The surroudings of Tokyo have undergone a remarkable change in their regional structure in the postwar period. The present paper deals with the results of the latest investigation made by our Regional Structure Research Group. Our purpose was to analyze the regional structure of the area including cities, towns and villages within the radius of 40km around Tokyo Central Station. (23 Wards constituting the Metropolis were excluded from the present investigation.) The uniqueness of the present analysis may be found in its statistical methods; we selected 16 indices which are expected to represent the regional structure of this area: area, population, time-distance, the ratio of commuting, the ratio of revenue, the ratio of employees, the ratio of industrial production, the ratio of per capita selling, the ratio of cultivated land, the ratio of increase in population, the ratio of primary industrial population, the ratio of
Factor Analysis
secondary industrial population, the ratio of tertiary industrial population, the ratio of managers and officers, the ratio of workers and the ratio of telephone subscribers.
The original figures were standardized after logarithmic transformation and a statistical method called Multiple Factor Analysis was applied to them using an electronic calculator. The calculated values were projected upon the area and each factor was designated according to the above table obtained in the process of calculation.
The results can be summarized as follows: the regional structure of this area is composed of three factors, namely, Urbanization, Residentilization, Industrialization and when they are projected, it is found that Urbanization is decreasingly sloping centrifugally while both Residentialization and Industrialization appear merely scattering around in the form of “detached estates.”
A distinction can be made between ‘higher order district’ comprising the western, eastern, southern and northern suburbs, and, lower order district' comprising Tamagawa valley, Arakawa valley, Tonegawa valley and Boso district. It will be noticed that the western suburbs are the most highly developed area and that Bôsô district possesses quite an independent regional structure which differs much from the rest of the surroundings.
We expect the present method of analysis to be the first of its kind in this country and we hope this will be a stepping-stone to further research in the field of regional structure anayliss.

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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.

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It has long been well known that types of flood vary greatly according to topographical elements mountains, basins, plains and so forth. Accordingly if we subclassify the topography of those flood-visited areas, we can tell their flood-types. For example, in the case of the plain, most of the plains in Japan are alluvial, formed by sand and gravel transported by the frequent flood-waters. So the micro-relief of the plain and the state of the accumulated sand and gravel show the history of the floods. From this point of view, the writer surveyed the topography of the City of Isahaya and its vicinity by means of aerial photographs and by a field survey. And he has drawn up a topographical survey map of the City of Isahaya and its vicinity showing the classification of flood stricken areas. Furthermore, he surveyed the great flood which visited the City of Isahaya on July 25th, 1957 and has drawn up a map of the City of Isahaya and its vicinity showing the state of flood there on July 25th, 1957. He found that the depth of the stagnate water, its period of stagnation, the direction of the current, the velocity of the current, the erosion and deposit vary remarkably according to its area. In such a case, the following factors—the valley plain, the fan and the delta—have a decisive influence upon deciding the type of flood. As a result of this research, the flood-types in this district were divided into the following six types.
(1) The flood-type seen in the valleys situated radially at the foot of the volcano.
A lot of land-slides occurred in the upper courses of the valleys made up of the following rocks—hypersthene andesite, lava and block-mud, and situated radially around the extinct crator of Taradake Volcano, and those land-slides have furnished debris to the valley-plains. The distance of the movement of the debris is not so long.
(2) The flood-type seen in the valley plains surrounding the foot of the volcano.
Here the Honmyo-gawa-river and Suzuta-gawa-river which run around the foot of the volcano have tributaries coming from the northeastern part but have no tributaries from the western and southern part. The velocity of the transmission of the flood-waves in these valleys was very fast and the water reached its highest level in almost all the places of the valley plains. This phenomenon is caused by the following factors: 1) it rained heavily along the valley, 2) the flood-water from the radiate valley plains flowed into the main stream about the same time, 3) the velocity of the transmission of flood-waves was very fast. No stagnation water was seen because the valley plain has steep slopes. The banks were destroyed in many places by the flood water and here and there the streams took short cuts changing their meander courses. The surface soil drifted away with the flood-water and the layer of sand and gravel was exposed here and there, but the distance of the movement of the gravel was very short.
(3) The flood-type seen in the fans at the lower part of the canyon.
The Honmyo-gawa-river cuts the foot of the volcano in the western part of the City of Isahaya and forms a small canyon there. At the lower part of the canyon where the City of Ishaya lies, the flood was very disastrous and the flood water flowed away radially to the east. The deposited gravel furnished from the canyon, banks and river-bed were seen at the area where banks were destroyed. Next to the deposited gravel was seen accumulated sand, and silt was seen beyond. The velocity of the flood-water was fast and the depth of the flood-water was 3 to 4 meters deep.
(4) The flood-type seen in the fan situated at the foot of the volcano.
There is a fan facing the coast of Omura Bay to the west of the Taradake Volcano. The fan is composed of the volcanic products when the Taradake volcano went into the eruption in ancient times, and is quite different from those fans formed by the frequent flood water.

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