Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 251-267

ArticleShallow Resistivity Structure of Sakurajima Volcano
Revealed by Audio-frequency Magnetotellurics
Wataru KANDA1）, Tomoya YAMAZAKI2）, Yasuo OGAWA1）, Takeshi HASHIMOTO3）,Shinʼya SAKANAKA4）, Koki AIZAWA5, 10）, Shinichi TAKAKURA6）,
Takao KOYAMA5）, Kenta YAMADA4）, Tsukasa KOBAYASHI7）
and Shogo KOMORI8, 9）(Received October 5, 2011 ; Accepted April 27, 2012)
An audio-frequency magnetotelluric (AMT) survey was conducted at the foot of Sakurajima volcano in November
2007. This survey was carried out within the framework of the 7th National Project for Prediction of Volcano
Eruptions. The main objective was to clarify the shallow layers of Sakurajima volcano for better understanding of thevolcanic activity. We measured electromagnetic fields at frequencies from 1 to 10400 Hz at 27 locations along only
three lines set on the northern, western, and southeastern flanks of the volcano. The three profiles enclosed the summit
area where access is prohibited because of frequent explosive vulcanian eruptions. The data quality was generallyconsidered to be good at frequencies higher than 2 Hz. Two-dimensional and three-dimensional (3-D) modelingapproaches were applied to the data.
As a result of 3-D modeling, we obtained the following features in the resistivity model. The surface layer showeda high resistivity from several hundreds to thousands of ohm-meters and corresponded to the lava. This surface layer
covered a highly conductive layer with a resistivity of less than tens of ohm-meters, interpreted as containing seawateror groundwater. However, the basement structure was not clearly detected, except for in an area of northern
Sakurajima. The first lava layer was distributed thickly in northern and western parts of Sakurajima, and the boundary
between the first and second layers was typically located at around sea level.
The conductive second layer was found at deeper levels beneath areas such as the Haruta-yama lava dome and theNabe-yama pumice cone. This depression of the low resistivity layer is probably caused by the fracture zone associatedwith previous eruptions, which is formed above the conductive layer. In the proximity of Hikino-hira lava dome andthe Taisho crater, the second conductive layer was found at a depth shallower than sea level, suggesting that ahydrothermal system has developed underneath this area. In contrast, elevation of the second layer was not seen on theside of Taisho crater located nearest to the edge of Nabe-yama, implying that a heat source is not present beneath theeastern foot of Sakurajima.
Key words : Sakurajima volcano, electrical resistivity structure, hydrothermal system, flank eruption, prediction ofvolcanic eruptions

Kusatsu-Shirane Volcano Observatory, Volcanic Fluid
Research Center, Tokyo Institute of Technology, 641-36
Kusatsu, Agatsuma, Gunma 377-1711, Japan.
2）
Disaster Prevention Research Institute, Kyoto University,Gokasho, Uji, Kyoto 611-0011, Japan.
3）
Faculty of Science, Hokkaido University, N10W8, Kitaku, Sapporo 060-0810, Japan.
4）
Graduate School of Engineering and Resource Science,Akita University, 1-1 Tegata gakuen-machi, Akita 0108502, Japan.
5）
Earthquake Research Institute, the University of Tokyo,
1-1-1, Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan.
6）
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba West, 16-1 Onogawa, Tsukuba,
1）

Ibaraki 305-8569, Japan.
Maizuru Marine Observatory, Japan Meteorological Agency,
901 Shimofukui, Maizuru, Kyoto 624-0946, Japan.
8）
Aso Volcanological Laboratory, Graduate School of Science,
Kyoto University, Minami-Aso, Kumamoto 869-1404, Japan.
9）
Now: Institute of Earth Sciences, Academia Sinica, 128
Academia Road, Section 2, Nankang, Taipei 115, Taiwan.
10）
Now: Institute of Seismology and Volcanology, Facultyof Sciences, Kyushu University, 2-5643-29 Shinʼyama,
Shimabara, Nagasaki 855-0843, Japan.
7）

Corresponding author: Wataru Kanda
e-mail: kanda@ksvo.titech.ac.jp