凝集粒子を用いた噴煙高度の推定浅間火山2004年9月23日噴火に伴う降下火砕物の堆積様式

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(3) . . . . ,**/ - -+ !" ,**/ +* + #$. Using Aggregated Particles to Estimate a Cloud Height Sedimentation Process of the September ,-, ,**., Pyroclastic Fall at the Asama Volcano Eruption Marekazu OHNOῌ, Shuji YAMAKAWAῌ, Masayuki O’ISHIῌῌ, Kou TAKAHASHIῌῌῌ, Tatsuyuki UENOῌῌῌῌ and Takafumi IDAῌ A cloud height generated by a volcanic eruption reflects the immensity and/or magnitude of the eruption ; thus a measuring of the height’s temporal variation during the event is very significant in judging whether the activity will become violent or decline. However, when a volcanic eruption occurs during bad weather, we must take information about the cloud’s height by means of the pyroclastic deposits. In general, the total time taken for pyroclastic materials to be ejected and deposited at a given distance from the source vent can be divided into three parts as follows : the time for the eruption cloud to ascend and reach its neutral buoyancy level (T+) ; the time for the pyroclastic materials to be transported laterally by the eruption cloud (T,) ; and the time for pyroclastic materials to fall and be deposited on the ground (T-). Since T- can be calculated from the settling velocity of pyroclastic materials, if the time that the pyroclastic materials fell at a given locality was observed and a given value for T+ is assumed, the most suitable wind velocity to explain T, can be determined. Thus the height at which pyroclastic materials separate from the eruption cloud can be determined by using the vertical profile of wind velocity around the volcano. These ideas were applied to the eruption occurred at +3 : .. (JST) on September ,-, ,**., at the Asama volcano, which produced a pyroclastic fall deposit with a minimum weight of 1.,%+*0 kg. Because this eruption occurred in bad weather, the pyroclastic materials fell as mud raindrops that were aggregate particles saturated by the rainwater. Based on the depositional mass, the number of impact marks of the mud raindrops in the unit area, and the apparent density and the equivalent diameter of these drops during their fall was estimated to be ,.,ῌ-.+ mm, which is consistent with the grain-size distribution of pyroclastic materials. According to some experienced accounts, mud raindrops several millimeters in diameter fell at ,* : * &+/0ῌ2//* '()*+ -ῌ,/ῌ.* ,-.#-/0123456-6 Department of Geosystem Sciences, College of Humanities and Sciences, Nihon University, -ῌ,/ῌ.*, Sakurajosui, Setagayaku, Tokyo +/0ῌ2//*, Japan.. &+3,ῌ*-31 789:;< +ῌ+ =>?@--A#-BC60#6-DE Department of Geography, Tokyo Metropolitan University, +ῌ+, Minamiosawa, Hachioji +3,ῌ*-31, Japan.. &-3*ῌ20*+ F,:G -ῌ+ῌ+ HI--AJ-KBC601LM23456DE Division of Environmental System Sciences, Graduate School of Science and Technology, Shinshu University, -ῌ+ῌ+, Asahi, Matsumoto -3*ῌ20*+, Japan.. &0/1ῌ2/*+ NO:P)QRST +ῌ+ NO--AUV6-BC601WX23456 -DE Department of Earth and Planetary System Sciences, Graduate School of Science and Technology, Kobe University, +ῌ+, Rokkodai-cho Nadaku, Kobe 0/1ῌ 2/*+, Japan. YZ[ : &+/0ῌ2//* '()*+ -ῌ,/ῌ.* ,-.#-/UV6-BCZ Present : The Institute of Natural Sciences, College of Humanities and Sciences, Nihon University, -ῌ,/ῌ .*, Sakurajosui, Setagayaku, Tokyo +/0ῌ2//*, Japan. Corresponding author : Marekazu Ohno e-mail : [email protected].

