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 0  +1+῍+3+ ῕ ῔ +3,0(   ,**2 H . I +0 *JK ,**2 H ++ I +* *JBRestudy of Stratigraphy and Paleomagnetic Characteristics of Taisho Lahar Deposit Associatedwith the +3,0 Eruption on Tokachidake Volcano, Central Hokkaido, Japan
Shimpei UESAWA῍
On May ,.th +3,0, the eruption of Tokachidake volcano, in central Hokkaido, e$ciently melted the snow packon the hill slope, triggering the Taisho lahar which killed +.. people in the towns of Kamifurano and Biei. A
geological survey and paleomagnetic and granumetric studies were conducted on the northwestern slope ofTokachidake volcano to reconstruct the sequence of the +3,0 eruption and decipher the triggering mechanism for
the Taisho lahar. The Taisho lahar deposits in the proximal area of the volcano are divided into ﬁve distinct units(unit L+, L,, and A through C, from oldest to youngest). Unit L+ is an older lahar deposit that underlies the+3,0 deposits. The +3,0 sequence consists of debris avalanche deposits (unit A and C), a laminated sandy debris
ﬂow deposit (unit B), and a lahar deposit including scoria clasts (unit L,). Each unit contains hydrothermallyaltered rocks and clay material with more than / wt.ῌ fragments smaller than , mm in diameter. The progressive
thermal demagnetization experiments show that the natural remanent magnetization (NRM) of all samples in unit
A, B and C have a stable single or multi-component magnetization. The emplacement temperatures are estimatedto be normal temperatures to 0,* for unit A, -** to ./* for unit B, and normal temperature to /** for unit
C. On the basis of geological and paleomagnetic data and old documents, a sequence for the eruption and themechanism of formation and emplacement of the Taisho lahar can be reconstructed. The ﬁrst eruption at +, :++ May ,.th triggered a small lahar (unit L,). Collapse of central crater at +0 : +1 May ,.th +3,0 then resulted
in a debris avalanche containing highly altered hydrothermal rocks with hot temperatures ranging from -** to0,* (unit A). The debris avalanche ﬂowed down the slope of the volcano, bulldozing and trapping snow.
Immediately following the collapse, a hot (approximately .**) hydrothermal surge (unit B) melted snow andtransformed into a lahar causing signiﬁcant damage and deaths in the towns downstream. Just after thegeneration of the lahar, another collapse occurred at the crater causing another debris avalanche (unit C).
Key words : debris avalanche, emplacement temperature, lahar, remanent magnetization, Tokachidake volcano+320 LMNO  PQRS T0 URVWX+ῌ ῐ ῎ ῑ ῏YZX[ RSU\]^S_ Z`ab

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Graduate School of integrated Basic Siences, Nihon
University, -῍,/῍.*, Sakura-jyosui, Setagaya-ku, Tokyo+/0῍2//*, Japan.

^nomxjyz{ |xi }lV (Major and New-(Present address Department of Natural History Sciences, Graduate School of Science, Hokkaido University, N+* W2, Kita-ku, Sapporo *0*῍*2+*)
Corresponding author : Shimpei Uesawa
e-mail : s-uesawa@mail.sci.hokudai.ac.jp