The Quaternary Research (Daiyonki-Kenkyu)

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Strong ground shaking and tsunamis caused by a large earthquake may trigger sediment resuspension and subaqueous slope failure, and may generate turbidity currents. Resuspension of deep-sea fine-grained sediments creates turbidity currents containing fine particles, and forms fine-grained turbidite. Therefore, the fine-grained (muddy) turbidite formed from such a turbidity current is a candidate for geological evidence of past earthquakes and tsunamis. A terminal basin, which is an isolated bathymetric depression, is a terminus of the turbidity current, and a preferable area for the deposition of fine-grained turbidites. Thus, deposits in a terminal basin may be a good recorder of past earthquake and tsunami events. For better understanding and interpretation of the recurrence and mode of past earthquakes and tsunamis, multidisciplinary Quaternary research on sedimentology, marine geology, tephra study, age determination and event detection for core correlation and event chronology are very important.

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The Shoubu volcanic ash is intercalated with the middle Pleistocene Iyoki Formation, western Shikoku, Japan. The ash has been petrographically correlated with several widespread tephras, but detailed geochemical correlations have yet to be made. We therefore examine the lithology, petrography, glass geochemistry, and fission-track (FT) ages of the Shoubu ash in an attempt to establish robust, detailed correlations. The petrography and chemical composition of glass show vertical trends in the Shoubu ash. The FT age of the lowest unit indicates that eruptions began at 0.53±0.12Ma. Although the Shoubu ash has previously been interpreted as a single, homogeneous volcanic ash, the results indicate that it either comprises several volcanic ashes or records a compositional change within a single eruptive sequence.

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