Land Subsidence and its Problems

Abstract

Taking the good opportunity of International Symposium on Land Subsidence held in Tokyo with excursion to Osaka and Niigata from September 17 to 23 in 1969, the writer tried to review the present situation of land subsidence in the world and its related problems.He gives definition of land subsidence as the comparatively progressive sinking in a limited area due to the artificial withdrawal of fluids such as ground water, gas, oil and so on. This phenomenon is different from the slower progressive downwarping movement in a tectonic basin or relative sinking due to eustatic movement during Quaternary age with the rate of 1-3 mm per year in Japan. The rate of land subsidence may differ by as much as one or two orders in magnitude. It may also be different from more rapid collapse due to mining of coal, gold and so on. According to R. Dolzal and M. Petersen's definition, subsidence, settlement and lowering are distinguished, the writer's definition corresponds to their settlement and in some case, to lowering.He reviewed the history of study on land subsidence in Japan, and grouped them to three categories. Leveling net-work covering all over Japan was established as early as the end of 19 centuries and precise geodetic leveling has been repeated at a fixed interval. This was very useful for making clear the phenomenological aspect of land subsidence.The first researcher, who gave attention to this problem was A. Imamura with his opinion of crustal movement. In an earlier stage of this survey, his successor, N. Miyabe might have the similar opinion, but soon later noticed that its cause was the contraction of the surface soil. Among these geophisicians, K. Wadati and his collaborators pointed out that there existed a proportional relation between the rate of subsidence and that of ground water pressure changes. S. Hayami added other elements such as tidal and artificial periodic changes to this.The second group was of soil mechanicians who applied Terzaghi's theory on compaction of clay layer. Y. Ishii and his collaborators estimated future amount of land subsidence and S. Murayama conducted laboratory experiments on land subsidence. The third group which is consist of geographers, geologists and hydrologist contributed to give the fundamental knowledges, for example, F. Tada and T. Nakano supplied subsurface topography of the land subsidence areas, S. Aoki and T. Shibasaki furnished Quarternary geology of these areas and I. Kayane and the present writer discussed water balance relations of the subsiding areas. These three groups are elaborating to proceed their researches and investigations with administrative officials. Now, Japan has many observation wells in land subsidence areas. (Tab. 1) He also summarized recent trend of subsidence in Japan (Tab. 2) collecting available data on the amount and rate of land subsidences in the world (Tab. 3) and pointed out the future problems and further studies.