Proximal Deposits of the Kuttara-Hayakita Tephra at Kuttara Caldera Volcano, Northern Japan: A Record of Precursor Volcanism

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  • Miura Daisuke
    Department of Physical Science, Graduate School of Science, Osaka Prefecture University
  • Yoshinaka Kouta
    Department of Physical Science, Graduate School of Science, Osaka Prefecture University
  • Takeuchi Shingo
    Geology and Geotechnical Engineering Division, Sustainable System Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI)
  • Uesawa Shimpei
    Geology and Geotechnical Engineering Division, Sustainable System Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI)

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  • 巨大噴火に先行する噴火活動の地質記録─クッタラカルデラ火山，クッタラ-早来テフラの近傍堆積物─

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<p>Identifying and characterizing a precursor volcanic episode that occurs prior to a large silicic eruption is important for the mitigation of volcanic hazards. Our key interest is the types of precursory phenomena that are manifested and how these relate to subsequent large eruptions. A sequence of tephra deposits can represent an invaluable record of precursors to subsequent large eruptions. Here, we report an investigation of the Kuttara-Hayakita (Kt-Hy) tephra (59-55 ka) on Kuttara caldera volcano based on geological field survey, granulometric measurements, and geochemical analyses. The eruption sequence revealed in proximal Kt-Hy is characterized by ejecta extending from earlier sub-Plinian (units Lpfa and Lpdc) to later phreatomagmatic (unit Mpdc) eruptions. Units Lpdc-Mpdc are valley-filling pyroclastic density current deposits. Subsequently, a poor-vesiculated silicic magma was erupted as an extensive and dilute pyroclastic density current (Updc) during the phreatomagmatic phase. On the basis of the inferred eruption sequence and detailed facies interpretations, the source craters for Mpdc and Updc are inferred to have been located in the southern apron of Kuttara stratovolcano and at the present caldera center, respectively. Estimation of the total volume of the Kt-Hy tephra remains uncertain because the evolution of the Kuttara stratovolcano is unclear. A comparison of proximal with distal facies and inferred crater migration suggests that the Kuttara stratovolcano could have grown before and during episode Kt-Hy. Including the volume of the stratovolcano, the estimated volume of Kt-Hy ejecta is approximately 7-8 km3 dense rock equivalent. These features of crater migration and the eruption style of Kt-Hy ejecta are consistent with a typical precursor episode characterized by simple, small-volume phreatomagmatic eruptions associated with stratovolcanoes and small calderas. The geochemical similarity of the precursor eruption products to those of the later large silicic eruption (Kt-3 eruption at 54 ka) may reflect the storage of a large volume of silicic magma during the precursor episode.</p>

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