IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1A2_2D-O11

Room B1

Date/Time: July 21 11:45-12:00

Continuous heat accumulation beneath Tokachi-dake volcano, northern Japan, as
inferred from magnetic total field changes
Ryo Tanaka1 , Takeshi Hashimoto1 , Kazuhiro Kawahashi2 , Eikichi Shioya2 , Teruaki Yamamoto2 , Jun’ichi
Miyamura2 , Tomofumi I3 , Shin Arita3 , Noritoshi Okazaki41Hokkaido University, Japan, 2 Sapporo District Meteorological Observatory, Japan MeteorologicalAgency, Japan, 3 Kakioka Magnetic Observatory, Japan Meteorological Agency, Japan, 4 Geological
Survey of Hokkaido, Hokkaido Research Organization, Japan
E-mail: ryo-10@mail.sci.hokudai.ac.jp

Tokachi-dake is an active volcano in the central Hokkaido, northern Japan. In historic age, it experienced
phreatomagmatic to magmatic eruptions in 1926, 1962, and 1988-89. Recent major geothermal anomalies andfumaroles have been concentrated on Taisho and 62-II craters. Geodetic monitoring based on GPS has revealed
continuous inflation localized at 62-II crater since 2007 (Japan Meteorological Agency, 2012). In contrast, seismicity
and fumarolic activity at 62-II crater were considerably low in this period. Most recently, after 2010, micro-seismicity
beneath the crater area has gradually revived and thermal activity at Taisho crater has measurably elevated. We
have performed magnetic repeated surveys since 2008 aiming for better understanding of ongoing subsurface
processes beneath the craters.
Numerous studies have reported magnetic changes associated with volcanic activity. In many cases, such changes
are attributed to the thermo-magnetic effect in which heating/cooling causes decrease/increase in magnetization of
volcanic rocks. If the localized ground inflation at Tokachi-dake accompanies thermal processes beneath the crater,
it should be detected as magnetic field changes.
In each survey, we recorded the magnetic total field at about 30 sites for 2 minutes on each with every 5 seconds
around the craters. Repeatability of the sensor position was typically several cm in a typical field gradient of
10 nT/m (but occasionally 100 nT/m). During the survey, another magnetometer was operated at a temporaryreference station that was located approximately 8 km north from the target area. Simple difference reduction wasapplied between each site and the reference in order to remove the variations of extra-terrestrial origins.
The first and second campaigns revealed a distinct dipolar pattern in magnetic changes, indicating magnetization
loss at a depth of roughly 200 m beneath the 62-II crater. Subsequent repeat surveys suggested the continuous
demagnetization at almost the same position with an approximately constant rate. The maximal cumulative changeexceeded 150 nT for the four years by 2012. The equivalent change in the magnetic moment amounted to 1.3×106
Am2 /yr. The most likely explanation of this demagnetization is the thermo-magnetic effect due to heating beneaththe crater. This result implies that heat supply from depth is larger than discharge from the vent. Throughseveral approximations and assumptions, we obtain an equivalent thermal accumulation as an order of 1014-15
J/yr; corresponding to several tens of MW. This thermal demagnetization accompanying the ground inflation may
be a part of the preparation process to forthcoming surface manifestation, although the estimated thermal energy
may also be accounted for the fluctuation of heat discharge from the vent. Further accurate measurements ofsurface heat loss may be a key to a better evaluation of the supply from depth.

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