An 18,000-year record of recurrent folding inferred from sediment slices and cores across a blind segment of the Biwako-seigan fault zone, central Japan

摘要

Closely spaced sediment slices and cores reveal evidence for recurrent folding in the past 18 ka across the tip of a blind thrust fault near Kyoto, central Japan. The slices, 0.4 m wide and up to 5.5 m long, collected with “geoslicer” technique, exposed the central 100 m of a stratigraphic cross section that extends across a fold scarp in the southern Biwako-seigan fault zone. A chronology inferred with 109 radiocarbon ages suggests that the most recent blind thrust earthquake occurred in the range A.D. 1060–1260, most likely around A.D. 1170. Historical accounts strongly support that this earthquake correlates with the 1185 M 7.4 Genryaku Kyoto earthquake. The earthquake produced a broad surficial fold more than 200 m wide with a vertical displacement of 3 m or more. Earlier paleoearthquakes lack well-defined event horizons, but we find that the deformation of the 18-ka unit is likely 4 times that from the most recent earthquake. Our results show the application of geoslicer technique to blind thrust paleoseismology, for which traditional trenching is rarely applicable or useful because of broad surficial deformation and commonly shallow groundwater at potential paleoseismic sites.