Changes on the Deltaic Plains in Japan

Abstract

Based on the recnt available data, the most of the present deltaic plains in the coastal areas of Japan probably began some 5, 000 years ago, following the virtual cessation of the eustatic rise in sea level. Subsequent sea level oscillations of small amplitude may have occurred since that. The sea level during the Yayoi period (about B. C. 300-A. D. 300) was lowered about 2 meters below present sea level and they would have had an effect on the rapid growth of the deltaic plains. In early historic times (about A. D. 4th century to 10th century), the sea level should have rised higher than the present one. In this time, consequently, the growth of the deltaic plains was slow. Generally, in the deltaic plains with altitude of less than 5 meters, the remains of the Yayoi period are now found at depths of about 1-2 meters under surface soil. The remains are overlaid with fluvial sediments of 1-2 meters thick, which were deposited under subaerial conditions. This fact shows that the topographical features of the fluvial deposits on the lower part of the great deltaic plains were formed after the Yayoi period. The minor regression is supposed to have taken place during the time from about 13th century to 19th century. The latter half of this period (Edo era, A. D. 1603-1867) constituted the most intensive delta-reclamation age in the history of Japan. In 20th century, the developed areas on the deltaic plains have suffered from many disasters. Among such disasters, those due to ground subsidence are of particular importance for their extensive area effected. This phenomenon is referred to as accelerated compaction of clayey stratum by lowering of underground water-level.