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+  +0 ῐGround Tilt Change Associated with the ,**. Eruption
at Asamayama Volcano, Japan
Masaaki CHUREI῍ and Hiroaki KATAYAMA῍῍
The eruptive activity of Asamayama Volcano started at the summit crater on September +, ,**.. The ﬁrst
eruption was explosive and accompanied with very strong air shock. Before the explosion, signiﬁcant tilt change
was detected by a tiltmeter installed at the northeastern ﬂank of the volcano by JMA. Tilting direction of thesignal was in the sense of ground-up toward the west side of the station. BH-type volcanic earthquakes drasticallyincreased at the same time as the start of the precursory tilt change. Including the ﬁrst one, four remarkable
explosive eruptions occurred. In all the cases, signiﬁcant tilt change and the BH-type volcanic earthquake swarms
were observed before the explosive eruptions. The starts of the precursory signals were about -./ῌ,3 hours before
the explosions and the amounts of tilt change were *.*-ῌ*.++ m radian. These features suggest that the predictionof explosion may be possible at Asamayama volcano.
We estimated the crustal deformation source using the tilt data associated with the explosive eruption
occurred on November +.. The location of the estimated dike intrusion source was about + km below sea-levelat the summit area. The volume of dike was ..0+*. m-.
Key words : Asamayama, explosion, tilt change, BH-type volcanic earthquake, precursory signal

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(")*+῎ 2 km ,-  ?1%IR&# ,**+

3 45

*q }~ (Pinnacle Technologies Series./012345*6789:;<=>-?1

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 ±3/*ῌ*3*2 ²³´P .ῌ.ῌ+² wµqrNiigata Local Meteorological Observatory, Saiwainishi.ῌ.ῌ+, Niigata-shi 3/*ῌ*3*2, Japan.
 ±+**ῌ2+,, ¶·¸¹`º» +ῌ-ῌ.
qrsw" ¼

Seismological and Volcanological Department, Japan
Meteorological Agency, Ohte-machi +ῌ-ῌ., Chiyoda-ku,
Tokyo +**ῌ2+,,, Japan.
Corresponding author : Masaaki Churei
e-mail : mchurei@met.kishou.go.jp

.Eº9)»!¼9

92

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Fig. +. Location map of observation points at
Asamayama. Solid circles indicate tilt observation
stations. A and F are operated by JMA, AVO
by ERI and TUMH and MYTH by NIED. Plus,solid sqare and solid triangle indicate the seismic
observation station B by JMA, Karuizawa
Weather Station (JMA) and the summit craterof Asamayama, respectively.

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93

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Table +.

Speciﬁcations of Pinnacle Technologies Series //** Tiltmeter at Asamayama.

Fig. ,. (a) Tilt changes at F, barometric pressure at KWS and hourly number of BH-type earthquakes at B before
and after the eruption on September +, ,**.. (b) Example of the tilt changes at F and barometric pressure at
KWS caused by the typhoon No. 0. (c) and (d) The trajectories of typhoons No.0 and No. +0, respectively.

r©ª«¬g­

94

Fig. -. Tilt changes at F and hourly number of BH-type earthquakes at B before and after the eruptions.
(a) September ,-, (b) September ,3.

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95

Fig. /. Tilt changes at F and hourly number of
explosion earthquakes at B during the period
from Sept. +- to Sept. +2. Small eruptions of
the Strombolian type occurred from ++ h /- m
Sept. +/ to ,+ h *- m Sept. +2, continuously.
Fig. .. Tilt changes at A and F, and hourly numberof BH-type earthquakes at B before and after
the eruption on November +., ,**..

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96

Fig. 0. Tilt changes accompanied with no explosive eruption and hourly number of BH-type earthquakes at B, (a)from October ., ,**. to 0, (b) from February ,*, ,**/ to ,-.

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97

Fig. 1. Comparison of the observation with the dyke model. Solid arrows show the vector diagrams of theobserved tilt change and gray arrows show the vector diagrams of the calculated tilt change using the dyke
model. Thick line, pluses (º) and open circles (») indicate the position of estimated dyke, hypocenters of
BH-type earthquake and explosion earthquakes from August + to December -+, ,**., respectively.

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Ishihara, K. (+33*) Pressure sources and induced grounddeformation associated with explosive eruptions at anandesitic volcano : Sakurajima volcano, Japan.In Magma
Transport and Storage (Ryan M. P. ed.), --/ῌ-/0. John
Willy & Sons, Chichester.
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+,.
Kamo, K. and Ishihara, K. (+323) A preliminary experiment on automated judgement of the stages of eruptive
activity using tiltmeter records at Sakurajima, Japan. In
Volcanic Hazards, Assessments and Monitoring (Latter, J.
H. ed.), /2/ῌ/32. IAVCEI Proceedings in Volcanology+, Springer-Verlag.
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Okada, Y. (+33,) Internal deformation due to shear andtensile faults in a half-space. Bull. Seism. Soc. Am., 2,,+*+2ῌ+*.*.
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Shimozuru, D. and Kagiyama, T. (+323) Some signiﬁcantfeatures of pre-eruption volcanic earthquakes. In Volcanic Hazards, Assessments and Monitoring (Latter, J. H.
ed.), /*.ῌ/+,. IAVCEI Proceedings in Volcanology +,Springer-Verlag.
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Tamura, Y., Sato, T., Ooe, M. and Ishiguro, M. (+33+) A
procedure for tidal analysis with a Bayesian information
criterion. Geophys. J. Int., +*., /*1ῌ/+0.
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