小海岸平野の水文地質構造と地盤沈下の関係 : 福島県原町市の例

A Study of Relationship between Hydrogeological Structure and the Land Subsidence in the Small Coastal Plains, with Special Reference to the Haramachi District, Fukushima Prefecture

CHUMAN Norichika

廣島大學地學研究報告

Geological report of the Hiroshima University

As to the land subsidence in the Haramachi district, the present paper clarifies its geological extent, stratigraphical horizons, mechanism, successive changes, a relation to social activities, and many other points by means of geological, hydrogeological, and soil engineering methods.
Data for the research were obtained by drillings, microfossil analyses of drilling cores, 14C-datings, electric soundings and loggings, soil tests, field survey, observation of aerial photographs, levelling of piezometric and phreatic surfaces and bench marks, pumping tests, and estimation of the pumped-up volume of groundwater.
The land subsidence of the Haramachi district occurs at the lowland area along the lower part of the Ota River, later extending to the terraces and hills. Stratigraphically, the subsidence concentratedly takes place in the "Alluvium", which consists mainly of silt, sand and peat, and the Tatsunokuchi Formation consisting of sandstone and siltstone is also affected. The land subsidence is chiefly caused by over-draft and influenced by soil-mechanical characters of confining layers and aquifers.
The land subsidence in the Japanese coastal plains is divided into two types, A and B, according to the respective characteristics of horizontal as well as vertical extent of subsidence, and the damage degree brought by the subsidence. The A type is observed in the large coastal plains such as the Kanto, Niigata, Nobi, Osaka, and Shiroishi plains. In these plains, Quaternary tectonic movements called the Island-arc tectonic movement has violently taken place, and many aquifers and confining layers are developed in many stratigraphic horizons on a large scale. Therefore, groundwater utilization has been progressed, and the land subsidence extensively occurred both horizontally and vertically, and its damage often becomes serious. The B type is found in the small coastal plains, where the "Alluvium" is relatively thin and underlain with the Tertiary formation (bed rock) with or without the thin "Dillivium". In these places, the land subsidence locally occurs in the surficial layers. The land subsidence of the Haramachi district belongs to the latter B type.
The present paper also proposes a new Holocene sea-level curve. According to the curve, the Early-Jomon transgression ranges from ca. 10,000 to ca. 6,000 years B. P., the small Jomon regression from ca. 6,000 to ca. 4,800 years B. P., the Late-Jomon transgression from ca. 4,800 to ca. 4,100 years B. P., the small Yayoi regression from ca. 4,100 to ca. 1,800 years B. P., and the recent sea-level from ca. 1,800 years B. P. to the present.

jpn

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676