Simulating a multi-phase tephra fall event: inversion modelling for the 1707 Hoei eruption of Mount Fuji, Japan

Abstract

Fuji Volcano last erupted in ad 1707 depositing approximately 40 mm of tephra in the area that is now central Tokyo. New high-resolution data describe 17 eruptive phases occurring over a period of 16 days (Miyaji et al., J Volcanol Geotherm Res 207(3–4):113–129, 2011). Inversion techniques were used in order to best replicate geological data and eyewitness accounts, and to estimate eruption source parameters. Inversion results based on data from individual eruptive phases suggest a total erupted mass of 2.09 × 10 12 kg. Comparatively, results based on a single data set describing the entire eruption sequence suggest a total mass of 1.69 × 10 12 kg. Values for total erupted mass determined by inversion were compared to those calculated using various curve fitting approaches. An exponential (two-segment) method, taking into account missing distal data, was found to be most compatible with inversion results, giving an erupted mass of 2.52 × 10 12 kg when combining individual phases and 1.59 × 10 12 kg when utilising a single data set describing the Hoei sequence. Similarly, a Weibull fitting method determined a total erupted mass of 1.54 × 10 12 kg for the single data set and compared favourably with inversion results when enough data were available. Partitioning extended eruption scenarios into multiple phases and including detailed geological data close to the eruption source, more accurately replicated the observed deposit by taking into account subtleties such as lobes deposited during transient increases in eruption rate and variations in wind velocity or direction throughout the eruption.