[S-VC36] Volcanic and igneous activities, and these long-term forecasting

[SVC36-P04] Evaluation of magma eruption potential of Kusatsu-Shirane volcano using a cumulative volumetric step diagram

*Wakako Numata¹, Nobuko Kametani², Yasuo Ishizaki³, Noriko Hasebe⁴, Naoto Ishikawa³ (1.Graduate School of Science and Engineering for Education, University of Toyama, 2.Mount Fuji Research Institute Yamanashi Prefectural Government, 3.Graduate School of Science and Engineering, University of Toyama, 4.Institute of Nature and Environmental Technology, Kanazawa University)

Keywords:Volcanic geology, Paleomagnetic Dating, TL dating

Kusatsu-Shirane is an active volcano located at the border between the Nagano and Gunma prefectures in Japan. Frequent phreatic eruptions have occurred here in recent years and would likely recur in the near future. Although the stratigraphy of eruption products from the Motoshirane (MPCG) and Shirane (SPCG) pyroclastic cone groups, the main sites of Holocene eruptions, has been clarified recently, by Ishizaki et al. (2020), Kametani et al. (2020, 2021) and Numata et al. (2021), the ages of most eruption products remain undetermined. Accordingly, we evaluated the volcano eruption potential by creating cumulative volumetric step diagrams for both pyroclastic cone groups based on additional dating and volumetric calculations using Quantum Geographic Information System (QGIS). To derive the eruption ages, we employed the thermoluminescence method for Kagusa lava (additional analysis will be conducted prior to the presentation) and compared the paleomagnetic directions to geomagnetic secular variations for others.
We obtained eruption ages of 10.7, 8.4–8.7, and 5.4–5.7 cal ka BP for the Motoshirane-nishi pyroclastic deposit, Ishizu lava, and Sessho lava, respectively, which constitute the MPCG, and 6.0–10.3, 6.2±0.3, and 2.0±0.3 cal ka BP for the Shirane-kita pyroclastic deposit, Kagusa lava, and Mizugama dome lava, respectively, which constitute the SPCG. The magma eruption rates estimated from the step diagrams during the last 10000 years were 0.04 km³/kyr for MPCG and 0.01 km³/kyr for SPCG. The difference in these rates could reflect differences in the conduit stability. Magma has been erupting from MPCG at intervals of several thousand years, with the crater shifting to the northeast, whereas eruptions have occurred from nearly the same vent of SPCG since the start of its activity. Usually, the excess pressure in a magma reservoir must be large enough for new vents to form (Tait et al., 1989). Therefore, the magma eruption rate could have been higher in the MPCG, where the eruption center differs for each eruption, and lower in the SPCG, where the location of the eruption center is approximately the same. An eruption of Heibeike lava, one order of magnitude larger in volume than other eruptions, occurred at the start of SPCG activity (ca. 18 cal ka BP), suggesting that substantial excess pressure was required to form such a stable conduit used repeatedly afterward.

Acknowledgements: This study was supported by the Earthquake and Volcano Hazards Observation and Research Program and the Integrated Program for Next Generation Volcano Research and Human Resource Development.