The flood-disaster that visited Isahaya city, Nagasaki Prefecture on the 25th of July, 1957, was caused by the torrential rain limited to that particular locality. Interrelation among the character of that torrential rain, the geological formation of Tara Volcano and its physical features had an important effect upon the character of the landslide, and the outflow of debris, etc.
The characteristics of landslides thet occurred in that area are summarized as follows;
(1) Landslides mainly occurred in the areas of the middle and lower parts of Tara Volcano composed of hypersthene andesite and of block mud.
(2) Each of the landslides forms a tadpole shape making a jet point its head, and the slided material is generally shallow.
(3) The greater part of these landslides broke out by the eruption of infiltrated water out of mountainslopes. These landslides are characteristic of landslip arising from ground water.
(4) Relations between the types of slopes and landslides will be summrized as follows;
Some landslides broke out the slopes of shallow valley-shaped hollows. Water gushed out of the place where it is convenient for some amount of ground water to get together, which gave rise to landslip. However, the greater part of landslides broke out on a part of even slope where water getting together on the slope is supposed to be small in quantity.
(5) Landslides arose from the divergence of physical properties to water between hypersthene andesite and block mud the volcano is composed of. From the genetic point of view, these landslides are classified into the following four groups.
(i) Landslides breaking out in the process of eroding the flat surface on the region composed of hypersthene andesitie.
(ii) Landslides breaking out on the region composed of block mud or of the andesitic lava flow intermingled with the former.
(iii) Old landslides.
(iv) Lateral erosion of river flow.
(6) Putting together the property of slided material on the slopes, types of slopes showing scar of old landslide, deposit of earthflow along the valley and deposit transported by the flood, the writer is led to the following supposition.
In that region the same sort of disastrous landslides in their character and scales occurred repeatedly both in prehistoric and historical ages.
And, the above-mentioned disaster caused by the torrential rain can be said to have shown a phase of disaster similar to those repeated before.