,. Comparative Study of the,. Eruption with Old Eruptions at Asama Volcano and the Activity Evaluation

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1 /* (,**/) / -*- -+-,**.,**/.,1,**/ 1 +/ Comparative Study of the,**. Eruption with Old Eruptions at Asama Volcano and the Activity Evaluation Setsuya NAKADA, Mitsuhiro YOSHIMOTO, Etsuro KOYAMA, Hiroshi TSUJI and Taku URABE The eruptive activity at Asama that started on + September,**. was characterized by several vulcanian small explosions, including a continuous stromblian-explosion period. About, +* 0 m - of magma (DRE) was issued in this eruption, most of which filled the summit crater floor. The sequence of eruptive activity was similar to that of the +31- eruption among previous eruptions at Asama Volcano. However, the two eruptions are di#erent in the following points. +) Both magnitude of seismic activity and scale of eruption products are small in the,**. eruption; about half or less than in the +31- eruption.,) A-type earthquakes occurred in the later stage of the,**. eruption, while in the early stage in ) Partial melted country rock (rhyolite tu#) is found among the,**. eruption products but not in the +31- ones. As the swarm of A-type earthquakes at Asama occurred in the period of inflation detected by GPS in this decade, intrusion of magma in the deep-seated magma reservoir (or dike) was associated with A-type earthquakes. It is likely that the di#erence of occurrence of the A-type earthquakes in the both eruptions reflect di#erent magma supply rates ; larger in the +31- eruption and smaller but long-lasting in the,**. eruption. Continuous inflation and occurrence of A-type earthquakes even in,**/ suggest not declining of the eruptive activity but renewal near future. Key words : Asama volcano,,**. and +31- eruptions, A-type earthquakes, sequence of eruptive activity, magma eruption + (Fig. +) +3,* +31/ ;,**- +3--,* + (Fig.,) +3-*. km (Minakami, +3.,),* +30* ++- **-, Earthquake Research Institute, University of Tokyo, Yayoi, Bunkyo, Tokyo ++- **-,, Japan , +32-,** ,** * A D Minakami (+3-/) (,**-) Minakami Corresponding author: Setsuya Nakada nakada@eri.u-tokyo.ac.jp

304 Fig. +. Index map of Asama Volcano. GPS baseline between Tsumagoi (3/*,,+) and Tobu (3/*,02) of Geographical Survey Institute is alsoshown. +31- - +* B +32-. 2 C,**.,**. 3,**.,,**., +,**.

2 304 Fig. +. Index map of Asama Volcano. GPS baseline between Tsumagoi (3/*,,+) and Tobu (3/*,02) of Geographical Survey Institute is alsoshown * B C,**.,**. 3,**.,,**., +,**.,*** SO,,**.,**-,.,**. 3 + Fig. - - S-P A Minakami et al., +31* +31- A +31/ GPS 1,**/ ;,**/ (Fig..) 3 +,* : *, Fig.,. Yearly numbers of erupting days and large/ medium explosions at Asama Volcano. Large and medium explosions were counted, referring tomiyazaki (,**-).,**/ (Fig. /) -./ /./ km,** ,-,3 +* +* ,**. +, 3 (,**/) +0 t (Table +),**/ +,**-,, Fig. 0a+, a, 3 + A, C,**. (Fig. 0a-) 3 +

3 ,**. 305 Fig. -. Temporal variations in monthly number of volcanic earthquakes during these +* years (top) and the daily number at Asama Volcano (bottom). Earthquakes triggered in automatic processing system (WIN system) of the Asama Volcano Observatory were counted. 3 +0,**/ +/ +0,**/ ;,**/ +0 +* + (Fig. 1a) +*,3 (Fig. 0b,) ++ 3 (Fig. 1c) +*,3,,.,* m (,,.3. m) 1* m +*,3,,-3/ m,* m 2* m,,-3/ m +3* m -.2* t,+* m - +0 t,** m -

306 Fig... Temporal variations in monthly number of the A-type earthquakes and in length of the baseline between Tsumagoi and Tobu in the GPS network by the Geographical Survey Institute, Japan (Fig.

