Identification of volcanic glass bearing Daisen-Kurayoshi tephra (DKP) from borehole core at Horinji, Nanto City, Toyama Prefecture, and its implications for estimating long-term slip rate on active faults in the Hokuriku region

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  • Maruyama Tadashi
    AIST, Geological Survey of Japan, Active Fault and Earthquake Research Center, AIST, Geological Survey of Japan, Institute of Earthquake and Volcano Geology
  • Saito Masaru
    Dia Consultants, Co., Ltd.

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Other Title
  • 富山県南砺市法林寺地区のボーリングコアにおける火山ガラスを含む 大山倉吉テフラ（DKP）の認定とその北陸地域の活断層の活動性評価における意義
  • トヤマケン ナントシホウリンジ チク ノ ボーリングコア ニ オケル カザン ガラス オ フクム オオヤマ クラヨシ テフラ(DKP)ノ ニンテイ ト ソノ ホクリク チイキ ノ カツダンソウ ノ カツドウセイ ヒョウカ ニ オケル イギ

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We report occurrence and petrological characteristics of the Daisen-Kurayoshi tephra (DKP; ≥55 ka), a late Pleistocene widespread tephra in Japan, identified from borehole cores at Horinji, Nanto City, Toyama Prefecture, drilled for defining shallow subsurface geometry of the Horinji fault. DKP found from the cores contains abundant volcanic glasses compared with the heavy minerals such as orthopyroxene and hornblende, whose occurrence is very rare because the glasses are commonly altered into clay minerals. Range of refractive index of the volcanic glass, reported for the first time in and around Hokuriku region including Toyama, as well as that of orthopyroxene and hornblende, is similar to that of the source area. Result of pollen analysis of organic silt just below DKP suggests that the silt was accumulated under cool climatic condition correlated with the period of marine oxygen isotope stage (MIS) 3. Identification of intact DKP from downthrown side of the Horinji fault implies that the precise correlation of the strata or terrace surfaces of MIS 3 separated by faulting might be possible through detailed examination of the strata with tephra and pollen analyses, which provides basis for determining more reliable long-term slip rate of active faults in the Hokuriku region.

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