IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS007 Poster presentation 1965
Magnetic and electric field variations associated with the tilt-step event
during the caldera formation of Miyake-jima volcano in 2000.
Dr. Yoichi Sasai
Disaster Prevention Division, Tokyo Metrop. Gvt. Disaster Prevention Specialist (Chief) IAVCEI
Makoto Uyeshima, Motoo Ukawa, Jacques Zlotnicki, Eisuke Fujita, Hideki Ueda
In the 2000 eruption of Miyake-jima volcano, central Japan, a new caldera was formed on the summit
of the volcano. It started from a sudden depression of the summit area in the existing Hatcho-Taira
caldera on July 8. The new sinkhole of 900 m in diameter and 200 m in depth enlarged up to 1.6 km in
diameter and 500 m deep until the August 18 largest eruption. A rapid ground deformation called the
tilt-step event took place once or twice a day during the caldera formation period (July 8 to August 18),
which was a sudden step-like inflation of the volcano edifice followed by a gradual shrinkage for several
hours (Ukawa et al., 2000). The velocity waveform of the ground motion was a single sinusoidal wave
of 50 seconds duration. The magnetic field showed a step-like change within one minute, which was
observed by proton magnetometers at several sites on the island with its measurement interval of 1
minute (Sasai et al., 2002). The electric field also varied with this event as detected by the long baseline
(a few km distance) SP measurement system using telephone cables with a sampling interval of 10
seconds. Unlike magnetic case, the electric field showed a single bay-like variation very similar to the
velocity waveform (Sasai et al., 2002). Recently, two more clear evidences have been found for the
electric and magnetic variations, namely the short-span (150 m) multi-channel SP measurement with 2
seconds sampling on the southwestern side (Zlotnicki et al., 2003), and 3 components magnetic data by
two flux-gate magnetometers with 1 second sampling on the north- and south-western side of the
volcano. All these EM data are compiled to clarify the generating mechanism of this unique volcanotectonic
phenomenon, which should be essentially related to the caldera formation process (Fujita et al.,
2002). Since the time-dependent behavior is different between magnetic and electric signals, the
generation source must be different. The magnetic variation is most probably ascribed to the
piezomagnetism of rocks due to stress changes, while the electric one to the electrokinetic effect due to
the groundwater movement induced by rapid stress changes and/or fluid injection from the pressure
source. Fujita et al., 2002, Cyclic jerky opening of magma sheet and caldera formation during the 2000
Miyakejima volcano eruption, Geophys. Res. Lett., 29, 10.1029/2001GL013848. Sasai et al., 2002,
Magnetic and electric field observations during the 2000 activity of Miyake-jima volcano, central Japan,
Earth Planet. Sci. Lett., 203, 769-777. Ukawa et al., 2000, The 2000 Miyakejima eruption: Crustal
deformation and earthquakes observed by the NIED Miyakejima observation network, Earth Planets
Space, 52, xix-xxvi. Zlotnicki et al., 2003, Resistivity and self-potential changes associated with volcanic
activity: The July 8, 2000 Miyake-jima eruption (Japan), Earth Planet. Sci. Lett., 205, 139-154.
Keywords: tilt step, piezomagnetic effect, electrokinetic effect