Tephra from large eruption in Kyushu area observed in UT-iwk core from Uwa basin, western Shikoku

Abstract

Large eruptions from caldera volcanoes in the Kyushu region during the Quaternary periods caused large amount of tephra over a wide area of the Japanese archipelago. Because the Shikoku region is close to the Kyushu region, many tephras were deposited relatively thickly, and it is important to identify these tephras and measure tephra thickness for disaster prevention. The UT core (Tsuji et al. 2018), collected in the Uwa Basin, western Ehime Prefecture, Japan, preserves tephra over the past 600,000 years, and Uwa basin is a good field for comparison of tephra thickness and magma storage conditions. Using UT-iwk core collected from 500 m north of the UT core site, we attempted to determine the precise layer thickness of tephra derived from the large eruption in Shikoku region, identify cryptotephra, and understand the magma storage conditions based on the glass and mineral chemical compositions for the visible tephra. In the UT-iwk core, visible widespread tephras are K-Ah, AT, Aso-4, K-Tz, Aso-ABCD, Ata, Aso-2, Ata-Th, Aso-1, Kkt, Oda resemble. For example, net thicknesses of Aso-1 and Kkt are about 15 cm and 20 cm, respectively, based on grain-size and component analyses. AT is 65 cm thick including the reworked layer. The net thickness of Aso-4 was estimated to be 20 to 30 cm (Nakamura et al. 2020). These results are expected to be useful for the estimation of the volume of eruptive products and the probability evaluation of the impact of a large eruption in western Shikoku.