IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1W_1E-P7

Date/Time: July 21 Poster

A physical model for the phreatic explosion on July 15, 1888 at Bandai volcano,northeastern Japan
Hamaguchi Hiroyuki1 , Ueki Sadato2 , Nakamichi Haruhisa3
1Tohoku Univ., Japan, 2 Graduate School of Sciences, Tohoku Univ., Japan, 3 Graduate School ofEnvironmental Studies, Nagoya Univ., Japan
E-mail: hamaguti@cello.ocn.ne.jp

The 1888 eruption at Bandai volcano, Japan, has been well known as one of the most gigantic historical phreaticexplosions. The precise nature of the explosion mechanism, however, has not been quantitatively clarified until
now. It is due to the statements and interpretations in the leading article of Sekiya and Kikuchi (1890) were mainly
based on the various circumstantial evidences that were collected by seeing of authors’ selves and local habitants.
They inferred that the hydrothermal fluid triggering the explosion and rock avalanche might have been locatedbeneath the exploded peak called Kobandai-san and that the depths of the earthquakes immediately preceding
the eruption might be very shallow in the volcano. These naive inferences have been left without any quantitative
examination for more than one century. We re-examined their ambiguous inferences about the explosion source
and the earthquakes preceding the eruption. We analyzed primarily the following two observational data: The
one is the shallow bulge structure beneath the old crater that is recently deduced from the seismic tomography
and the other is the isoseismal map drawn from questionnaire by Sekiya and Kikuchi. The map showed that theperceptible area of the biggest preceding earthquake had a shape of an ellipsoid elongated in the direction of N25E
and its intensity at the boundary was assigned to 1 on the JMA scale which was equivalent to ca. 2 gal. The
characteristics of the observed isoseismal were compared with those calculated as a function of source depth anddirectivity under several reasonable assumptions. Referring to the observed 3D bulge structure beneath the oldcrater, an explosion source was modeled by a pressurized spherical cavity with a radius of 0.5 km located at adepth of 1.0 km. The numerical analysis revealed that the fluid pressure would escape from the cavity through two
tensile fractures oriented in different directions; one is oriented upward from the apex of the cavity and the other is
at an oblique angle of 30 degrees with the ground surface at a horizontal distance of about 1.8 km from the apex.
We concluded that the explosion source with a radius of 0.5 km was located beneath Numanotaira (the old crater)
at a shallow depth of 1.0 km but not beneath Kobandai-san (the collapsed mountain) and that two tensile fracturesin directions of N20W and N100E were simultaneously opened due to an excess pressure, resulting the dynamiccollapse with large rock avalanche into the north and the steam-jet burst with mud flow into the south-east. The
seismic moment and magnitude of the biggest earthquake immediately preceding the explosion were estimated tobe 10**16 Nm and M4.6, respectively. These results provided a counter-evidence for the traditional inference bySekiya and Kikuchi.

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