Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Koki Aizawa Last modified date:2022.04.26

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

1. 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, 10.1029/2021GL096555, GRL63677, 2022.01, [URL].
2. 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, 10.1038/s41598-021-00481-6, 11, 22407, 2021.11, [URL].
3. 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, 10.1093/gji/ggab368, 228, 755-772, 2021.09, [URL], Magnetotelluric (MT) observations have revealed subvertical electrical conductors that extend
from shallow depths into the mid-crust at various geothermal zones, active volcanoes and
active faults worldwide. These deeply rooted subvertical conductors have typically been interpreted
to represent entire zones of dedicated fluid transport through the crust. We estimate
the high-resolution 3-D crustal resistivity structure below the Kuju Volcanoes, Japan, using
dense observations from 153 broad-band MT measurement sites and 40 telluric measurement
sites. The resistivity structure highlights subvertical conductors that merge into a deep conductor
to the north of the volcanoes, with deep low-frequency earthquakes occurring near
the southeastern edge of this subvertical conductor at 10–30 km depth. This deep conductor
branches into several subvertical conductors at 2–10 km depth, coinciding with a shallow zone
where tectonic earthquakes rarely occur. The surface expressions of active geothermal areas
and past volcanic eruptions are all located above the edges of the conductors at 2–6 km depth.
Widespread conductive layers exist around the volcanoes above 2 km depth, and their distribution
approximately corresponds to a low-gravity-anomaly zone.We discuss the nature of these
subvertical conductors, the potential causes of their complex structure and their relationship
to local magmatic fluid transport. These subvertical conductors, a shallow clay-rich layer, developed
fracture systems and high-strength solidified magma may all contribute to magmatic
fluid transport to the surface at the Kuju Volcanoes. In this study, we add the possibility that
the edges of these subvertical conductors act as important magmatic fluid pathways..
4. Marceau Gresse, Makoto Uyeshima, Takao Koyama, Hideaki Hase, Koki Aizawa, Yusuke Yamaya, Yuichi Morita, Derek Weller, Tawat Rung-Arunwan, Takayuki Kaneko, Yoichi Sasai, Jacques Zlotnicki, Tsuneo Ishido, Hideki Ueda, Maki Hata, Hydrothermal and magmatic system of a volcanic island inferred from magnetotellurics, seismicity, self-potential, and thermal image: an example of Miyakejima (Japan), Journal of Geophysical Research -Solid Earth-, 2021.05, [URL].
5. Usui Y., Uyeshima M., Ogawa T., Yoshimura R., Oshiman N., Yamaguchi S., Toh H., Murakami H., Aizawa K., Tanbo T., Ogawa Y., Nishitani T., Sakanaka S. Mishina M., Satoh H., Goto T., Kasaya T., Mogi T., Yamaya Y., Shiozaki I., Honkura Y., Electrical resistivity structure around the Atotsugawa fault, central Japan, revealed by a new 2‐D inversion method combining Wideband‐MT and Network‐MT datasets, J. Geophys. Res. Solid Earth,, 126, e2020JB020904, 2021.02, [URL].
6. 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], Crustal earthquake ruptures tend to initiate near fluid-rich zones. However, it is relatively unknown whether fluidrich
zones can further promote or arrest these ruptures. We image the electrical resistivity structure around the focal
area of the 2016 Kumamoto earthquake sequence by using 200 sites broadband magnetotelluric data, and discuss
its quantitative relationship to earthquake initiation, growth, and arrest processes. The ruptures that initiated along
the outer edge of the low-resistivity fluid-rich zones (< 30 Ωm) tended to become large earthquakes, whereas those
that initiated either distal to or within the fluid-rich zones did not. The ruptures were arrested by high-temperature
(> 400 °C) fluid-rich zones, whereas shallower low-temperature (200–400 °C) fluid-rich zones either promoted or
arrested the ruptures. These results suggest that the distribution of mid-crustal fluids contributes to the initiation,
growth, and arrest of crustal earthquakes. The pre-failure pressure/temperature gradient (spatial difference) of the
pore fluids may contribute to the rupture initiation, propagation, and arrest..
7. 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.
8. Sabry Abdallah, Mitsuru Utsugi, Koki Aizawa, Makoto Uyeshima, Wataru Kanda, Takao Koyama, Taro Shiotani, Three-dimensional electrical resistivity structure of the Kuju volcanic group, Central Kyushu, Japan revealed by magnetotelluric survey data, Journal of Volcanology and Geothermal Research,, 400, 106898, 2020.05, [URL].
9. Yao-Chong Sun, Makoto Uyeshima, Hengxin Ren, Qinghua Huang, Koki Aizawa, Kaori Tsukamoto, Wataru Kanda, Kaori Seki, Takahiro Kishita, Takao Ohminato, Atsushi Watanabe, Jiangjun Ran, Xiaofei Chen, Numerical simulations to explain the coseismic electromagnetic signals: A case study for a M5.4 aftershock of the 2016 Kumamoto earthquake, Earth Planets and Space,, 71, 143, 2019.12, [URL].
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. Usui Y., Ogawa Y., Aizawa Koki, Kanda W., Hashimoto T., Koyama T., Yamaya Y., Kagiyama T., Three-dimensional resistivity structure of Asama Volcano revealed by data-space magnetotelluric inversion using unstructured tetrahedral elements, Geophysical Journal International, 10.1093/gji/ggw459, 208, 3, 1359-1372, 2016.12, [URL].
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16. 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].
17. C. Cimarelli, M. A. Alatorre-Ibargüengoitia, Koki Aizawa, A. Yokoo, A. Díaz-Marina, M. Iguchi, D. B. Dingwell, Multiparametric observation of volcanic lightning: Sakurajima Volcano, Japan, Geophysical Research Letters, 10.1002/2015GL067445, 43, 4221-4228, 2016.04, [URL].
18. 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].
19. 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].
20. 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].
21. Kanda W., Yamazaki T., Ogawa Y., Hashimoto T., Sakanaka S., Aizawa Koki, Takakura S., Koyama T., Yamada K., Kobayashi T., Komori S., Shallow Resistivity Structure of Sakurajima Volcano Revealed by Audio-frequency Magnetotellurics, Bulletin of Volcanological Society of Japan, 58, 1, 251-267, 2013.01.
22. 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
23. 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].
24. Yoshimura, R., Oshiman, N., Uyeshima, M., Toh, H., Uto, T., Kanezaki, H., Mochido, Y., Aizawa Koki, Ogawa, Y., Nishitani, T., Sakanaka, S., Mishina, M., Satoh, H., Goto, T, Kasaya, T., Yamaguchi, S., Murakami, H., Mogi, T., Yamaya, Y., Magnetotelluric transect across the Niigata-Kobe Tectonic Zone, central Japan: A clear correlation between strain accumulation and resistivity structure, Geophysical Research Letters, 10.1029/2009GL040016, 36, L20311, 2009.10.
25. Yamaguchi, S., Uyeshima, M, Murakami H., Sutoh S., Tanigawa D., Ogawa T., Oshiman N., Yoshimura R., Aizawa Koki, Shiozaki I., Kasaya T., Modification of the Network-MT method and its first application in imaging the deep conductivity structure beneath the Kii Peninsula, southwestern Japan, Earth, Planets and Space, 61, 8, 957-971, 2009.10.
26. 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].
27. 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].
28. 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].
29. 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].
30. 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].
31. 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].
32. 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].
33. 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].