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[en] The 2014–2015 eruption of the Nakadake first crater at Aso Volcano in southwestern Japan was characterized by continuous ash emissions and intermittent strombolian eruptions. In this paper, we present the distribution, discharged mass, and components of tephra-fall deposits to examine the sequence of activities. We installed 21 ash samplers around the crater (SW crater rim to 9 km in all directions) and calculated the mass of ash-fall deposits based on 28 isomass maps. From November 25, 2014, to the end of January 2015, the cumulative erupted mass increased at a high discharge rate (2.2 × 104 tons/day). After February 2015, the cumulative erupted mass decreased to a low rate of 0.6 × 104 tons/day, although this rate rose slightly in March and late April 2015. The 2014–2015 tephra-fall deposits consisted of glass shards, crystal, and lithic grains. In the November 25–27, 2014 ash-fall deposits, lithic fragments, which are interpreted to be derived from lavas or pyroclasts of previous eruptions, were dominant (59–68%). Thereafter, the proportion of glass shards, which are probably juvenile materials of newly ascending magma, gradually increased with time, and the December 21–23, 2014 ash contained abundant glass grains (63%). The proportions of glass shards ranged from 29 to 50% until February 25, 2015. Subsequently, they decreased with time and reached 14% on March 17. Afterward, the proportions increased again prior to April 27 and ranged between 20 and 30% in May 2015. The total erupted tephra mass from the November 2014–May 2015 activity of Nakadake first crater was 2.1 × 106 tons (1.2 × 104 tons/day), which was less than the tephra deposits of previous activities that have occurred within the past few decades. .
[en] Two volcaniclastic deposits - a debris-avalanche deposit(NgDA) and a lahar deposit (NgL) were discovered along the Nigorikawa River on the western slope of Aso central cones, southwestern Japan. The Nigorikawa debris-avalanche deposit, having a maximum thickness > 3 m, contains numerous plastically deformed debris-avalanche blocks (< 3.7 m) of volcanic ash and soil layers in a poorly-sorted silty to clay matrix. The Nigorikawa lahar deposit contains many subangular to subrounded lithic clasts (< 0.7 m) and shows clast-supported and matrix-rich depositional structures. We obtained a 14C age of 2.230±70 years BP from a wood fragment in NgDa, which corresponds to 400-100 cal BC (2σ). The date is consistent with the age of cultural remains (the Yayoi period: 300 BC-300 AD) underlying the debris-avalanche deposit. We obtained a 14C age of 4,100±60 years BP (2880-2480 cal BC) from a wood fragment in NgL. These 14C ages indicate that major volcaniclastic flows have inundated the Nigorikawa River multiple items in the past 4,000 years. Although the source and cause of the volcaniclastic flows remain unsolved, this discovery provides important information about volcanic hazards in the western part of Aso central cones. (author)
[en] Holocene environmental changes and vegetation history are constructed using phytolith and macroscopic-charcoal analyses of a 23-m-deep drilling core obtained at the Senchomuta marsh in Asodani Valley, northern part of Aso caldera, SW Japan. An intra-caldera lake existed in the Asodani Valley prior to approximately 9 cal ka (calibrated 14C age). Multiple large flood events occurred during the period 8.9-8.1 cal ka and emplaced thick sandy deposits in the valley basin. Thereafter, the center of the Asodani Valley (northern part of caldera floor) changed to swampy and fluvial environments. Sasa (cool-temperature dwarf bamboo) grasslands and/or forests with understory Sasa covered slopes of the Asodani Valley basin between 11 and 9 cal ka. Sasa phytoliths significantly increased at ca. 7.3-6.5 cal ka, but thereafter decreased. Miscanthus (Japanese pampas grass) grasslands existed continuously on the slopes. Macroscopic-charcoal particles were abundant during the last 6000 years, and the peak (6.1 cal ka) amount of charcoal particles is consistent with that of Miscanthus phytoliths. This indicates that the existence of Miscanthus grassland might be related to fire events. Inside the Asodani Valley, Phragmites (reed) became established continuously along the shore of the intra-caldera lake (prior to ca. 9 cal ka) and in subsequent marshes. Gramineae phytoliths were detected predominately through all horizons of the drilling core, whereas a small amount of arboreal phytolith was observed at most horizons. We, therefore, believe that forests existed on steep slopes such as the caldera wall where human impacts were small, although Sasa and Miscanthus grasslands were maintained by human activity outside Aso caldera. (author)