Late Quaternary Activity of the Uozu Fault Zone Inferred from Fluvial Terrace Surfaces in the Eastern Part of Toyama Prefecture

Abstract

We evaluate the late Quaternary activity of the Uozu fault zone in the eastern part of Toyama Prefecture. The Uozu fault zone dislocates fluvial terrace surfaces severely, but the slip rate is not well known because the terrace surfaces are not well dated. We estimate the formation ages of fluvial terrace surfaces from aerial photo interpretation, field survey, and chemical analysis of tephra particles in loess deposits. Loess deposits on fluvial terrace surfaces include cryptotephras such as U-1 (120-140ka), K-Tz (Kikai-Tozurahara : 90-95ka), Aso-4 (85-90ka), DKP (Daisen-Kurayoshi : 55-60ka), and AT (Aira-Tn : 25-30ka). LH1 terrace surface is slightly older than the 55-60ka DKP tephra. M1 terrace surface is older than the 90-95ka K-Tz tephra. H2, H3, and M1 terrace surfaces are estimated to be 280-310ka, 240-260ka, and 155-165ka, respectively, as long as the accumulation rate of loess deposits above each surface is constant. Vertical slip rates of faults are calculated using these terrace formation ages : over 0.24-0.46m/ky at the central part of the fault zone, and 0.13-0.24m/ky at the southern part of the zone. Maximum vertical slip rate of the Uozu fault zone has been considered to be over 1m/ky. But the rate might be overestimated because terrace ages are being revised to be older than previously reported. Future study is required to clarify the underground geological structure which records vertical displacement on faults for evaluation of the activity of the Uozu fault zone.