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Koki Aizawa Last modified date:2024.05.23

Associate Professor / Seismology and Volcanology
Institute of Seismology and Volcanology
Faculty of Sciences

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Research Interests
  • Process prior to phreatic eruption
    keyword : Phreatic eruption, self potential
  • Electromagnetic signals by a volcanic lightning
    keyword : volcanic lightning
  • Relationship between resistivity structure and earthquakes
    keyword : resistivity earthquakes
  • Temporal change of resistivity structure
    keyword : resistivity magnetotellurics structure change
  • hydrothermal system of volcanoes
    keyword : self-potential resistivity hydrothermal system
  • Resistivity structure beneath active volcanoes
    keyword : Resistivity magnetotellurics
Academic Activities
1. , [URL].
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2. , [URL].
3. , [URL].
4. Aizawa K., Muramatsu D., Matsushima T., Koyama T., Uyeshima M., Nakao S., Phreatic volcanic eruption preceded by observable shallow groundwater flow at Iwo-Yama, Kirishima Volcanic Complex, Japan, Communications Earth & Environment,, 3, 187, 2022.08, [URL], It is difficult to forecast phreatic eruptions because they are often characterised by an abrupt
onset at shallow depths beneath volcanoes. Here we show that temporal changes in the tilt,
tremor, and horizontal electric field have occurred repeatedly near the vent of a small phreatic
eruption at Iwo-Yama, Kirishima Volcanic Complex, Japan. Such geophysical changes were
observed 13 times, with one of these events occurring immediately before the onset of the
2018 phreatic eruption. These observations suggest that shallow hydrothermal intrusions,
which are observed as tilt changes with tremors, commonly induce near-surface cold
groundwater flow, which is observed as electric-field changes. Near-surface groundwater
flows towards the active vent, potentially inhibiting a phreatic eruption. However, explosive
phreatic eruptions occur when the intrusion is shallow and cold groundwater flow is depleted.
The near-surface groundwater is key in controlling the occurrence of phreatic eruptions and
can be monitored using electric-field measurements..
5. Aniya S., Aizawa K., and Matsushima T., Infrasound–electric-field coupling associated with the 2018 Shinmoe-dake eruptions, Kirishima volcanic complex, Japan, Geophysical Research Letters,, GRL63677, 2022.01, [URL].
6. Miyano K., Aizawa K., Matsushima T., Shito A., and Shimizu H., Seismic velocity structure of Unzen Volcano, Japan, and relationship to the magma ascent route during eruptions in 1990–1995, Scientific Reports,, 11, 22407, 2021.11, [URL].
7. Aizawa, K., Ustugi, M., Kitamura, K., Koyama, T., Uyeshima, M., Matsuhima, N., Takakura, S., Inagaki, H., Saito, H., Fujimitsu, Y., Magmatic fluid pathways in the upper crust: Insights from dense magnetotelluric observations around the Kuju Volcanoes, Japan, Geophysical Journal International,, 228, 755-772, 2021.09, [URL].
8. Koki Aizawa, Shinichi Takakura, Hisafumi Asaue, Katsuaki Koike, Ryokei Yoshimura, Ken’ichi Yamazaki, Shintaro Komatsu, Mitsuru Utsugi, Hiroyuki Inoue, Kaori Tsukamoto, Makoto Uyeshima, Takao Koyama, Wataru Kanda, Tohru Yoshinaga, Nobuo Matsushima, Kazunari Uchida, Yuko Tsukashima, Takeshi Matsushima, Hiroshi Ichihara, Dan Muramatsu, Yoshiko Teguri, Azusa Shito, Satoshi Matsumoto & Hiroshi Shimizu, Electrical conductive fluid-rich zones and their influence on the earthquake initiation, growth, and arrest processes: observations from the 2016 Kumamoto earthquake sequence, Kyushu Island, Japan, Earth Planets Space,, 1, 12, 2021.01, [URL].
9. Tasuku M Hashimoto, Koki Aizawa, Yuto Hayashida, Yuhei Yuasa, Takeshi Matsushima, Yuto Yamamoto, Kaori Tsukamoto, Kanta Miyano, Satoshi Matsumoto, Hiroshi Shimizu, Joint seismological–magnetotelluric investigation of shallow and implosive non-DC and DC earthquakes beneath the gravitationally unstable Heisei-Shinzan Lava Dome, Unzen Volcano, Japan, Journal of Volcanology and Geothermal Research,, 2020.09.
10. Triahadini A. Aizawa K., Teguri Y., Koyama T., Tsukamoto K., Muramatsu D., Chiba K., and Uyeshima M., Magnetotelluric transect of Unzen graben, Japan: Conductors associated with normal faults, Earth Planets Space,, 71, 28, 2019.02, [URL].
11. Dan Muramatsu, Koki Aizawa, Akihiko Yokoo, Masato Iguchi, Takeshi Tameguri, Estimation of Vent Radii From Video Recordings and Infrasound Data Analysis: Implications for Vulcanian Eruptions From Sakurajima Volcano, Japan, Geophysical Research Letters, 10.1029/2018GL079898, 45, 23, 12829-12836, 2018.12, [URL].