4 306 Fig... Temporal variations in monthly number of the A-type earthquakes and in length of the baseline between Tsumagoi and Tobu in the GPS network by the Geographical Survey Institute, Japan (Fig. +). The latter data were from Geographical Survey Institute (,**/). The former is the same as in Fig. -. Fig. /. Distribution of tephra from major explosive events in the,**. eruption at Asama Volcano. Curves for the distribution limit and contours of tephra weight (in unit area) are shown ; see Yoshimoto et al. (,**/) for the detail.

5 ,**. 307 Table +. Whole-rock compositions of products of the +31- and,**. eruptions4 Eruption +31-,**. Smpl ID +, -. / 0 1 Major element (wt. : recalculated into +** ) SiO, 0*4., 0*4-/ 0* /* 0+4/2 104+/ TiO, *41, *41- *41- *41, *41+ *41+ *4/, Al,O - +04/2 +/ /1 +/ ,* +04,, +-4** FeO* 04., / * 04,1-42/ MnO *4+- *4++ *4++ *4++ *4*- MgO.4* *1-42/ CaO 14+/ * *420 Na,O -4, ,* ,+ -4,* +422 K,O , ,4+- P,O / *4++ *4+* Original sum 334/* 334/3 334/* / ,2 Trace element (ppm) Ba Co Cr Cu Nb Sc V Ni Zn Rb Zr Sr Y,/3, ,+ +2..* 02,3 3, -,2 +3,/.,1 01 /3 -,/ +3/., 1-,2 3, -*1,+,20,0 0* ,2 3, --* +3,02,, 0,.0.,* , 33 -*+,+,12,, /0.1.,* +1* , +*+ -*0,*,3.,+ //.1.,, +10 -/ ** -*0, * /.,0 +* +* +**,3 1. 2, / -+ * total iron as FeO + : Dense lapilli erupted on + February +31-, Manzanbo (ASM1-*,*+), : Dense lappili erupted on +* March +31-, Manzanbo (ASM1-*-+*-*+) - : Pumiceous lapilli on +* March +31-, collected in AVO (ASM1-++W). : Pumiceous bomb erupted on + September, Maekakeyama (AS*.*3+--,) / : Glassy dense bomb erupted on + September, Maekakeyama (AS*.+*,3-/) 0 : Dense lapilli erupted on +. November, near AVO (ASO B) 1 : White xenolith in glassy dense bomb, Maekakeyama (AS*.+*,3--.) /*,**.,* /* Fig. 2 /* +3/* +3/2 +31-,**. +3/* ,- 0 km 3 km 0 cm ; -,*** t +, +3/. +3// +3/2 +3/3 +30+

Color scale of thermal image a- covers *ῌ2**. b) Northern to northwestern part of the crater wall and bottom from the southwestern crater rim. Dash lines show the margin of lava cake.

6 308 Fig. 0. Photographs and infrared camera images of the summit crater at Asama on +- September,**. (a+ῌ-) and,3 October,**. (b+ῌ,). a) Southeastern part of crater wall and bottom from the western crater rim. Dash-line rims the crater of the + September eruption in the close-up view (a,). Color scale of thermal image a- covers *ῌ2**. b) Northern to northwestern part of the crater wall and bottom from the southwestern crater rim. Dash lines show the margin of lava cake. Color scale of thermal image b, covers,*ῌ-** Temperature image was taken with an infrared camera (AVIO Neothermo TVS-0,*). Fig. 1. Air views of the summit crater on + October (upper left) and 3 November (upper right), and infrared camera (stereographic) images on 3 November (lower). North is lower left. The crater ﬂoor is convex downward with the vent (hottest spots) in the center and fumaroles (bright spotted line) along the periphery of the crater ﬂoor (lava cake). Taken from helicopters of Tokyo Shimbun and Asahi Air Survey Co. Ltd.

