Distribution and Eruptive Volume of Aso-4 Pyroclastic Density Current and Tephra Fall Deposits, Japan: A M8 Super-Eruption

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) and tephra fall deposits derived from the last 87–89 ka caldera-forming eruption (named Aso-4) 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 a M8.1–8.4 (VEI8) super-eruption. The Aso-4 results to be the largest eruption in Japan and the 2nd largest eruption in the world (after the 74 ka Toba eruption) in the last 100 ka.