Identification of stratigraphic duplication using radiocarbon dating: An example from geological units at a road-cut exposure near the Kumamoto Earthquake Museum KIOKU, southwest Japan

Abstract A large-scale outcrop was exposed along the newly constructed road access to the Kumamoto Earthquake Museum “KIOKU” (former Tokai University Aso Campus). Multiple layers of tephra and paleosols cover the Sawatsuno lava (27 ± 6 K-Ar ka) in this outcrop. Three (3) characteristic tephra layers: Kusasenrigahama Pumice (Kpfa: ca. 32.5 cal ka BP) from Kusasenrigahama crater in Aso caldera, Aira Tn (AT: ca. 30 cal ka BP) and Kikai Akahoya (K-Ah: ca. 7.3 cal ka BP) from southern Kyushu are intercalated between thick sandy volcanic ash layers erupted from the post-caldera volcanoes of Aso caldera. Thirteen (13) radiocarbon ages were obtained from soil samples and charcoal fragments. Among these, a sample just below AT shows a younger age, indicating that the upper soil/tephra sequence including AT, was re-deposited on ground surface at the time immediately after ca. 13.4 cal ka BP indicated by this age. This suggests that the duplicated sequence, confirmed by our detailed dating is a product of either near-surface hidden faulting or a small local landslide associated with one of the paleoseismic events similar to the 2016 Kumamoto Earthquake.