12. K Tsukamoto, K Aizawa, K Chiba, W Kanda, M Uyeshima, T Koyama, M Utsugi, K Seki, T Kishita, Three-Dimensional Resistivity Structure of Iwo-Yama Volcano, Kirishima Volcanic Complex, Japan: Relationship to Shallow Seismicity, Surface Uplift, and a Small Phreatic Eruption, Geophysical Research Letters, 10.1029/2018GL080202, 45, 23, 12821-12828, 2018.12, [URL], Abstract Iwo-yama volcano, part of the Kirishima Volcanic Complex, has recently shown signs of unrest.
We conducted a hypocenter relocation of shallow earthquakes and broadband magnetotelluric
measurements around Iwo-yama. Three-dimensional inversion of magnetotelluric data revealed an
electrically conductive layer that is interpreted as a hydrothermally altered clay-dominated unit. Shallow
earthquakes occur beneath this layer, suggesting that it controls the location of seismicity. The base of the
layer corresponds to the depth of a pressure source identified by a leveling survey. These observations
suggest that the supply of high-temperature fluids has increased over time beneath Iwo-yama, causing an
increase in pore pressure beneath the clay-rich layer and resulting in tectonic earthquakes and ground
inflation. Increased upwelling of fluids through a fracture in the clay-rich layer may have caused a vigorous
liquid-gas phase transition near the surface, which in turn might have led to the small phreatic eruption on 19
April 2018.

Plain Language Summary
Phreatic eruptions result from the sudden release of pressurized fluid
into the air. Imaging the pressurized region is important to predict future phreatic eruptions. We studied
Iwo-yama volcano, Kirishima, Japan, where recent volcanic unrest has led to a small phreatic eruption. By
imaging the subsurface electrical resistivity structure using the magnetotelluric technique, we have identified
a low-permeability clay-rich layer at depths of 200–700 m below the surface. Earthquake hypocenters and
one of the sources of surface uplift are located beneath this clay layer, so we conclude that the layer caps
deep hydrothermal fluids and allows the buildup of pressure beneath it. If this clay-rich layer suddenly breaks,
or if the fluid supply through the deeper conductor suddenly increases, a larger phreatic eruption could occur
in the future. Imaging and monitoring of such clay-rich layers is important to our understanding of shallow
activity beneath volcanoes..
13. Aizawa Koki, H. Asaue, K. Koike, S. Takakura, M. Utsugi, H. Inoue, R. Yoshimura, K. Yamazaki, S. Komatsu, M. Uyeshima, T. Koyama, W. Kanda, T. Shiotani, N. Matsushima, M. Hata, T. Yoshinaga, K. Uchida, Y. Tsukashima, et al., Seismicity controlled by resistivity structure: the 2016 Kumamoto earthquakes, Kyushu Island, Japan, Earth Planets and Space, 10.1186/s40623-016-0590-2, 69, 4, 2017.01, [URL].
14. , [URL].
15. Aizawa Koki, C. Cimarelli, M. A. Alatorre-Ibargüengoitia, A. Yokoo, D. B. Dingwell, M. Iguchi, Physical properties of volcanic lightning: constraints from magnetotelluric and video observations at Sakurajima volcano, Japan, Earth and Planetary Science Letters, 10.1016/j.epsl.2016.03.024, 444, 45-55, 2016.06, [URL].
16. Aizawa Koki, H. Sumino, M. Uyeshima, Y. Yamaya, H. Hase, H. A. Takahashi, M. Takahashi, K. Kazahaya, Masao OHNO, T. Rung-Arunwan, Y. Ogawa, Gas pathways and remotely triggered earthquakes beneath Mt. Fuji, Japan, Geology, 10.1130/G37313.1, 44, 127-130, 2016.02, [URL].
17. Aizawa Koki, T. Koyama, H. Hase, M. Uyeshima, W. Kanda, M. Utsusgi, R. Yoshimura, Y. Yamaya, T. Hashimoto, K. Yamazaki, S. Komatsu, A. Watanabe, K. Miyakawa, Y. Ogawa, Three-dimensional resistivity structure and magma plumbing system of the Kirishima Volcanoes as inferred from broadband magnetotelluric data, J. Geophys. Res. Solid Earth, 10.1002/2013JB010682, 119, 1, 198-215, 2014.01, [URL].
18. Aizawa Koki, T. Koyama, M. Uyeshima, H. Hase, T. Hashimoto, W. Kanda, R. Yoshimura, M. Utsugi, Y. Ogawa, K. Yamazaki, Magnetotelluric and temperature monitoring after the 2011 sub-Plinian eruptions of Shinmoe-dake volcano, Earth Planets Space, 10.5047/eps.2013.05.008, 65, 6, 539-550, 2013.07, [URL].
