秋田臨海平野の形成史  [in Japanese] Late Quaternary geology and development of the coastal alluvial plain of Akita prefecture, northeast Japan  [in Japanese]

Most of the damage by the Nihonkai Chubu Earthquake of 1983 was caused by the liquefaction of subsurface sediment in the alluvial plain along the Japan Sea coast of Akita and Aomori Prefectures, northeast Japan. Geological observations were carried out to find the relation between the damage and the geological conditions. This paper describes the geology of this area and the late Quatemary development of the coastal alluvial plain in the Akita area based on geomorphological analysis and sedimentological study using abundant drill-hole data. In the coastal region of central to north Akita Prefecture, so-called Green Tuff formations of Early to Middle Miocene age overlie pre-Tertiary granitic rocks intwo uplifted zones, the northernmost and central areas of the Prefecture, and in the western part of the Oga Peninsula. The other part is an area of subsidence where Neogene oil-producing formations and Quaternary sediments are widely distributed. The late Pleistocene Katanishi Formation is present in the area surrounding the alluvial plain forming a terrace of 20-60 m above sea level. Fluvial plains develop in the lower catchment areas of the Yoneshiro River in Noshiro City and the Omono River in Akita City, and in small areas along the eastern side of the Hachirogata lagoon. The lagoon is now mostly reclaimed. The coastal plain is narrow and is continuous with the fluvial plains. Sand dunes are well developed along the coast in a few rows, and cover the shore-side area of the coastal plain. During the last glacial epoch about 18,000 years ago, sea level fell to more than 100 m below the present level and large valleys were excavated by the ancient Yoneshiro and Omono Rivers. Thereafter, these valleys were gradually filled by gravel, sand, mud and peat during the rise of the sea level. About 6,000 years ago, sea level rose to more than 6 m higher than the present level (the Jomon Transgession), and the sea invaded the river mouth areas of that time, forming the highest Holocene terrace of the alluvial plain. Since the Jomon Transgression, two minor regressions and two minor transgressions occurred, and lower terraces were formed cutting the higher terraces in the alluvial plain. On the other hand, the Hachirogata area has a different geologichistory than the fluvial plains. After the early stage of deposition in the late glacial age, the area became brackish to shallow marine enclosed by sand bars on both the northern and southern sides between an island (the present Oga Peninsula) and the mainland. Owing to tectonic subsidence of this area, thick Holocene mud accumulated under the brackish water or partly shallow marine conditions controlled by the development of sand bars in accordance with the sea-level change. Development of coastals and dunes was controlled by the sea-level fluctuation, and at least three major ages of development were detected.