Residential Districts on Mountain Slope
Figure A wooden house at risk of steep slope failure
Rokko Mountain is composed of heavily weathered rock. Many active faults are widely distributed in this area, which cause innumerable cracks. Rain water infiltrates deep into the ground through the cracks weathering granite rocks into brittle, sandlike rocks. Mt. Rokko, having been formed by the uplift movement of faults, has many steep slopes, where many cases of slope failure and landslide have been observed. In the Rokko mountain area, over 571 cases of slope failures and cracks were recognized to have occurred due to the disaster (Tainosho et al, 1996). The damage was spread out by rainfall after the earthquake and the number was increased to more than double. The mountain area is at risk of collapsing with even a small amount of rainfall. A research was conducted to measure the degree of the risk at about 830 steep slopes, designated as steep slopes at risk of collapsing, and the result revealed that at least 39 steep slopes were at an extremely high risk (Tainosho, 1998). In and around Kobe, where there is not much flat ground, many people live on steep mountain slopes in danger of collapsing (Figure).
Dangerous Areas Increase by Development
According to research conducted by the Hyogo prefectural government and the Kinki Bureau of Construction, there were 1,500 places in danger of earth flow disaster on steep slopes. Improvement works have started at 1,140 places, such as an erosion control dam, but it has been completed in only 62 sites. The number of dangerous spots has been increasing, however, regardless of the continuous improvement works, since there are more and more residential sites developed at the more dangerous areas on the steep mountain slopes. It is essential and critical to recognize that the best disaster prevention is to stop the development called "Mountain goes to Sea", learning lessons from the great earthquake.