Late quaternary vertical average slip rate of the southern part of the Uozu fault using a drilling survey, Toyama prefecture, northern part of central Japan

Abstract

Fluvial terraces along the lower reaches of the Kamiichi River to the Hayatsuki River, eastern margin of the Toyama Plain, are widely distributed. These terraces are divided into three levels, the Tohfukujino terrace, Nakano terrace, and Oosakino terrace, in descending order of elevation. In the Kamiichi and Hayatsuki river areas, the age of the terraces has not reported, as volcanic ash is not visible within terrace deposits and overlying loamy soil, and there are few outcrops in this area. In this study, we carried out a drilling survey on the terrace to obtain overlying loamy soil. We extracted some well-known widespread volcanic ash within the loamy soil collected in the drilling samples and estimated the approximate age of the terraces. Below are the main results obtained in this study. 1) The Tohfukujino terrace and Nakano terrace are overlain by the Aira-Tanzawa tephra (22-25 ka), Daisen-Kurayoshi tephra (43-55 ka), and Kikai-Tozurahara tephra (75-95 ka). From the tephrochronology and the sedimentary facies of terrace deposits, we estimate that the Tohfukujino terrace was formed at oxygen isotope stage 6, and the Nakano terrace was formed at oxygen isotope stage 5-d. 2) The Oosakino terrace is overlain by the Aira-Tanzawa tephra and Daisen-Kurayoshi tephra. The Daisen-Kurayoshi tephra contains the lowest part of loamy soil overlying the terrace. We estimate that the Oosakino terrace was formed at oxygen isotope stage 4 or at the stage 4-3 boundary. 3) On the basis of the approximate age of terraces and vertical displacement of active faults, we estimated the average vertical slip rates for the Uozu fault at 0.4-0.5 mm/yr (Tohfukujino terrace), ca. 0.4 mm/yr (Nakano terrace), ca. 0.4 mm/yr (northern part of Oosakino terrace), and ca. 0.2 mm/yr (southern part of Oosakino terrace), respectively.