Increased Risk with Large Typhoon… Giant Mudslide “Five Major Warning Areas in Japan

A giant typhoon is about to overrun Japan. The Japan Meteorological Agency (JMA) has called for the utmost caution regarding Typhoon No. 14, describing it as “the most dangerous typhoon we have ever experienced.

When we think of typhoons, damage from wind storms and heavy rains are the first things that come to mind, but the accompanying risk of “mudslides” cannot be overlooked. In its July 23, 2009 issue, “FRIDAY” published an article titled “Five Major Warning Areas in Japan: Prepare for Giant Mudslides! Five major warning areas in Japan,” which introduces areas in Japan that are at particularly high risk of mudslides. It is reproduced below.

Global warming has raised sea surface temperatures near Japan and increased the amount of water vapor in the atmosphere, making abnormal weather events such as giant typhoons and record-breaking long rains more likely to occur. We are now in an era when typhoons can strike as far as Hokkaido while still maintaining their strength. The risk of rain-related disasters is increasing nationwide, and landslides like the one in Atami could occur anywhere,” said Nobuyuki Tsuchiya, technical advisor of the Riverfront Research Institute.

This publication mapped out “five major warning areas” where steep slopes and mountain stream areas are densely populated and where dangerous mudslides are likely to occur. The following is a brief description by Tsuchiya of each of these areas.

Northern Morioka City, Iwate Prefecture
Cracks in the ground caused by the aftershocks of the Great East Japan Earthquake are still accumulating rainwater. In winter, rainwater freezes and widens the cracks, which accelerates the weathering of the ground.

Around Shimotsu Port, Wakayama Prefecture
The Kii Peninsula is frequently hit by typhoons, and rainfall tends to be concentrated. Steep slopes are also scattered in urban areas, making them highly hazardous.

Around Matsue City, Shimane Prefecture
The Chugoku Mountains to the south are volcanic in nature and very prone to collapse, making them one of the top three most dangerous areas in Japan.

Kochi City, Kochi Prefecture
Kochi Prefecture is subject to heavy rainfall exceeding 1,000 mm every year. The urban area of Kochi City is densely populated with hazardous areas.

Around Sakurajima, Kagoshima Prefecture
The area is located on the Shirasu Plateau, which was deposited by volcanic ejecta, and has many steep slopes. The ground is made of volcanic ash and is prone to collapse due to rainwater.

The hazard map for landslides produced by the Ministry of Land, Infrastructure, Transport and Tourism is actually incomplete. Local residents protested vehemently because the property value of their land would decrease if it were designated as a hazardous area. Fearing protests, many municipalities have not yet produced the maps. In the case of Atami, we have learned that mudslides occur not only in short periods of torrential rains, but also in long rains. In the future, mapping of landslide disasters should be made mandatory, just as it is for floods and earthquakes.

The “common sense” of the past no longer applies to natural disasters.