Landslides occur when, as a result of a typhoon, intense torrential rain, an earthquake or melting snow, weakened sloping ground breaks up and causes a large volume of sediment to suddenly collapse. Caution is particularly necessary when there is heavy rain caused by the rainy season or a typhoon. When rain falling at a rate of more than 20mm an hour reaches more than 100mm since the time when it started falling, the risk of a landslide grows; persons living on sloping land, mountains or valleys should take care at such times.