Estimation of ballistic block landing energy during 2014 Mount Ontake eruption

Kae Tsunematsu,Yasuhiro Ishimine,Mitsuhiro Yoshimoto,Toshitsugu Fujii,Takayuki Kaneko,Koshun Yamaoka

doi:10.1186/s40623-016-0463-8

Abstract

The 2014 Mount Ontake eruption started just before noon on September 27, 2014. It killed 58 people, and five are still missing (as of January 1, 2016). The casualties were mainly caused by the impact of ballistic blocks around the summit area. It is necessary to know the magnitude of the block velocity and energy to construct a hazard map of ballistic projectiles and design effective shelters and mountain huts. The ejection velocities of the ballistic projectiles were estimated by comparing the observed distribution of the ballistic impact craters on the ground with simulated distributions of landing positions under various sets of conditions. A three-dimensional numerical multiparticle ballistic model adapted to account for topographic effect was used to estimate the ejection angles. From these simulations, we have obtained an ejection angle of γ = 20° from vertical to horizontal and α = 20° from north to east. With these ejection angle conditions, the ejection speed was estimated to be between 145 and 185 m/s for a previously obtained range of drag coefficients of 0.62–1.01. The order of magnitude of the mean landing energy obtained using our numerical simulation was 104 J.

Highlights

Ballistic projectiles are ejected during explosive eruptions, follow a parabolic trajectory in the air that is minimally affected by wind, and land on the ground (Wilson 1972)
Dt where m is the mass of a ballistic block, A is the crosssectional area of the block perpendicular to the flow direction, CD is the drag coefficient, ρa is the air density, g is the acceleration due to gravity, v is the velocity of the block, and u is the velocity of the ambient gas flow
Simulations were performed with the direction and rotation angles that best matched the observed particle dispersion to assess the effect of the drag coefficient and ejection speed on the deposition of ballistic blocks

Summary

Introduction

Ballistic projectiles are ejected during explosive eruptions, follow a parabolic trajectory in the air that is minimally affected by wind, and land on the ground (Wilson 1972). These blocks can cause significant damage, such as penetrating roofs (Blong 1981, 1984; Ui et al 2002), demolishing mountain huts, injuring humans (Blong 1984; Baxter and Gresham 1997), and causing fires if they are still hot when they land (Pistolesi et al 2011). The high number of Tsunematsu et al Earth, Planets and Space (2016) 68:88

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