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S U M M A R YA self-potential (SP) survey is conducted in Izu-Oshima volcano to reveal subsurface fluid flows during a dormant period. At the southwestern slope outside the caldera, terrain-related SP is observed. Higher potentials are observed in the southern caldera, where fumarolic and thermal activities are found. Contrarily, lower potentials are observed in the northern caldera. This region coincides with an area where lavas pile thicker than that on the outside. 3-D simulations of the groundwater flow due to meteoric water infiltration and induced SP are performed with a heterogeneous electrical resistivity model, to understand the fundamental groundwater flow regime and causes of the observed SP. The observed SP variation is strongly affected by the subsurface resistivity structure and streaming potential within the unsaturated zone, and an overall pattern of the observed SP is reproduced without including an effect of thermally driven upward flows. The terrain-related SP at the southwestern slope is caused by the downward flow of the meteoric water in the unsaturated zone as proposed by previous researchers. The primary cause of the high SP observed in the southern caldera is shallow conductors connecting with a lower conductive layer. Contribution from the upward fluid flows due to the thermal activities, which was believed to be the main cause of the high SP at active areas, is thought to be smaller. The lower potential at the northern caldera is interpreted by the greater streaming potential as compared to that of the surroundings. This can be caused by the difference in the electrical and/or hydraulic parameters of lavas compared with those of pyroclastic rocks. One of the probable reasons of the greater streaming potential is the greater magnitude of zeta potentials of lavas. Probably, lower vertical permeabilities of lavas also lead to the greater streaming potential.

Emission of large amount of volcanic gas occurs at the summit crater of Satsuma-Iwojima volcano located south of Kyushu, Japan. Sakurajima Volcanological Observatory (SVO), DPRI, Kyoto University has conducted seismic observation at a permanent station 1.5 km WNW of the summit crater since June 1995. Volcanic earthquakes at the volcano are classified into A-type (high frequency), B-type (low frequency), C-type (monochromatic) and tremor. A-type earthquake generated by shear fracture and B-type earthquake with volumetric source are dominant at the volcano. Volcanic activity of Satsuma-Iwojima was evaluated from seismicity of the volcanic earthquakes. During the period of 1995-1998, the seismicity has remained at a constant level with seismic energy release rate of 7×10 14 erg/year. The seismicity was as high as that during 1975-1977. The energy release rate of volcanic earthquakes at the volcano has been stable at least for recent 20 years. Within the stable activity, a minor increase in activity occurred in mid-1990's, as shown by a felt earthquake (M2

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