Terrestrial phenomena disasters/Earthquakes/Slope disasters. Slope disasters/Slope collapse and landslides
Disaster name
Large-scale landslide on Mt. Ontake Caused by the Earthquake of Western Nagano Prefecture in 1984
Author of WEB conversion
Morohoshi Toshikazu

Case Study

No. 11

1. Analysis objective

The occurrence of a M6.8 earthquake with its epicenter near Otaki village in the county of Kiso, Nagano Prefecture resulted in a large-scale collapse, rock slide and debris flow at the foot of Mt. Kiso-ontake. The toll of dead and missing persons eventually reached 29 people. The maximum cubic volume of the collapse was estimated to be 36,000,000m3, and the total length of its flow and volume area reached 12km. The total amount of physical damage reached 42.5 billion Yen, including roughly 24.4 Yen billion for Nagano Prefecture and related municipalities, and about 18.1 billion Yen for Forestry Agency-related losses.

In situations where a field survey of a mountainous area disaster would be difficult, uncover the full details of a disaster from an analysis of LANDSAT data, which is faster than interpreting aerial photographs or conducting field surveys.

Map the detected collapse and debris flow region onto a topographical map, and use the map for various practical applications such as measures to prevent secondary disasters, support of rescue operations and restoration activities.

2.Analysis procedure Analysis flow chart

The analytical method is shown as a flow chart in the appendix, but the unique characteristics of the method are described below.

1) As a method for detecting disaster situations using LANDSAT data, a procedure for comparing data that are as closely related as possible to the situations before and after a disaster and that can capture the change in the covering of the earth's surface during this period is effective.

2) Investigates the change in spectral reflection characteristics for each type of change in the covering of the earth's surface (from forest to sediment, from forest to submerged land, from lake water region to sediment), and determines the area of the change in the covering of the earth's surface.

3) Because disaster situation detection results begin to have a practical use only after their positional relationship with topographic maps is clarified, plot the area of the change on a topographic map.

3. Analysis results

Although we were unable to detect the origin of the collapse because of clouds or cloud shadows, we were able to plainly detect the large-scale area of earth surface changes caused by the rock slide and debris flow, and the area submerged under water along the Otaki river that was dammed up by the lower reaches of the collapse. We were also able to detect the conditions where the collapse sediment buried a dam lake.

4. Results from using the analysis results

The results are effective as reference material prior to a field survey in a mountainous area where a field survey of the disaster will be difficult, and as primary data for secondary disaster prevention, rescue support, and restoration measures, including times when a field survey will be conducted.

By mapping the results to a topographic map, the results can provide even more precise reference material for needs such s quantifying the scale of a disaster.

5. Sources

Kishi S., Natural Disaster Research Report by the National Research Center for Disaster Prevention, No. 25, pp. 76-79, 1985