AMS C-14 CHRONOLOGICAL STUDY OF HOLOCENE ACTIVITIES IN ACTIVE FAULTS IN JAPAN

A submarine active fault, the Toyooka-oki fault, located on the northw est end of Beppu Bay, northeast Kyushu, was investigated by observing the fault structure preserved in the submarine sediments using a seism ic profiler, and by collecting 15-20 m long sediment cores using a pis ton corer, on both sides of the fault. A distinct accumulation of the vertical displacements induced by seismic events is recognized by corr elating specific layers using the seismic profile data between the two cored sediments. Vertical displacements of offset horizons on both si des of the fault are larger for the deeper sediment layers as compared with the upper layers. AMS C-14 dating was performed on a total of 32 calcareous samples collected from both cores and two seismic events a re clarified using the age-depth profiles of sediment samples on both sides of the fault. We estimate vertical displacement and timing of th e 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 C-14 dating was applied to i dentify the previous activities of the Nojima fault, located on the no rthwest coast of Awajishima, Hyogo Prefecture, which was active during the 1995 Hyogoken Nanbu Earthquake. A trench survey conducted just af ter the earthquake confirmed that the Nojima fault has repeated displa cements over in the past 2000 years. A definite one occurred about 200 0 years ago and a possible one about 400 years ago, which may have bee n caused by the 1596 Keicho earthquake of estimated magnitude 7.5.