7 ,**. 309 Fig. 2. Comparison of daily number of explosions in recent four major eruptions. Arrows show dates of intermediate explosions. Data except for the,**. eruption were after Miyazaki (,**-).,**- +3/2 - +* +, ++ +* +, / ++ +* 0 2 km 0 t -.2 km ,,. +31-,**. (Fig. 2) ,.,0 +*, , +/ t ,**. 3 + /* +31-,**. +3/ / , , , + +31, +, ; +31/ , +31, +31/ Fig (Fig. 3) A +31, / Fig.., +.,0, * +31., + 3 / km,*, +0 +2,**. - +* +./ cm (+31/) B A B

eruption were counted in the automatic processing system, WIN-system. The data in the Sannotorii station for the +31- eruption (counted on the drum recorder) were from Shimozuru et al. (+31/).

8 310 Fig. 3. Temporal variation in daily number of earthquakes around the +31- and,**. eruptions. Shaded zones represent eruptive periods in which dates for explosions of intermediate scale are shown by dashed lines. Earthquakes during the,**. eruption were counted in the automatic processing system, WIN-system. The data in the Sannotorii station for the +31- eruption (counted on the drum recorder) were from Shimozuru et al. (+31/). See Fig. A+ for comparison of these two recording systems. Fig. +*. Weight comparison of tephra from the +31- and,**. eruptions, which were deposited along the national road about. km east-northeast of the crater. The data for the +31- eruption were from Shimozuru et al. (+31/).,**. +/* m +32,.,0 +32, ,**. +31-,**.,**. : Fig. 3,** A +31/,**. A 1 + A

9 ,**. 311,**. Fig. 3, +31-,**. WIN +31-,**., Fig. A+ -,** : / km - +31/,**. (Fig. +*) +31-,**.,** ,**. SiO, (Table +),**.,**/ *.. + A,**. A + km +./ km A 1 (Fig. -) (,**/) GPS,**. 1 A, km A Fig. - +* A A GPS Fig.. 3,**/ A +31- A,**., +, GPS,**/,**/ +*,**. +31-,**. +31-,**. +,**.,**..,,** * SO,,** ,**.

10 ,** , A,*,* +32, +32- A +32,,* /,**. +30* +31- A +31-,**. +*,**.,** m - +, A GPS,**/,**.,* +31- Fig.. GPS,**. : +02***, (,**/),**.,**/ /* (+32-) ,2,-,/. (,**.),** (,**/) 3* Minakami, T. (+3-/) The explosion activities of Volcano Asama in +3-/ (Part +). Bull. Earthq. Res. Inst., +-, 0, Minakami, T. (+3.,) On the distribution of volcanic ejecta (Part +). The distributions of volcanic bombs ejected by the recent explosions of Asama. Bull. Earthq. Res. Inst.,,*, 0/ 3,. Minakami, T., Utibori, S., Hiraga, S., Miyazaki, T., Gyoda, N. and Utsunomiya, T. (+31*) Seismometrical studies of Volcano Asama Part +. Bull. Earthq. Res. Inst.,.2,,-/ -*+. (,**-) 12, (+31.) ,1 +0,. (,**/)GPS

11 /* (,**/),**. /* -+/ --,. (+31/)+31- /* ++/ +/+. (+32,)+32,.,0 /1 /-1 //3. (,**-) -2 0/ 22. +/ (,**/),**. /*,**. 313 WIN Fig. A+ Fig Fig. A+ - Fig ,**. Fig. 3 Fig. A+ Fig. A+. Comparison of volcanic earthquakes on a drum recorder with those triggered in the automatic processing system,win system,at the Asama Volcano Observatory. The drum records are at the Sannotorii station. WIN system is more sensitive than the drum recording.

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