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1 /- (,**2) / ,**2, /,** Relationship Between the Explosive Activities and the Associated Volcanic Tremors observed at Nakadake Summit of Aso Volcano The Temporal Variation in Amplitude of Tremors Recorded by Wiechert Seismograph Hironori S AKAGUCHI, Yasuaki S UDO, Yoshihiro SAWADAand Shin YOSHIKAWA Aso Volcano is one of the active volcanoes in Japan. Seismic wave associated with volcanic activity had been recorded by Wiechert seismograph at Aso Volcanological Laboratory, Kyoto University from +3,2 to,***. Some records of volcanic tremors related to large-scale explosive events had been already analyzed, however, many events were not yet examined. In this study, the previous volcanic activities with explosion are re-examined and classified into the following four types based on the seismographic record : (A) The amplitude of tremor was small prior to an explosion. After the explosion, the amplitude of tremor increased ; (B) A phreatic explosion suddenly took place without any precursory signal. The tremor amplitude was less than - mm before the phreatic explosion, and then decreased to be less than */. mm after the explosion; (C) An explosion occurred after decreasing in amplitude of volcanic tremor. After explosion, volcanic activity had been increasing; (D) The volcanic tremor was increasing and changed into continuous tremors. An explosion occurs among continuous tremor. In Nakadake crater, types C and D are major types of volcanic activity after The above classification could be an important criteria for the prediction of eruption at Aso Volcano. Key words : volcanic tremor, volcanic eruption, Wiechert seismograph, Aso Volcano, phreatic explosion +,/ km +2 +3,2 km +1 1 km Aso Volcanological Laboratory: AVL +32/ + 1 (Fig. + ) 1 +,*** 1, +3.* +.,**1 0,** *./ mm 03* 2/*. +*0* *. Interdisciplinary Faculty of Science and Engineering, Shimane University, +*0* Nishikawatsu, Matue, 03* Aso Volcanological Laboratory, Kyoto University, 2/*., Japan. Kawayo, Minami-Aso-son, Aso-gun, *., Japan. 203,,-, +3-* + Corresponding author: Hironori Sakaguchi Aso Volcano Museum, +3-* + Akamizu, Aso, Kuma- s *13,*0@matsu.shimane-u.ac.jp moto, 203,,-,, Japan.

2 144 Fig. +. Locations of AVL and Nakadake Crater. AVL is Aso Volcanological Laboratory of Kyoto University. Solid lines are topography contour intervals of,** m (Geological Survey Institute in Japan,,**+). AVL +3-, +, ** Sassa ( +3-/, +3-0)., +3,2,*** AVL,, * +3/2 +, 1* 0,, / Sassa EW EW ( +3-/, +3-0 ) +3-*, (Sassa, +3-/ ) 1* + +,. +3-*,*** 1* +3-*,, * Type A Type B Type C Type D.

3 145 Table +. Classification of Nakadake explosive events +3,2,***, based on the amplitude variation of volcanic tremors by Wiechert seismograph(ew component)at AVL. * represents the case that heavy rains were observed before explosions. and in the column of crater lake indicates the presence and absence of lake water, respectively, at the explosive activity. (Table. + ).,+ Type A Type A +3-* ,1 (Table + ) Fig., Periods. Fig.,. Amplitude variation of volcanic tremors which were recorded by Wiechert seismograph (EW Period component) at AVL from January to April, The vertical axis is the daily average amplitude. The period I shows the phreatic explosions from the +3.* small crater lake remaining at + st Crater. The period II shows the Strombolian eruptions at + st Crater. The period III shows the Strombolian eruptions at, nd Crater. The period IV shows the, mm activity of explosion activity at + st and, nd Craters. +,1 +. The arrow indicates the phreatic explosion at + st + Crater which is classified into Type A. Period +,1

