,3** A Large-scale Collapse Event at the Eastern Slope of Fuji Volcano about,3** Years Ago. Naomichi MIYAJI, Shigeko TOGASHI and Tatsuro CHIBA

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1 .3 (,**.) /,-1,.2,3**,**- +*,,**. 2,- A Large-scale Collapse Event at the Eastern Slope of Fuji Volcano about,3** Years Ago Naomichi MIYAJI, Shigeko TOGASHI and Tatsuro CHIBA A large-scale collapse occurred at the eastern slope of Fuji volcano about,3** years ago, based on calibrated +. C age of a wood sample collected in the resulting debris avalanche deposit. The collapsed slide deposit, called Gotemba debris avalanche deposit (Goda), is distributed on the eastern foot of the volcano covering an area of more than /- km,. The source amphitheater is not preserved because it became covered by younger tephra erupted from the summit crater. This avalanche deposit is overlain by the Gotemba mudflow deposits (Gomf) emplaced repeatedly after the avalanche. Some flow units of the Goda and Gomf entered pre-existing rivers and were finally emplaced as fluvial deposits. The Goda is composed of debris-avalanche blocks, showing jigsaw cracks, along with smaller blocks ranging from several tens of centimeters up to + m in diameter. The debris-avalanche matrix is a mixture of smaller pieces of blocks and ash-sized materials due to mainly shearing and fragmentation of large blocks. Igneous rocks include fresh and altered gray basaltic lava, weathered tephra including red scoria and white clay. Petrographical and geochemical data indicate that most blocks were derived from the Older Fuji volcano. The volumes of the Goda and Gomf are about +.*/ km - and *.1+ km -, respectively, based on presently available geological and borehole data. Since the blocks of Goda are composed mostly of the products of the Older Fuji volcano and the older stage lavas of Younger Fuji volcano do not extend to the eastern foot of Fuji volcano, a bulge of Older Fuji volcano must have existed in the eastern flank of Fuji volcano preventing the older stage lavas to flow to the east. This bulge collapsed in the form of three blocks from the foot of the mountain. The abundance of hydrothermally altered deposits in the Goda and the absence of fresh volcanic products within the Goda suggest its origin as a rupture inside the altered deposits possibly triggered by a large earthquake or phreatic eruption. Key words : altered deposits, debris avalanche, Fuji volcano, geochemical data, mudflow ; +32. Gomf: Gomf +/0 2//* -,/.* College of Humanities and Sciences, Nihon University, -,/.* Sakurajousui, Setagaya-ku, Tokyo +/0 2//*, Japan. -*/ 2/ Geological Survey of Japan, AIST, Central 1, Goda Fig. + Goda +320 ;,*** Higashi, Tsukuba -*/ 2/01, Japan.,+/ ***. +,, Asia Air Survey Co., LTD., +,,, Manpukuji, Asou-ku, Kawasaki,+/ ***., Japan. Corresponding author: Naomichi Miyaji miyaji@chs.nihon-u.ac.jp

2 Goda Goda Goda +* +.+,.. Tn +310.,**, Ui et al.,,*** ;,**, Debris flow +31. Goda Goda Gomf Goda Fig., Goda debrisavalanche block; Ui et al.,,*** debris-avalanche matrix ; Ui et al.,,*** +. / X S-+., Gomf, S-+/ +311 Goda Gomf S-+. Gomf S-+/ Goda Gomf, Goda Gomf /** +/** m Goda Gomf Goda +32-3* Fig. +. Study area and distribution of the Gotemba debris avalanche deposit (Goda). Dotted area shows the distribution of the older stage lava flows of Younger Fuji Volcano (after Miyaji, +322).

3 ,3** 239 A: N-/,, +/ E +-2 /0 +/ Goda Goda, m /* cm 2 cm AMS,1+*./ yrs. B.P. PLD-,,,. d +- C,+.,,11/./ yrs.b.p. INTCAL32 Stuiver et al., +332 CALIB..- Stuiver and Reimer, +33- cal BC 3+/ +s cal BC 3.* 23*,3** Goda Gomf,-0* +** yrs. B.P. Gak- 3.-, : +30.,.0 B: N-/,+ +0, E+-2 /1 +2 Gomf,/2* 0/ yrs. B.P. N-.*20 d +- C Goda Goda,** -** Gomf -, Goda Gomf Goda /** +*** m Gomf /** m Goda Fig., Goda / Fig. - Goda Goda Goda Goda m Goda Fig. - Gomf Gomf /** m Goda,* -* m / m Fig.,. Locality of outcrops and geological map of the Goda and the Gotemba mudflow deposits (Gomf). Gomf are subdivided into Gomf-m (mud flow deposit) and Gomf-f (fluvial deposit).

4 240 /* m +* m./* m Goda +*cm m Goda Goda Goda, - m Fig.. FB: +30. FB Goda Gomf FB Goda Gomf Goda Goda N /* / -* cm Goda -* m Fig. / Gomf Gomf-m Gomf-f Fig., Gomf-m Goda Gomf-f Gomf / +* cm N -* Gomf-m N,* /* Gomf-f Gomf-m +* m Gomf-f.2 m Fig. / Gomf-f * ; Gomf +30. ; +320 Gomf-f Gomf-f.* m a: b: c: d: e: / a + mm, - mm b +, mm,. / mm c +, mm,, - mm +* cm d

5 ,3** 241 Fig. -. Map showing the extent of flow units of Goda, based on landform analysis of aerial photographs. Gomf covering Goda are stripped in this map. Fig... Outcrop of a hummock of Goda at Loc.-, Kamikobayashi, Gotemba City. Symbols, a, c, d and e show the di#erent types of debrisavalanche blocks. +, mm,, - mm e +, mm, +, mm e +* cm X +*+ B- d e, Table + Table + +*ppm +** ppm / Fig a e G+ G- - G+ e /* d G, c /*