(4) )BµOsj¶·s)<¸ks¹ ºsBlksu[»¼. 536. in the Kitakaruizawa area (about 3 km north-northeast from the source). Assuming ,ῌ/ minutes for T+ and ++./ῌ+,.* m/s of average lateral wind velocity, the height at which the mud raindrops separated from the eruption cloud can be estimated at -,.-*ῌ-,20* m (-,0+* m on average). From this conclusion, the transportation and depositional process of the pyroclastic materials generated on September ,-, ,**., at the Asama volcano can summarized as follows : the explosion occurred at +3 : .. and the eruption cloud rose to -,0+* m while blowing ,..3 km downwind from the source. The cloud moved laterally for ../+ km with generating raindrops. At +3 : /., mud raindrops separated from the cloud 1.* km north-northeast from the source, then fell to the ground at ,* : *- after being blown ,.* km downwind by a lateral wind. Key words : cloud height, pyroclastic fall deposit, mud raindrops, sedimentation process, the Asama volcano. p&m9

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(86) . Fig. .. An isopleth map of the pyroclastic fall deposit that accompanied with the eruption which occurred in September ,-, ,**. at the Asama volcano. This map also shows localities where the diameters of mud raindrops were estimated (I, II, and III on this map). The pyroclastic fall deposit distributes in a north to northeast direction and has a secondary thickening at about 3 km away from the source. The amount of the deposit within the isopleth contour of 2 g/m, is estimated to about 1.,+*0 kg. The meanings of the abbreviations in this map are as follows, Hg : Higashine city (Yamagata Prefecture), Yg : Yamagata city (Yamagata Pref.), Ym : Yamato town (Niigata Pref.), Sm : Shima Hot Spring (Gunma Pref.), Wj : Wajima weather station (Ishikawa Pref.), Tt : Tateno, Kk : Kitakaruizawa, Sz : Suzuranzaka crossroad, Fj : Fujiwara, On : Onioshidashien park..

(87) µ¶'·+2QR¸¸ ABCD ,**. 3 ,- Cp¹Cmº»(¼ ¸¸. Fig. /. Grain-size distribution of pyroclastic deposits derived from the eruptions that occurred on September ,-, ,**. at the Asama volcano. The distribution is bimodal, showing one grain size that is of a coarse mode, *ῌ, phi (+ῌ*.,/ mm in diameter), and another of a finer mode, -ῌ/ phi (+,2ῌ-, mm in diameter). With increasing distance from the source, the coarser mode migrates toward finer fractions, whereas the finer one is fixed.. 543. Fig. 0. A surface synoptic chart at ,+ : ** (JST) on September ,-, ,**.. A stationary front was stagnating south of Japan, and an atmospheric ridge expanded to the Kanto area from a high atmospheric pressure existing southeast of Hokkaido. This atmospheric pattern indicated that, in the Kanto area, the surface wind direction was north-northeast and its velocity was weak.. I4(t [. 3 ,- C. -ῌ/ . * #V' 0 2QR

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(175) Y$#A12. Fig. +*. The relationship between an average lateral wind velocity and the height at which the mud raindrops separated from the eruption cloud. Assuming ,ῌ/ minutes of T+ and +*ῌ,* m/s of average lateral wind velocity, we find that an average lateral wind velocity of +-ῌ+. m/s corresponds to the real wind velocity described in Fig. 1 (the gray area in this diagram). Considering +* m/s of wind velocity near the Asama volcano summit, we find that the actual lateral wind velocity should average a value ranging from the average lateral wind velocity (+-ῌ+. m/s) to the wind velocity near the summit (+* m/s), or ++./ῌ+, m/s. The height at which the mud raindrops separated from the eruption cloud is therefore estimated to range from -,.-* to -,2** m. See the text for a detailed discussion.. 12+, (T+) P ! ,**. 3 C +/E+2 $%"p2 , &312 '! Y$ +3 &. 6(7~!a] (1,/.* m) '8 ,* mῌs. 8K(nP

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