ELECTROMAGNETIC MONITORING OF MIYAKE-JIMA VOLCANO, IZU-BONIN ARC, JAPAN - A PRELIMINARY-REPORT

Miyake-jima Island, about 150 km south of Tokyo in Izu-Bonin Arc, is o ne of the most active basalt volcanoes in Japan. Big eruptions took pl ace in 1940, 1962 and 1983. In this volcano, magma ascends towards a d epth of a few km below the summit without any significant earthquakes or deformation, then gives rise to flank fissure eruptions because of the blockaded vent just beneath the summit crater. Hence eruption fore casts are very difficult to make with mechanical methods (i.e., seismi c and deformation measurements) alone. We have developed an electromag netic monitoring system of the volcano that combines magnetic, resisti vity and electric field (SP) measurements. We expect that magma inject ion and the hydrothermal materials dispatched from it will result in t hermal demagnetization, resistivity change and SP variations together with the electrokinetic-magnetic effect. Since October, 1995, we have continuously operated eight well distributed proton magnetometers over the island as well as two SP measurement systems on the NE and SW fis sure zones. SP surveys brought to light distinct anomalies, which stro ngly suggest a close relation to the eruption mechanism. They are a po sitive anomaly up to 700 mV centered around the summit, and two negati ve ones amounting to -250 mV on the north and -100 mV on the southwest ern mountainside. These anomalies can originate from a common mechanis m: Rainwater penetrates from fissure zones along fractures toward the center of the volcano, a few km deep, where it is warmed by the heat s upplied from deep-seated magma to rise through the summit vent. The do wn flow makes the negative, while the upwelling the positive SP anomal ies, respectively. Miyake-jima island is located near the path of the Kuroshio, the most dominant ocean current in the western Pacific. A la rge magnetic variation amounting to several nT was observed to result from the meander of the flow path. This phenomenon produces serious EM noise and complicates monitoring for volcanic activity. It is crucial to investigate the characteristics of motionally-induced EM fields an d to properly eliminate their effects.