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Reconsideration of the +3** Explosive Eruption at Adatara Volcano, Northeastern Japan
Akihiko FUJINAWAῌ, Takeshi KAMOSHIDAῌῌ, Atsushi TANASEῌῌ, Kazuki TANIMOTOῌῌ,
Youichi NAKAMURAῌῌῌ and Kazuo KONTANIῌῌῌῌOf the historic eruptions at Adatara volcano in northeastern Japan, the +3** eruption caused the worst
volcanic hazard taking a toll of over 2* human lives. Although the eruption was thought to be phreatic, detailsof the volcanic events have had remained uncertain. In order to reveal the course of the +3** eruption, historic
documents, particularly the reports of newspapers published by three local newspaper companies, were reexamined
and mutually crosschecked. By correlating the revealed eruption sequence to the facies of the +3** eruption
deposits at outcrops, characteristics of the fatal blasts that accompanied the eruption were examined.
The sequence of the eruption is revealed as follows. (+) The ﬁrst explosion occurred at +0 : ** of +1th July.
Because the explosion was very small, all the workers stayed and kept working at the sulfuric mine located in thevery bottom of the Numanotaira crater. (,) Immediately after the second explosion occurred at about +2 : **,one boy ran away and over .* workers commenced evacuation toward west. (-) When the workers reached thewestern rim of the crater, they witnessed the third explosion which violently damaged the sulfur reﬁnery andlodging houses in the crater. A small pyroclastic surge accompanied the third explosion which occurred near theeastern rim of the crater, blowing away the pieces of pottery, wooden sandals and clothes, together with thehousing materials. (.) Around +2 : -*, the largest explosion entirely destroyed the sulfur reﬁnery and lodging
houses. The fourth explosion caused strong pyroclastic surge and broadcasted dense ash in the crater area, andwas also accompanied with the wet surge which ran down through the Iou river. The strong surge hit theevacuating workers near the western rim of the crater. Among the workers, ,, people were killed and the others
were seriously scalded. Probably, most of the fallout ash erupted during the fourth eruption.
Characteristics of the surge are summarized as follows. (+) total volume of the surge was ,.3&+*/ m-,including 0*** m-, ,.. km long and .ῌ+* cm thick deposit along the Iou river. (,) The discharged energy to form
the new crater was +*+. J, roughly +*',ῌ+*'- of the energy discharged at the Bandai +222 or St. Helens +32*eruptions. (-) The velocity of the surge near the old Numajiri spa (+./ km west of the crater) is estimated to be
2*ῌ+-* m/s. (.) The temperature of the surge near the crater is estimated to be over +**( but lower than .**(.
(/) At its margin, the surge swept straight up the side slope of the valley at the corner of the Iou river, anddelivered thin surge deposits at the top ﬂat area.
Key words : explosion, blast, surge, phreatic eruption)-+*ῌ2/+, *+,-. ,ῌ+ῌ+/012$2345678929
Department of Environmental Sciences, Faculty ofScience, Ibaraki University, ,ῌ+ῌ+, Bunkyo, Mito,-+*ῌ2/+,, Japan)++*ῌ**+. :.;<:=>?@ ,ῌ+2ῌ. ABCDE
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Sumiko Consultants Co. Ltd., ,ῌ+2ῌ., Kita-ueno,Taito-ku, Tokyo ++*ῌ**+., Japan)-,+ῌ2/*/ J;K,LM -/*J;K12NO2342NP
Faculty of Education, Utsunomiya University, -/*Mine-cho, Utsunomiya, -,+ῌ2/*/, Japan)32*ῌ2/12 Q<,RS=TURS:>1212V$2WX9
Graduate School of Science, Tohoku University, Aoba,Aramaki, Aoba Ward, Sendai 32*ῌ2/12, Japan
Corresponding author : Akihiko Fujinawa
e-mail : fujinawa@mx.ibaraki.ac.jp