Back marsh sediments in the northern area of Nagano City, Japan and earthquake damage induced by the Naganuma Earthquake in 1941

Abstract

The Nagano basin is a typical Quaternary sedimentary basin in the northern part of the Fossa Magna in central Japan. Along the western marginal area of this basin, the tectonic movements continue since the late Pleistocene. As a result of tectonic movement, fan, natural levee and back marsh sediments were formed along the Chikuma River in the Nagano basin. The study is summarized as follows; (1)The back marsh sediments in the northern area of Nagano city are classified into the upper clay and lower clay by the about ten-hundreds drilling data. The back marsh sediments have a thickness of a maximum of 40m. (2)The back marsh and natural levee sediments were classified with N values. The upper clay of the back marsh sediments is very soft with the N values 4 or less, and the sand of the natural levee sediments is loose with the N values 10 or less. (3)The earthquake damage of the Naganuma Earthquake in 1941 was concentrated on the soft ground area. The earthquake damage area caused by the seismic vibration was concentrated on the connection part of the upper clay area. The liquefaction damage is located on the loose sand area.