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Yamada, Ryuji*; Kokubu, Yoko; Wakatsuki, Tsuyoshi*; Yasue, Kenichi
no journal, ,
Mass movements and fault movements are natural phenomena to change the local topography. Reconstruction of the movement histories and evaluation of long-term stability of topographies are crucial to assess the geological environment in future. History reconstruction based on radiocabon dating of plant material recovered from sediment deposited by the past movements requires the compilation and the comparison of existing data measured by different analysts with different methods. We compared the results of radiocarbon ages measured at different laboratories with various chemical pretreatments, using plant material samples. For older samples (about 45000 BP), dispersion in ages is greater than measurement uncertainty. However the order of weighted mean ages for each sample agrees well with stratigraphic sequence at the sampling site. It is therefore thought that the influence of the systematic error due to a specific experimental conditions is small.
Wakatsuki, Tsuyoshi*; Yamada, Ryuji*; Kokubu, Yoko
no journal, ,
Many soil slips and debris flows on the welded-tuff slopes were induced by heavy rain on July 28, 2013 in Tsuwano, Shimane. The debris flows eroded sidewalls or streambeds in river channels and the sediments driven by earlier slope movements were exposed in places. In six outcrops, we collected 53 dead wood chip samples presumably buried at the sediment deposition, and radiocarbon dating was performed. At around 418 m asl in Nayoshi district, Tsuwano, the outcrops of the black-clayey sediments are found, which are supposed to have deposited in a past dammed lake. They are distributed at upstream of a topographically-estimated deposit zone for an ancient large-scale slope failure, and the calendar-calibrated ages of these sediments ranges from 41 to 55 ka BP. It is therefore conjectured that the slope failure had occurred before 55 ka BP and the dammed lake had burst after 41 ka BP. An average deposition rate of about 0.3 m/1000 y was estimated at an outcrop of the dammed lake sediments