沖積地盤の傾動とその自由地下水の水位および水質の変化

Abstract

The Alluvial ground undulates incessantly at Tegata in the Akita Oil field in the North Japan. The tiltings are composed of (1) annual cyclic variation, (2) progressive inclination to ENE, and (3) short period seesaw movements.The annual cyclic variation, to incline relatively to SE in winter and to reduce the angle in summer almost regularly, is related with the fluctuation of free groundwater table. The inclination increases in general as the water table gets higher. The two are especially intimate in winter. It may de due to both the reduction of permeability and the variations of permeability by temperature at the lower temperatures than probably 12° C.The progressive inclination to ENE is certainly related with the movements of the Miocene silty bed rocks.The short period undulation reflects surely the movements of the bed rocks, and related to the occurrences of earthquakes and the volcanic eruption of Mt. Koma in 1970.In the case, the inclination increases to some extent, the groundwater generally gets higher in resistivity and becomes lower in Redox potential. Resistivity, Redox potential and pH sometimes vary to large degree even without remarkable variation of inclination. Although the mutual relations of tilting, resistivity, Redox potential and pH are complicate, their variations suggest the squeezing-out of groundwater by the crustal lateral pressure, occasionally relating to the occurrences of earthquake.