Abstract
Estimations of the distribution and eruptive volume of large-scale pyroclastic density current (PDCs) and tephra fall deposits are essential for evaluation of the affected area, long-term volcanic hazards assessments, volcanic activity, and geophysical and petrological quantitative analysis at caldera volcanoes. For this study, the original distributions and eruptive volumes of large-scale PDC (up to 166 km runout distance PDC) and tephra fall deposits derived from the 87-89 ka caldera-forming eruption of Aso volcano in Japan were reevaluated. The original distributions and volumes of PDC deposits just after the eruption were estimated using 3,600 data from geological maps, published research papers, and borehole thickness. The original distributions and volumes of tephra falls were estimated from new isopach maps based on thickness and distribution data of submarine, lacustrine, and subaerial tephra fall deposits. The estimated original volume of the Aso-4 PDC deposits is 340–935 km3 (5.6–14.8×1014 kg). The estimated original volume of the Aso-4 tephra fall deposit is 590–920 km3 (6.0–9.3×1014 kg). The total eruptive volume of the Aso-4 eruption was 930–1,860 km3 (1.2–2.4×1015 kg). This estimation result is about 1.5 to 3 times larger than the previous estimation (>600 km3). Thus, the Aso-4 eruption is now defined as an M8.1–8.4 (VEI8) super-eruption.
Highlights
Accurate determinations of the eruptive volumes of large-scale caldera-forming eruptions provide an essential parameter for assessing long-term eruption records, eruption activity evaluations, and quantitative geophysical and petrological investigations
The distribution of the current exposure of Aso-4 pyroclastic density current (PDC) deposits (Figures 1, 4) was compiled based on geological maps, such as 1:50,000 and 1:200,000 Geological Maps published by Geological Survey of Japan (GSJ), 1:50,000 Surface Geological Maps published by Ministry of Land, Infrastructure and Tourism (MLIT), and maps in published papers
The Aso-4 PDC distributions are not shown in areas that have been covered subsequently by thick tephra fall deposits and the younger Kuju volcanic area (30 km NE of Aso caldera)
Summary
Accurate determinations of the eruptive volumes of large-scale caldera-forming eruptions provide an essential parameter for assessing long-term eruption records, eruption activity evaluations, and quantitative geophysical and petrological investigations. Several studies to estimate the eruptive volume of tephra falls derived from large-scale volcanic eruptions were made (e.g., Costa et al, 2014; Kandlbauer and Sparks, 2014). A large number of studies have been published to estimate the eruptive volume of tephra fall deposits We estimated the original distributions and eruptive volume of the large-scale Aso-4 PDC deposit, based on more than 3,600 boreholes, topography, and outcrop data with a 7 km × 5.5 km mesh interval. The eruptive volume of the co-ignimbrite ash fall deposit derived from the Aso-4 eruption was estimated using new isopach maps based on thickness data from about 70 locations, including submarine and lacustrine sample data
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