  .3  (,**.)
 /  ,33ῌ-*.   +320      , ,**-  ,  ,2  ,**.  2  + Flashing Arc on Izu-Oshima +320 Eruption
Akihiko YOKOO, Mie ICHIHARA, and Hiromitsu TANIGUCHI
Izu-Oshima volcano, one of the most active volcanoes in Japan, started a series of eruptions on Nov. +/, +320.
One of the characters of the eruptions is the small-scale explosions with ﬂashing arcs, which were the visualized
shock waves as a phase change of H, O in the air, occurred at the summit lava-lake. The ﬂashing arcs, seen on
the movie, show several characteristics ; +) they spread and propagate semi-spherically, and ,) the launching ofsemi-spherical ballistics follows the ﬂashing arcs. Analysis of the movie indicates that the velocities of ﬂashingarcs were -**ῌ..* m/s and their sources were located almost on the surface of the lava-lake. We paid attention
to the ballistics in the light of scaling laws, which were established from ﬁeld explosion experiments, so that wewere able to estimate explosion energy as about 2+*3 J. Additionally, assuming that the ﬂashing arcs wereproduced by an explosion, such as a bursting of pressurized-bubbles, we could get information of their condition,a relationship between inner-pressure and the size.
Key words : Izu-Oshima +320 eruption, ﬂashing arc, bubble bursting, explosion energy+  m0!fHo<78 =4c 7j "  !"#$% &'({d$$m ! nfI,!n)* +,-./0!' 1 -2345!fo6D 4- L +320 z!6/  789 km,6 :;<4"#=>'?@,!AB'C0 (Johnson, ,**-D =>EF'<U!n$m?0 1$|
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 Etna  f Perret, +3+, +3/1ῌ/2  Capelinhos% fH!D 8-$_H=F78 H>

 (Machado et al., +30, ; Richards et al., +30- +3/.

 ¢32*ῌ2/12 £¤¥¦§¨¥
©$LªLª«ª¬­®Eª¯°
Institute of Mineralogy, Petrology and EconomicGeology, Graduate School of Science, Tohoku University, Aramaki-Aza-Aoba, Aoba-ku, Sendai 32*ῌ
2/12, Japan.
 ¢32*ῌ2/10 £¤¥¦±²©$Lª©$³´³¬­µ¶ihCenter for Northeast Asian Studies, Tohoku Univer-sity, Kawauchi, Aoba-ku, Sendai 32*ῌ2/10, Japan.
 n·¸ : ¢++-ῌ**-, ©¹º%¹¦& +ῌ+ῌ+©¹LªE»¬­·¼"¬­µ¶ih
Present address : Earthquake Research Institute, University of Tokyo, +ῌ+ῌ+, Yayoi, Bunkyo-ku, Tokyo++-ῌ**-,, Japan.
Corresponding Author : Akihiko Yokoo
E-mail : yokoo@cneas.tohoku.ac.jp

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(Fig. +-A 0¡b +, ¢6) +320 ++ £ +/ 12¤¥w¦§#9 ¦§¨03 4©ª¥w«#¥w 5w6P)<B7¬ + km D4©­wUV#9 ++£ +3 1®

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b0*+, "wBµ %¶'()<B·´ +321 ; ¸= +322 ;  +321+3229  (+322 <( *+, "¤0Fig. +. A : Map showing the location of the TV
camera, Observatory (O), and Miharayama (M)at Izu-Oshima, with ,/ m contour. The
original map was made in August +33, by GSI.
The gray zone indicates the range of the screenwidth. B : The topography around Miharayama(the bottom square area in Fig. +-A) on
November ,2, +320, is shown with / m contours.
C : A schematic cross section of the planeincluding the TV camera, Observatory (O), andMiharayama (M). The solid and broken lines
show the topography before +320 eruption(September, +32+) and during the eruption(November, +320), respectively.

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Fig. ,. Snapshots of a ﬂashing arc (FA1 : A-H,white arrows) and ballistics (B1 : I, broken line)on November ,+, +320, captured from a NHK
special TV program. Each time interval
between the sequential images (A-H) is +/-*
sec. M and O denote same meaning in Fig. +.

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Fig. -. The source regions of the ﬂashing arcs(circles) and ballistic fragments (cubes)
determined by analysis of the movie (errors are
smaller than the size of symbols). They are
projected onto the plane passing through thecrater center in Fig. +-B. The part with theoblique lines shows the surface of the lava-lake.
M and O denote same meaning in Fig. +.

J RJ! Ohba et al. (,**,E ³¬´F#! :;bcd{°z¥±` *.*0 m ῌ J+ῌ- :;#<Rj«µbc` (+,* mE R!

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 $NBΐῒS{|}) ~  TU#S   5 4ῐ ῕ ῔ ῑghij (+322) V4#$ +320 DQ /SkE#lmno ,- .3 00.
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x ¡Rv¢£&y5 (+322) ¤¥¦§¦¨¦©ª+ «¬ +320­+321 DV4#$
Q z{| QR -- [® S-, S/+.
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v^ (+321) V4#$ +320 DQz z
 i·&; 3 -00 -1+.
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K (+321) QR.¼ } 0+ D +*­+, i ,0 . 2/p.
K (+322) QR.¼ } 0, D +­- i ,1 + ++3p.
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304!"#LMNO

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waves within the ash-laden plume on Aso +323 eruption.
$%&'()*+), ,**. -./, V*//ῌP*-*.
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:;<=> (,**,) 
?@ABCD ,*** E   .1 ,.-ῌ,/-.
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