Radiocarbon Wiggle-matching Age and Depositional Characteristics of the Ippongihara Debris Avalanche Deposit, Iwate Volcano: An Example of a Small-scale (<0.1km<sup>3</sup>) Debris Avalanche Deposit Induced by Flank Collapse on an Active Volcano

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  • ITOH Jun’ichi
    Research Institute of Earthquake and Volcano Geology, Geological Survey of Japan, AIST Tsukuba Central

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  • 岩手火山，一本木原岩屑なだれ堆積物の<sup>14</sup>Cウィグルマッチング年代と崩壊堆積物の特徴：活火山における小規模な表層崩壊堆積物
  • 岩手火山,一本木原岩屑なだれ堆積物の¹⁴Cウィグルマッチング年代と崩壊堆積物の特徴 : 活火山における小規模な表層崩壊堆積物
  • イワテ カザン,イッポン キハラ ガンセツナダレ タイセキブツ ノ ¹ ⁴ Cウィグルマッチング ネンダイ ト ホウカイ タイセキブツ ノ トクチョウ : カッカザン ニ オケル ショウキボ ナ ヒョウソウ ホウカイ タイセキブツ

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Abstract

<p>Hazard evaluation of the Ippongihara Debris Avalanche Deposit (Di), which is the most recent slope failure deposit at Iwate Volcano, NE Japan, requires a careful and precise assessment of the age and characteristics of the deposit. Geological and geomorphological investigations have yielded a volume of ca. 4.6×107m3, a travel distance of ca. 8km, and an elevation change of ca. 1.6km. AMS radiocarbon dating was carried out on a buried tree trunk sample that was recovered from the deposit. The trunk displayed 83 annual tree-ring layers, which allowed eight 14C ages for the outer 3 annual layers of the tree trunk. Using the IntCal 13 calibration curve, the wiggle-matching method was applied to the age data and provided a suitable and consistent age of cal. AD 1333-1346 (95.4%), with the exclusion of the sample for the outermost sample. While the hummocky surface is not observed for Di, a block facies composed of unconsolidated tephra and paleosol units is recognized for this deposit. Internal facies of the debris avalanche blocks include 1) minor normal-faults observed as offset tephra layers caused by stretched deformation during sliding on the volcano slope, and 2) repetitions of tephra layers caused by compressional deformation during the depositional processes. The features indicate that Di is a ridged and extensional type of debris avalanche deposit. Di was formed by the flank collapse of a wide area of the volcano, but the trigger of this avalanche remains unclear.</p>

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