6 242 Fig. /. Isopach map of the Goda and Gomf based on borehole and field data. Table +. Major and trace element concentration of debris-avalanche blocks in the Goda. a, d e G- d /* c e G+ G, G- 0 Zr Y Zr Y Zr Y,.. Tn Zr Y -.* +331 a, b, c Zr Y -.*,.. d, e Zr Y -.*,.. Fig. 1

7 ,3** 243 SiO, TiO, K, O Fig. 2, Table + SiO, TiO, K,O +331 Goda a e SiO, TiO, K,O +331 a d, e b, c a d e,3** a ++*** 2*** ; +322./** -*** ; +322 a d, e Zr Y SiO, TiO, K, O,.. b, c Zr Y d, e / mm + mm +332 b, c b, c 1 Goda Gomf Fig., Fig. - Fig. / Fig. 0. Percentages of debris-avalanche blocks for each type at the studied eight outcrops. Gomf covering Goda are stripped in this map. Types of debris-avalanche blocks; type-a : fresh and porous lavas (olivine basalt, two-pyroxene basalt), type-b: fresh and dense lavas (olivine basalt), type-c : weakly altered tephra (olivine basalt), type-d: weakly altered lavas (clinopyroxene bearing olivine basalt, olivine basalt), type-e : heavily altered tephra and lavas (clinopyroxene olivine basalt, olivine basalt)

8 244 Fig. 1. Zr vs. Y of lavas from debris-avalanche blocks. Data of volcanic rocks from Older and Younger Fuji volcano are after Togashi et al. (+331). Symbols show the types of debrisavalanche blocks. Goda /- km, +.*/ km -, Gomf /* km, *.1+ km - Goda Gomf Fig. - Fig. / Goda -*.* +* +/, Table, Fig. 0 d *../ km -.- e *.,3 km -, c *.,, km - a, b *.*2 km -, *.*+ km - Table, Fig. 2. TiO, and K,O vs. SiO, of debris-avalanche blocks in the Goda. Outlined areas of volcanic rocks from Older and Younger Fuji volcano are after Togashi et al. (+331). Symbols show the types of debris-avalanche blocks. Table,. Volume of the Goda for all units and types of debris-avalanche blocks. 2 +/** -*** m,*** +, -***

9 ,3** 245 1** m,**,,3** -*** m L +32** m, H,//* m H L *.+- + km - H L *.*/ *.+- Siebert et al., */ km - H L +/** -*** m Fig., 2.2* km, +.*/ km m a b, c d, e Fig. 3 a Fig. + c Goda, Mf-, Y-+,,-- +33, Mf Y ,,..,.* +./ ,,..,.* +./ +.., Goda,.. Goda, ,**+ Fig. 3. Schematic profile of Fuji volcano at the time of the landslide about,3** yearsago.

10 Goda G+ G, G- Fig. 3 - d.- +32* Voight et al., +32+ c G, Fig. 3 d e e S-+- +/ -/** -*** S-3 + S-+* ++ Os S-3 -/** -*** e -***,***,*** ; -***,**, +33* G+ - e G- e G+ e Gomf-m Gomf-f Goda,** -** +*,3** Goda Gomf Goda +.*/ km -, Gomf *.1+ km - H L *.+- Goda

11 ,3** Goda Goda +.+ Schmincke, H. +/ B, : +/-**-*2,**,,**, *-, P*+/ * ,/ Tephrochronology +, 1-,3- -* /* , p.,***,, /*1 / Tn / 3, /1A -1 / *. +32.,3 +./ /, /2 +*/ +,* Siebert, L., Glicken, H. and Ui, T.(+321) Volcanic hazards from Bezymianny- and Bandai-type eruptions. Bull. Volcanol.,.3,.-/./3. Stuiver, M. and Reimer, P. J. (+33-) Extended +. C database and revised CALIB-.* +. C age calibration program, Radiocarbon, -/,,+/,-*. Stuiver, M., Reimer, P. J., Bard, E., Beck, J. W., Burr, G. S., Hughen, K. A., Kromer, B., MacCormac, F. G., van der Plicht, J. and Spurk, M. (+332) INTCAL32 radiocarbon age calibration,,.,*** * cal BP. Radiocarbon,.*, +*.+ +*2-.,**, +32* ,..+.,***,*** 0/. +32+

12 248 / 3, * ,.*3.,+. Togashi, S. (+323) XRF analytical Report +/23, Determination of major elements in igneous rocks using Sc/Mo dual anode tube. Geological Survey of Japan, Open-file Report, +-,, + -/. +33* + :S,/ Y ,2. +33, Ui, T., Takarada, S. and Yoshimoto, M. (,***) Debris avalanche. In Encycloperia of volcano (Sigurdsson, H., Houghton, B. F., McNutt, S. R., Rymer, H. and Stix, J. eds.), Academic Press, London, 0+1 0,0. Voight, B., Glicken, H., Janda, R. J. and Douglass, P.M. (+32+) Catastrophic rockslide avalanche of May +2. U.S. Geol.Surv., Prof.Pap., +,/*, ,,* p.,**+ :,**+ Jn*+2.,**,,. 0.* ** ,, * B- Togashi +323 XRF +***, - +** X Togashi +323 JB+A / 32./3 + *.+/ X JB +A Togashi +323 JB+A root mean square + *.*2 XRF SiO, X + *., Al,O -, MgO, Na, O SiO, SiO,, *.2 +** *.2 JB- - +**.-/ JB- +**./. JB- +**