International Workshop on the mechanism of Phreatic Eruption

Temporal variation in the shallow resistivity structure of Aso volcano
before the phreatomagmatic eruption in September 2015, as inferred byACTIVE system
Takuto Minami1, Mitsuru Utsugi2, Hiroyuki Inoue2
1 Earthquake and Volcano Research Center, Nagoya University, 2 Aso Volcanological Laboratory, Kyoto University
E-mail: tminami(at)seis.nagoya-u.ac.jp

In Aso volcano, Central Kyushu, Japan, the latest two phreatomagmatic eruptions in
September 2015 and October 2016 followed intermittent magmatic eruptions that started inNovember 2014 after 20 years of the quiescent stage. A research group of Aso VolcanologicalLaboratory, Kyoto University, has conducted five campaigns of ACTIVE (Utada et al. 2017)
observations from May 2014 to August 2015, including the magmatic eruption period (fromNovember 2014 to May 2015). ACTIVE is a controlled-source electromagnetic volcano
sounding system consisting of transmitters with earthing electric dipoles and an array ofinduction-coil receivers that measure the vertical component of the induced magnetic field.
Minami et al. (2018) revealed temporal variation in the three-dimensional (3-D) resistivity
structure from August 2014 to August 2015, by focusing on the two campaign data sets.
However, there remained the other three campaign data sets that have not used in theanalysis, i.e. those obtained in May 2014, November 2014, and February 2015.
In this study, we performed three additional inversions to reveal detailed evolution of thehydrothermal system of Aso volcano before the phreatomagmatic eruption in September
2015. Using the five ACTIVE data sets (Fig. 1e), we resolved the evolution of the change inthe 3-D resistivity structure from May 2014 to August 2015 into four snap shots of 3-D
resistivity change models for Period 1 (May 2014 to Aug. 2014), Period 2 (Aug. 2014 to Nov.
2014), Period 3 (Nov. 2014 to Feb. 2015), and Period 4 (Feb. 2015 to Aug. 2015). In Period
1, which corresponds to the rainy season, the resistivity decreased in the surface layer,probably due to heavy rainfall. Furthermore, a resistive change was modelled just beneaththe bottom of the first crater of Nakadake, which is consistent with the lowered water leveland drying of the crater lake in this period. In Period 2, the surface resistivity in turn increased.
It is likely because the surface layer dried after the rainy season. In Period 2, a resistivechange 400 meter beneath the crater bottom was modeled in the same zone as Minami etal. (2018). This implies drying of the hydrothermal system driven by ascending magma justbefore the start of magmatic eruptions in November 2014. From Period 3 to 4, the trend ofresistive change is reversed to conductive change at the depth of 400 m. We interpreted thisInternational Workshop on the mechanism of Phreatic Eruptionchange as recovering of ground water in the hydrothermal system after the peak of themagmatic activity in February 2015. Our results imply that phreatic eruptions can occur withvery slight change in the hydrothermal system (Fig. 4d) compared to the magmatic eruption(Fig. 4b).

Fig. 1 3-D temporal variation models of the resistivity structure of Aso volcano before thephreatomagmatic eruption in September 2015 (Period Ⅰ to Ⅳ ). Blue and red areas
indicates zones that become resistive and conductive, respectively. Noticeable differences
occur mainly before the start of the magmatic eruption in November 2014, possibly due todrying of the hydrothermal system. Just before the phreatomagmatic eruption, Period Ⅳ,
light red color implies recovery of ground water in the hydrothermal system.

Main References
Minami, T., Utsugi, M., Utada, H., Kagiyama, T., & Inoue, H. (2018). Temporal variation in theresistivity structure of the first Nakadake crater, Aso volcano, Japan, during the magmatic
eruptions from November 2014 to May 2015, as inferred by the ACTIVE electromagnetic
monitoring system. Earth, Planets and Space, 70(1), 138. https://doi.org/10.1186/s40623-018-0909-2.
Utada, H., Takahashi, Y., Morita, Y., Koyama, T., & Kagiyama, T. (2007) ACTIVE system formonitoring volcanic activity: A case study of the Izu-Oshima Volcano, Central
Japan. Journal

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https://doi.org/10.1016/j.jvolgeores.2007.05.010.

research, 164(4),217-243.