VANISHING TOKIO

Te Awamutu Courier, Volume 52, Issue 3756, 15 May 1936, Page 8

VANISHING TOKIO

ALARMING POSITION. SLOW SUBSIDENCE GOES ON. FACING INUNDATION. About the middle of the Pacific coast on the main island of Japan a broad inlet, known as Sagami Bay, runs northwards into the country for thirty miles or more, Writes Dr. Charles Davison in the “Manchester Guardian.’’ At its north-west end it contracts into a narrow channel that widens out northwards into another bay, Tokio Bay, of about the same length, along '.*< western shore of which lie the great seaport of Yokohama and the capital city Tokio. Sagami Bay was the chief seat of the great earthquake that, on September 1, 1923, ruined the two cities and many others in the neighbourhood. During or after the earthquake the bed of the bay was strangely distorted, one portion sinking as much as 1312 feet, and another close by rising 755 feet. To a less extent the land was also a&ected, and the shore of Tokio Bay was lowered by from six inches to a foot. A small amount, if we think of the movements elsewhere, but the eastern portion of Tokio is so low that even a movement on this scale might be of importance, and in consequence the ground of the whole district was

of the whole district was raised in 1923 so as to be at least four feet above the mean sea-level of the bay, while there is no point more than about ten feet above that level.

Fortunately the tides in Tokio Bay ! are usually of small account. The J greatest rise is not much more than * three feet, so that inundations at ! high tide should not occur unless 1 there is some abnormal rise of the r water surface. Nevertheless, on ’ several occasions in the autumn of 1931 houses in the eastern district were flooded during high tides. I A Foot Drop. : It is the custom in Japan after jvery great earthquake to carry out I s new survey of the stricken district. ■ Fresh series of levellings are made I dong former routes, and occasiontlly the trigonometrical survey of the district is repeated. Such resurveys have been made on several occasions since the great earthquake, and they have revealed the important fact that the depression of Tokio still continues. Anxiety is naturally celt, since, with the further continumce of the depression, a large part of Tokio may lie beneath the sea before many years have passed. Within this area there are no fewer than forty-two bench-marks. (These are marks place* on posts, walls, etc., to indicate points in a line of levels, for the determination if altitudes. In taking readings an angle-iron is fixed at the mark as a ‘bench’’ or' support for the surveyor’s levelling staff.) The heights of chese points above sea-level were measured in November, 1929, and again in March, 1932. In the interval, with two exceptions of little consequence, every one of the benchmarks were found to be lowered, .hree of them more than a foot, ten letween eight inches and a foot, and ifteen between four and eight nches. Since 1923 one point had oeen lowered by more than five feet. The zone of greatest depression lies between the rivers Sumida and Yedo. This happens also to be the region in which the covering of soil is thickest and lies above a well-marked old valley that dates back to Tertiary times. Closely Placed. The bench-marks are so closely scattered over the area that Professor Miyabe, who has been engaged in studying these depressions, has found it possible to divide Eastern Tokio into six sub-regions, each of which seems to be moving on its own account. Each sub-region is the surface of a block of the earth’s :rust, divided from those adjoining tby fissures. The blocks vary in size and are of somewhat irregular shape. Roughly, their sides are ibout\a mile or a mile and a half ong. One of the blocks occupies the srea of greatest depression, and it Is remarkable that the tilting of the four surrounding crust-blocks, though slight, should be directed approximately towards its centre. Later

measurements in the same area show that at many points the rate of unking has increased perceptibly rinCe 1932, and that so far it has ihown no signs of slackening. Further light is thrown on these movements by the records of two tide-gauges in the southern half of ■.he area. From these records the height of the sea level of Tokio Bay was read every hour, and the average heights during successive intervals of a lunar month, or twentynine days, were calculated. This particular interval was chosen so as to lessen the effect of the varying height of the sea-level from one spring tide to another. Professor Miyabe found that the average height was subject to two variations. One of these lasts for one year, the average level being lowest at the end of the winter and I higher by about two feet at the end of summer, and this ho doubt is the

reason why the lower parts of Eastern Tokio are occasionally subject to inundations during the summer and autumn months. The other change is of more importance, for it shows that the sea level is gradually rising —in other words, that the shore is being lowered. At one of the two tide-gauge stations the sea level in 1926 was about eight inches higher than in 1923. At the other, it rose by about five inches in 1932, and 5i inches in 1933, but in the following year the upward movement ceased, at any rate for a time. Thus, unless measures are taken to raise the level of the ground, the waters of Tokio Bay will before many years are passed have spread over the present lower part of the city. Professor Miyabe has given some atten'on to the cause or causes of this remarkable depression. There can be little doubt that it is chiefly due to the subsidence and tilting of the crust-blocks that exist in this district. But there may be other partial factors that may increase slightly the rate of sinking. The load of the earth that was added in 1923 to raise the level of eastern Tokio would help to depress the crust below. So, also, would the reduction in pressure of the underground gases and the contraction of the surface soil due to the lowering of the underground water level, both of which are known to have occurred. The changes resulting from such causes would, however, be small. We are left with the conclusion that the eastern part of Tokio is sharing in the widespread lowering of the whole north-eastern coast of Japan.