Consideration of the Origin of the Sag-Shaped, Closed Depression in the Fukui Plain

Abstract

The author investigated the characteristics and the origin of the sag-shaped closed depression along the lower reach of the River Arakawa in the Fukui Plain.
The results are summarized in the following manner.
1) In the sag-shaped closed depression, the deltaic sediments are thickly deposited as the upper final layer.
2) From the geomorphological characteristics of the sag-shaped closed depression, its formation has its origin in the subsiding activity following the formation of the deltaic plain.
3) The sag-shaped depression has the following characteristics.
(i) The sag-shaped closed depression is located just above the earthquake fault caused by the Fukui earthquake, and a leftslip together with west side down-thrown have occurred.
(ii) Breaking out of the crack and the formation of the depression have occurred at the time of the Fukui earthquake.
Based on the above mentioned characteristics, the author has come to the conclusion that in the sag-shaped closed depression, faulting in the bed rock has occurred due to earthquakes in the past, and a subsiding activity associated with this faulting gave rise to the depression of a thick upper final layer and also to the sag-shaped closed depression on the deltaic plain.