4 * -+ Mori et al.,,**2 (Takagi et al.,,**0),.,* mm, * + Period Fig. -. Amplitude variation of volcanic tremors from, +*, April to July, +3/2. The vertical axis value is the,- daily average amplitude. From April to June, the /* mm, activity of boiling and phreatic explosions had been occurring in the small crater lake remaining at + st Crater. The arrow indicates the time of the,,. -. phreatic explosion which is classified into Type B. +3-3,,0,1, / 1/* 0 -, +3.* mm 0,.,, +/ Period - Fig. -; *, mm +3-- (Fig. -) Type A +3-- (,**0) - mm +3/2,, Type B Type B ( * -mm) + +3-* 3. / +3/*. +/ +3/2 0, ,+.,- Type C (Table + ) Type B Type C +3/2 0,. * mm +3/2. 1 Fig. - (Fig..) +3/ ,,0 +3/- +3/3 * + mm.,1 +3/1 +, ,* +313 / , +32* +,0 +33*.,* +33, 2,+ 3 - mm (Table + ) +313

5 147 +,** *0 Fig.. ; Period Fig... Amplitude variation of volcanic tremors from 3 0 +* July to October, The vertical axis is the daily average amplitude. The period I shows the, / mm activity of volcanic ash emissions and fragment +313 ejections. The period II shows the activity of strong ash emissions and strong fragment ejections Type C with continuous tremors. The period III shows the attacking of Typhoons 13++ and 13+, in the Aso region. The period IV shows small explosions, ash emissions and fragment ejections. The +313 arrow indicates the phreatic explosion which is classified into Type C. Type C * Fig (Table + ; / mark),. Type D 0 Type D Type D 1 Type C Period. +3// 1,/ +3/2 +* Period / +* ,0 2 Type D (Table + ) Period +32* , Fig. / 1 +* 2 +* mm Period +32* Period 2,0, , Period,20 mm ,0 +* mm

6 148-1,,, Type A Type B Type C Type D. Type A Fig. /. Amplitude variation of volcanic tremors from August to December, The vertical axis is the daily average amplitude. The period I shows the Type B activity of ash emissions and fragment ejections. The period II shows the activity of Strombolian Type C eruptions. The period III shows phreatic explosions and Strombolian spatterings. The arrow indicates the phreatic explosion with pyroclastic Type D surge which is classified into Type D.. Type C 3 Type D 2 Type B. Type A Type C 3 Type D Type A,**1 Period +** +3-- Type A +* +/ mm ++ Type C Type D,+,/,**1 ++,0 Fig. / ; Type C Period +, 2 mm Table +,**1 +, +, / mm (Fig. /) Type D Ono et al., +33/ ;,**0 Type D

7 149 Sassa, K. ( +3-/ ) Volcanic micro-tremors and eruptionearthquakes (Part + of the geophysical studies on the Volcano Aso). Mem, Coll, Science, Kyoto Imp. Univ., Ser., A, +2, /,,/.,3-. Sassa, K. ( +3-0) Micro-seismometric study on eruptions of the Volcano Aso (Part, of the Geophysical studies on the Volcano Aso). Mem, Coll, Science, Kyoto Imp. Univ., Ser., A, +3, +, ++ /0. ( +3-3) -,+/,,0. (,**0) ( +3/2) 0 /,3+,. 3 p. -*3. ( +3.* ) 2, (,**1) -+3 p Takagi, N., Kaneshima, S., Kawakatsu, H., Yamamoto, (,**+) /* m M., Sudo, Y., Ohkura, T., Yoshikawa, S. and Mori, T. (,**0) Apparent migration of tremor source synchronized Mori, T., Sudo, Y., Tsutsui, T. and Shin, Y. (,**2) Charserved with the change in the tremor amplitude ob- acteristic of isolated hybrid tremor (HBT) during a calm at Aso volcano, Japan. J. Volcanol. Geotherm. activity period at Aso Volcano. Bull. Volcanol., 1*, +*-+ Res. +/., +2+,**. +*.,. ( +32/ ) ( +3/3 ) +3/2 0 2 p Ono, K., Watanabe, K., Hoshizumi, H. and Ikebe, S. ( +32* ) ( +33/ ) Ash eruption of the Naka-dake crater, Aso volcano,,/,./,/-. southwestern Japan. J. Volcanol. Geotherm. Res., 00,

Seismic signature of fluid motion in a shallow conduit system beneath Aso volcano, Japan

Seismic signature of fluid motion in a shallow conduit system beneath Aso volcano, Japan Mare Yamamoto, Hitoshi Kawakatsu Earthquake Research Institute, University of Tokyo, Tokyo, Japan Satoshi Kaneshima,