抄録
In the Kanazawa City area, northern part of central Japan, fluvial terraces are widely distributed along the Sai to the Asano rivers. The absolute age of these terraces has reported few, as primary visible volcanic ashes are not recognized within terrace deposits and overlying loamy soils, and outcrops are quite few in this area. In this study, we have carried out drilling survey on the terrace to obtain the whole sections of overlying loamy soil. We have extracted some concentrated zones of volcanic glass and mineral derived from well-known wide-spread volcanic ashes within the loamy soil and could estimate the approximate age of terraces.
Late Quaternary fluvial terraces in this area are divided into seven levels: Noda I terrace to Kasamai II terrace in descending order. Noda I terrace is overlain by the Kikai-Tozurahara tephra (75-95ka) contained at the lower part of the loamy soil. The Kodatsuno terrace is overlain by the Daisen-Kurayoshi tephra (43-55ka) and Kasamai II terrace is overlain by the Aira-Tanzawa tephra (22-25ka). Especially, each tephra is contained the lowest part of loamy soil in each terrace. We estimate that the Noda I terrace, the Kodatsuno terrace and Kasamai II terrace were approximately formed at 85,000∼95,000 years ago,50,000∼60,000 years ago,25,000∼30,000 years ago, respectively.
The Kasamai II terrace is vertically deformed about 15-20m by the Nomachi flexure along the Morimoto-Togashi fault zone. Considered from the approximate age of terraces and vertical displacement of active faults, average vertical slip rates for the Nomachi flexure is estimated to be 0.5∼0.8mm/yr.
