1997 Volume 49 Issue 11-12 Pages 1293-1316
Miyake-jima Island, about 150 km south of Tokyo in Izu-Bonin Arc, is one of the most active basalt volcanoes in Japan. Big eruptions took place in 1940, 1962 and 1983. In this volcano, magma ascends towards a depth 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 forecasts are very difficult to make with mechanical methods (i.e., seismic and deformation measurements) alone. We have developed an electromagnetic monitoring system of the volcano that combines magnetic, resistivity and electric field (SP) measurements. We expect that magma injection and the hydrothermal materials dispatched from it will result in thermal 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 fissure zones. SP surveys brought to light distinct anomalies, which strongly suggest a close relation to the eruption mechanism. They are a positive anomaly up to 700 mV centered around the summit, and two negative ones amounting to -250 mV on the north and -100 mV on the southwestern mountainside. These anomalies can originate from a common mechanism: Rainwater penetrates from fissure zones along fractures toward the center of the volcano, a few km deep, where it is warmed by the heat supplied from deep-seated magma to rise through the summit vent. The down flow makes the negative, while the upwelling the positive SP anomalies, respectively. Miyake-jima island is located near the path of the Kuroshio, the most dominant ocean current in the western Pacific. A large 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 and to properly eliminate their effects.