19. Aizawa Koki, Kanda W., Ogawa Y., Iguchi M., Yokoo A., Yakiwara H., Sugano T., Temporal changes in electrical resistivity at Sakurajima volcano from continuous magnetotelluric observations, Journal of Volcanology and Geothermal Research, 10.1016/j.jvolgeores.2010.11.003, 199, 1-2, 165-175, 2011.01, [URL], Continuous magnetotelluric (MT) measurements were conducted from May 2008 to July 2009 at Sakurajima,
one of the most active volcanoes in Japan. Two observation sites were established at locations 3.3 km east and
3 km west–northwest of the summit crater. At both observation sites, the high-quality component of the
impedance tensor (Zyx) showed variations in apparent resistivity of approximately ±20% and phase change
of ±2°, which continued for 20–180 days in the frequency range between 320 and 4 Hz. The start of the
period of changes in apparent resistivity approximately coincided with the start of uplift in the direction of the
summit crater, as observed by a tiltmeter, which is one of the most reliable pieces of equipment with which to
detect magma intrusion beneath a volcano. A 2D inversion of MT impedance suggests that the resistivity
change occurred at a depth around sea level. One of the possible implications of the present finding is that the
degassed volatiles migrated not only vertically through the conduit but also laterally through a fracture
network, mixing with shallow groundwater beneath sea level and thereby causing the observed resistivity
20. Aizawa Koki, Yokoo A., Kanda W., Ogawa Y., Iguchi M., Magnetotelluric pulses generated by volcanic lightning at Sakurajima volcano, Japan, Geophysical Research Letters, 10.1029/2010GL044208, 37, L17301, 2010.09, [URL].
21. Aizawa Koki, Ogawa Y., Mishina M., Takahashi K., Nagaoka S., Takagi N., Sakanaka S., Miura T., Structural controls on the 1998 volcanic unrest at Iwate volcano: Relationship between a shallow, electrically resistive body and the possible ascent route of magmatic fluid, Journal of Volcanology and Geothermal Research, 10.1016/j.jvolgeores.2009.08.009, 187, 1-2, 131-139, 2009.10, [URL].
22. Aizawa Koki, Ogawa Y., Ishido T., Groundwater flow and hydrothermal systems within volcanic edifices: Delineation by electric self-potential and magnetotellurics, Journal of Geophysical Research, 10.1029/2008JB005910, 114, B01208, 2009.01, [URL].
23. Aizawa Koki, Classification of self-potential anomalies on volcanoes and possible interpretations for their subsurface structure, Journal of Volcanology and Geothermal Research, 10.1016/j.jvolgeores.2008.03.011, 175, 3, 253-268, 2008.08, [URL].
24. Aizawa Koki, Ogawa Y., Hashimoto T., Koyama T., Kanda W., Yamaya Y., Mishina M., Kagiyama T., Shallow resistivity structure of Asama Volcano and its implications for magma ascent process in the 2004 eruption, Journal of Volcanology and Geothermal Research, 10.1016/j.jvolgeores.2008.01.016, 173, 3-4, 165-177, 2008.06, [URL].
25. Aizawa Koki, Yoshimura R., Oshiman N., Splitting of the Philippine Sea Plate and a magma chamber beneath Mt. Fuji, Geophysical Research Letters, 10.1029/2004GL019477, 31, 9, L09603, 2004.05, [URL].
26. Aizawa Koki, Yoshimura, R., Oshiman, N., Yamazaki, K., Uto, T., Ogawa, Y., Tank, S. B., Kanda, W., Sakanaka, S., Furukawa, Y., Hashimoto, T., Uyeshima, M., Ogawa, T., Shiozaki, I., Hurst, A. W., Hydrothermal system beneath Mt. Fuji volcano inferred from magnetotellurics and electric self-potential, Earth and Planetary Science Letters, 10.1016/j.epsl.2005.03.023, 235, 1-2, 343-355, 2005.06, [URL].
27. Aizawa Koki, Uyeshima M., Nogami K., Zeta potential estimation of volcanic rocks on 11 island arc-type volcanoes in Japan: Implication for the generation of local self-potential anomalies, Journal of Geophysical Research, 10.1029/2007JB005058, 113, B2, B02201, 2008.02, [URL].
28. Aizawa Koki, A Large Self-potential Anomaly and its Changes on the Quiet Mt. Fuji, Japan, Geophysical Research Letters, 10.1029/2004GL019462, 31, No. 5, L05612, 2004.03, [URL].
1. , [URL].
2. Koki Aizawa, Takao Koyama, Hideaki Hase, Makoto Uyeshima, Magnetotelluric monitoring at Sakurajima volcano, Japan, IAVCEI 2013 Scientific Assembly, 2013.07.
3. Koki Aizawa, et al., Three dimensional resistivity structure of Kirishima volcanoes inferred from magnetotelluric data, IAVCEI 2013 Scientific Assembly, 2013.07.
Membership in Academic Society
  • The seismological society of japan
  • AGU
  • The volcanological society of Japan
  • sgepss