著者

小山 順二

出版者

北海道大学大学院理学研究院

雑誌

北海道大学地球物理学研究報告 (ISSN:04393503)

巻号頁・発行日

vol.78, pp.53-68, 2015-03-19

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After the 2011 Tohoku-oki megathrust earthquake of Mw 9.0, one of the great concern is whether or not the megathrust earthquake induces large volcanic eruptions and disastrous aftershocks in Japan. There exist 110 active volcanoes in Japan and large eruptions including historical events of each volcano have been compiled in terms of Volcanic Explosivity Index (VEI; Newhall and Self, 1982) from 0 (Non-explosive) to 8 (Mega-colossal). According to “Volcanoes of the World” by Siebert et al. (2010), we could find 50 eruptions in Japan since 600 A.D. classified as VEI 4 (Large) and 5 ~ (Very Large). Speaking about great earthquakes whose earthquake (moment) magnitude M (Mw) is equal to or larger than 8.0, there occurred 34 times since 684 A.D. in Japan. I have compared the time of occurrences of Large volcanic eruptions and large earthquakes (M>7.5) at first, taking examples of Mt. Fuji, Tokachidake, Hokkaido Komagatake and Sakurajima volcanoes. Since I could not find any systematic correlation between them, I decided to compare all the Large (and Very Large) volcanic eruptions and the great earthquakes (M>8.0) in Japan. In order to develop the quantitative discussion, we need to introduce cross-correlogram analysis (Perkel et al., 1967; Grun, 2009) for these two discrete point-processes. All that I could find was very poor correlation between great earthquakes and Large volcanic eruptions in Japan. It is true that many papers have been showing us the correlation between large earthquakes and volcanic eruptions. However, speaking about Large eruptions (not speaking about volcanic disasters), it is not true for the correlation between them. The reason is that some small-scale volcanic activity out of many volcanoes has been activated by great earthquakes almost all the time in the world. Even just after the 2011 Tohoku-oki megathrust, it has been reported that the volcanic activity of about 20 volcanoes in Japan has been enhanced. It is also true that some volcanoes are activated in the present, about 4 years after the megathrust. The evidences should be added, which are the eruptions of Mt. Fuji in 1707 (VEI 5), 49 days after the 1707 Hoei earthquake of M 8.6 and of Bezymianny in Kamchatka in 1956 (VEI 5), 4 years after the 1952 Kamchatka earthquake of Mw 9.2. Both of the volcanoes have spent about 900 ~ 1200 years of quiescence in the volcanic activity. All these would suggest that great earthquakes might induce Large volcanic eruptions nearby, however it is totally dependent on its own potential and preparedness to erupt of each volcano and the earthquakes just play a role of a trigger.