1888年磐梯山水蒸気爆発に関するノート

—(4）諸大家の誤謬に関する考察—

抄録

An abnormally vast phreatic eruption occurred at Bandai volcano in northeastern Japan on July 15, 1888, a time when Japan was on the cusp of Western Learning and knowledge was beginning to be influenced by the outside world. Sekiya and Kikuchi (1890) summarized the evidence and observations of the eruptive activity and published details of the scenario for the eruption. However, immediately after the publication, the veracity of their inference and scenario, which were mainly guided by the senior author Prof. Sekiya, was questioned not only by Mr. Kobayashi (1892), who was an eyewitness to the explosions among those who inhabited the nearby foot of the volcano, but also by some researchers who surveyed the same eruption site in more detail. Although more than 120 years have elapsed since the eruption, neither of the researchers had cleared the suspicions around their inference nor has provided any critical review of their scenario. In contrast, many later researchers agreed with the inferences and scenario of Sekiya and Kikuchi regarding the mechanism of the 1888 phreatic explosion.

In order to settle this controversy, we tried to re-evaluate several eyewitness reports and sketches, which were constructed from direct observations by the local habitants. Next, we examined carefully whether or not these earlier inference and scenario were grounded in reliable fact. We reached the following conclusions:

(1) Kobayashi (1892) pointed out that the comprehensive scenario provided by Sekiya and Kikuchi (1892) was not at all in agreement with what the local habitants had observed at the scene of the explosion. Kobayashi revealed that the outbursts from the first 15-20 phreatic explosions of columnar steam and dust occurred not at Kobandai-san, but at the old crater now known as Numanotaira, 2 km away from Kobandai-san. (2) During the deductive process, the principal investigator Prof. Sekiya did not adopt Kobayashi’s key eyewitness data. Instead, he incorporated Turumaki’s fallacious eyewitness account that all explosions erupted from the slope of Kobandai-san. In logical conclusion, he decided fallaciously that a series of eruptions occurred at one particular place, Kobandai-san, which disappeared from view following the last eruption. (3) Prof. Sekiya’s implicit faith had been influenced by Humboldt’s hypothesis (1849) about the origin of the volcano in the 19th century. This background led him in making the fallacious inference about the phreatic explosion in 1888. (4) We have demonstrated in this paper that the origin of fallacious inferences and the scenario in Sekiya and Kikuchi (1890) derived from the notion entrenched in the acknowledged authority of Humboldt for that day and age.