Noboru Chida

AMS 14C chronological study of holocene activities in active faults in Japan

Abstract A submarine active fault, the Toyooka-oki fault, located on the northwest end of Beppu Bay, northeast Kyushu, was investigated by observing the fault structure preserved in the submarine sediments using a seismic profiler, and by collecting 15–20 m long sediment cores using a piston corer, on both sides of the fault. A distinct accumulation of the vertical displacements induced by seismic events is recognized by correlating specific layers using the seismic profile data between the two cored sediments. Vertical displacements of offset horizons on both sides of the fault are larger for the deeper sediment layers as compared with the upper layers. AMS 14 C dating was performed on a total of 32 calcareous samples collected from both cores and two seismic events are clarified using the age-depth profiles of sediment samples on both sides of the fault. We estimate vertical displacement and timing of the events of ca. 2.8 m around 1900 Cal yr BP and ca. 2.0 m around 4800 Cal yr BP, for the Toyooka-Oki fault. AMS 14 C dating was applied to identify the previous activities of the Nojima fault, located on the northwest coast of Awajishima, Hyogo Prefecture, which was active during the 1995 Hyogoken Nanbu Earthquake. A trench survey conducted just after the earthquake confirmed that the Nojima fault has repeated displacements over in the past 2000 years. A definite one occurred about 2000 years ago and a possible one about 400 years ago, which may have been caused by the 1596 Keicho earthquake of estimated magnitude 7.5.