Ground Tilt Change Associated with the,**. Eruption at Asamayama Volcano, Japan

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1 /+ (,**0) *+,**.,**/ / +0,** 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 the signal was in the sense of ground-up toward the west side of the station. BH-type volcanic earthquakes drastically increased 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 -./,3 hours before the explosions and the amounts of tilt change were *.*- *.++ m radian. These features suggest that the prediction of 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-level at the summit area. The volume of dike was..0 +*. m -. Key words : Asamayama, explosion, tilt change, BH-type volcanic earthquake, precursory signal +,**. 3 +,* *,,+ 2 km, km,*/ Pa,- -,./ km F ; Fig /. 3/* *3*2.. + Niigata Local Meteorological Observatory, Saiwainishi.. +, Niigata-shi 3/* *3*2, Japan. +** 2+,, + -., m,**+ 3 (Pinnacle Technologies Series //** Tiltmeter) +* m 3 +,3 BH B F,* *, - 3,- 3, BH BH Seismological and Volcanological Department, Japan Meteorological Agency, Ohte-machi + -., Chiyoda-ku, Tokyo +** 2+,,, Japan. Corresponding author: Masaaki Churei mchurei@met.kishou.go.jp

2 92 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 crater of Asamayama, respectively. F,**, ;,**+ (Kamo and Ishihara, +323) BH,, +. km A F +, m,**. +* Table + +nano-radian, + Baytap-G (Tamura et al.,+33+) + F A *., m radian +,, ,* Pa BH 3 +/ +1,**/,,,,- BH BH,, +,**. 3 +,* *,,**-, /*,** ,* *, Fig., (a) BH F 2 -* (Fig., (d)) /

,**. 93 BH 3 + -,* *, BH BH 3 + 3 + 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 +,,**.

3 ,**. 93 BH 3 + -,* *, BH BH 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.

4 94 Fig. -. Tilt changes at F and hourly number of BH-type earthquakes at B before and after the eruptions. (a) September,-, (b) September, F + Fig., (b) +0 0 (Fig., (c)) +0 BH Fig., (a) 0 *.+ m radian,,,, 3, BH (Fig. - (a)) - / BH + *.*- m radian,, - 3,3 +, +1, BH (Fig. - (b)) +../ 3,2,+ -*. BH,3 * - BH 3,3 0 BH BH *./ *.*2 m radian,, ,* /3 A A Fig.. F A BH F BH +. / A F F +.. BH +. 3

5 ,**. 95 Fig... Tilt changes at A and F, and hourly number of BH-type earthquakes at B before and after the eruption on November +.,,**.., +. +3,1 F *.++ m radian, A *.*2 m radian, -,** , + +/ ++ / ,+ *- +,*** +,/** m 3.. Pa Fig. / *- -* +, +/ 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 / +2 * +., m radian, *.- m radian,. Fig. 0 BH Fig. 0 (a) +* 0 F BH 0 *0 +*. + 0 *0 *0 +* *.*/ m radian BH *2 +* Fig. 0 (b),**/,,+,- F

6 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,-. BH,+ +1,,,,,3,- - BH,+,,,+ +1,,,, *.+ m radian *.*- m radian A F A,+ +0,, 0 *.+. m radian +, F,,,, 2 *.3 m radian,,, +,,,+,- F BH. A,, +, F ,** ,,. A A A F (AVO) (TUMH) (MYTH) Fig. +. F F,./km +320 Fig

7 ,**. 97 Fig. 1. Comparison of the observation with the dyke model. Solid arrows show the vector diagrams of the observed tilt change and gray arrows show the vector diagrams of the calculated tilt change using the dyke model. Thick line, pluses ( ) and open circles( ) indicate the position of estimated dyke, hypocenters of BH-type earthquake and explosion earthquakes from August + to December -+,,**., respectively. A F TUMH MYTH *.*, m radian *.*+ m radian AVO A F BH Fig. 1 Okada (+33,) 0** m 0** m *.+ km +.- km *.+ km *., km -.2 km,,* m -.* m, +. +* +* Pa MaGCAP-V,**/ Table, Fig. 1 /** m + km +, km

98 -,,**. 3 +. +2, - F 3 +/ +1 3* m -,**/ ;,**/ F Fig. 2 F 3 +- +, ** 3 +2 ** ** F Table,.

8 98 -,,** , - F 3 +/ +1 3* m -,**/ ;,**/ F Fig. 2 F , ** 3 +2 ** ** F Table,. Optimum parameters for the model in Fig. 1. F Fig.., / +2 * 0 m /* m - 3* m - (,**.) (,**/),./ km + - km F +, km F Fig. 2. Solid arrow shows the vector diagram of the observed tilt change accompanied with continuous Strombolian type eruption from +, h ** m Sept. +- to ** h ** m Sept. +2,,**..

9 ,**. 99 Table -. Major eruptions and significant tilt change accompanied with each eruption. GPS ; Shimozuru and Kagiyama, +323 ; +31/ (+320). Table -,**. 3 +,**/, F. *.*- *.++ m radian, -./,3 BH +* +320 Ishihara (+33*) inflation deflation, 0 km /** m -./,3 Fig. - 3,- Table - 3 +, ,3 (Ishihara, +33*)

10 100 F,**. +*,- 0.2 inflation deflation (+31.) BH. BH : +32/ B (+323) B + - Hz BL / 2 Hz BH BH BL, - -, BL BL,**/,,,,- BH (Fig. 0 (b)),,,,,- *+ BL (+323) Fig. 3 BH BL BL BH / BH BH. BH,**/,,+,-

11 ,**. 101 (,**.),**.,**.,*/. (,**,) /. (,**/) (,),**/ V *//-P*--. Ishihara, K. (+33*) Pressure sources and induced ground deformation associated with explosive eruptions at an andesitic volcano: Sakurajima volcano, Japan.In Magma Transport and Storage (Ryan M. P. ed.), --/ -/0. John Willy & Sons, Chichester. (+323) (+) -,B (+320),3B-+ + +,. 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/ /32. IAVCEI Proceedings in Volcanology +, Springer-Verlag. (,**/) 3* -3.,. (,**/) GPS /* (,**+),*** GPS 3/ 33 +*/. (+31.) ++ -./ -.0. Okada, Y. (+33,) Internal deformation due to shear and tensile faults in a half-space. Bull. Seism. Soc. Am., 2,, +*+2 +*.*. (,**/) (SAR),**. /*.*+.+*. Shimozuru, D. and Kagiyama, T. (+323) Some significant features of pre-eruption volcanic earthquakes. In Volcanic Hazards, Assessments and Monitoring (Latter, J. H. ed.), /*. /+,. IAVCEI Proceedings in Volcanology +, Springer-Verlag. (+31/) +31- /* ++/ +/+. Tamura, Y., Sato, T., Ooe, M. and Ishiguro, M. (+33+) A procedure for tidal analysis with a Bayesian information criterion. Geophys. J. Int., +*., /*1 /+0.

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