Reconstruction of the Holocene seismic history of a seabed fault using relative sea-level curves reconstructed by ostracode assemblages: Case study on the Median Tectonic Line in Iyo-nada Bay, western Japan

Abstract

Relative sea-level curves of cores K1, K2, S1, and S2 collected from Iyo-nada Bay, western Japan, were reconstructed using ostracode assemblages. These curves clearly show a Holocene history of seismic subsidence. Comparing the curves with sea-level histories from a tectonically stable area of Japan demonstrates that Iyo-nada Bay has undergone subsidence through the Holocene. This subsidence was probably caused by movements of the Median Tectonic Line (MTL) fault system in Iyo-nada Bay. Comparison of relative sea-level curves of the lower and upper plates elucidated a detailed seismic history of the Kaminada-oki-kita and Shimonada-oki-minami Faults. The Kaminada-oki-kita Fault moved vertically about 7 m at ca. 5500–0 cal yr BP, in addition this fault may have moved vertically about 5 m, and 9 m at ca. 7500–6000 cal yr BP and ca. 10,000–9000 cal yr BP, respectively. The Shimonada-oki-minami Fault has moved vertically about 15 m over the last 6500 years. This fault also might have had a large movement around 10,000 cal yr BP. This study provided a robust method for reconstruction of the Holocene seismic history using ostracode assemblages found in seabed drilling cores.