This study examined depictions of volcanic disasters in Japanese mass media utilizing Latent Dirichlet allocation (LDA) a method of text mining, to explore how scientific findings in the field of volcanology are portrayed in newspaper media. In all, 1841 articles published between 1990 and 2015 in Asahi Shimbun, Yomiuri Shimbun, and Mainichi that contained references to scientific stories on volcanology were collected and analyzed. The content of the newspaper coverage was classified into 18 topics using the statistical properties of word frequencies analyzed according to LDA, and the number of these words in articles belonging to each topic was calculated. A time-series analysis of articles shows that a very large number of volcanic reports are released immediately after a volcanic crisis occurs but this number rapidly decreases within a few months. The results of text mining show that newspaper media have detailed descriptions of only Mt. Fuji in articles on the topic of volcanic disaster mitigation, and volcanic mechanisms tend to be reported most frequently as a related event in the context of the seismic activity. This means that newspaper media may normally consider disaster prevention stories regarding volcanos other than Mt. Fuji as minor news. These findings suggest that civilians can systematically absorb little scientific knowledge of volcanos and may only acknowledge volcanic risk as a regional problem within a volcanic area, which results in a significant barrier to volcanic risk communication. To mitigate potential volcanic risk, mutual communication through which opportunities for better interaction between experts, journalists, and citizens would be possible should be promoted.

The Aso volcano produced four gigantic ignimbrites (Aso-1, Aso-2, Aso-3 and Aso-4) between 270 and 90 ka, after which it produced Aso central cones. Previously the volcanic products formed after the Aso-1 ignimbrites were defined as Aso volcanic products. However, Pre-Aso volcanoes had been formed until 400 ka around the Aso region. Pre-Aso volcanic rocks and Aso volcanic products have distinct SiO₂-K₂O chemical trends. In this study, we surveyed outcrops above and below the Aso-1 ignimbrites sequence in the eastern sidewall of the caldera at Sakanashi. Using volcanic stratigraphy and K-Ar dating, we identified a lava flow erupted just before the Aso-1 eruptions. This lava shows a chemical character similar to that of the Aso volcano. It is suggested that the activity of Aso volcano started before the emplacement of the Aso-1 ignimbrites. We also found that Aso-1B, consisting of scoriaceous (basaltic andesite) ignimbrite, overlies Aso-1A, which consists of welded ignimbrite, in the northwestern caldera wall at Matoishi. This succession indicates that the Aso-1 eruption cycle began with Plinian-type silicic eruptions (Aso-1P). This was followed by the major silicic ignimbrite Aso-1A and the minor mafic ignimbrite Aso-1B.