation: 200-400 m) within an area of approximately 400 km 2 in Atsuma (landslides caused 36 deaths), Abira, and Mukawa, and the number of landslides and the total area of the landslides were the largest in Japan ever since the Meiji Era . The catchments where shallow landslides were concentrated were severely devastated.

This is a review of the national policy related to hazard maps for sediment-related disasters in Japan. Until the 1980s, we focused mainly on rainfall-triggered sediment-related disasters, including debris flow, deep-seated landslides, and steep-slope failures, and until 2001, Japan did not have any laws related to hazard maps for sediment-related disasters. The Sediment-related Disaster Prevention Act became effective in 2001 and required all prefectural governments to publish hazard maps. Hazard mapping for volcanic sediment-related disasters started in 1991, and hazard maps have now been published for all 29 volcanoes that would have significant social impacts in the event of eruption. Investigations aimed at assessing the susceptibility to massive collapse and shallow seismic landslides recently started based on new methods.

Countermeasures to prevent or mitigate sedimentrelated hazards, debris flow in this paper are classified as structural measures or nonstructural measures. The structural measures are Sabo dams, leading dikes, and channels for debris flow control. The nonstructural measures are the designation of areas prone to debris flow, proper land use in the areas, the reinforcement of houses, the creation of warning systems and the evacuation of the inhabitants in case of emergency. A technical standard on debris flow control structures of Japan was revised in 2007. Major revision is that check dams called Sabo dams constructed near houses are changed from closed type Sabo dams to open type Sabo dams because the occurrence frequency is low as once for longer than one hundred years, natural torrent environment should be conserved in ordinary days and sediment trap capacity should be kept until debris flow occurs. Points changed are explained such as the opening of open type dams is as equal as the maximum stone size.

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