ALBERT

Description: 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 measurem nt 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 arc 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 crucil to investigate the characteristics of motionally-induced EM fields and to properly eliminate their effects. © 1997, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

Description: Intensive observations of the geomagnetic field have been carried out since 1976 in the Izu Peninsula, Japan. An array of observation sites covers the area of anomalously high microearthquake activity persisting since 1975. The array also extends over the region of crustal uplift amounting to 15 cm or thereabouts which was revealed at the beginning of 1976. At present an observation system consisting of four types of stations is in operation. The total intensity of the geomagnetic field has been measured continuously by proton precession magnetometers at four stations. At two other temporary stations the total intensity is to be measured whenever earthquake activity becomes high in their vicinity. Three components of geomagnetic field variations have also been measured by a flux-gate magnetometer at another station. Finally, 45 observation sites have been established for repeated surveys of the total intensity. In addition to the repeated surveys at these sites, a small-scale experiment based on a synchronized measurement of difference in the total intensity between station pairs has been under way at an array of 14 sites located near the center of the crustal uplift. During the period from 1976 to 1978, four major earthquakes took place within a distance of 30 km from the array: a M 5.4 event of August 1976, a M 7.0 one of January 1978, a M 5.0 one of November 1978, and a M 5.4 one of December 1978. In association with these earthquakes, some changes were detected in the total intensity and also in short-period geomagnetic variations. In the case of the largest event of magnitude 7.0, the difference in the total intensity between the two stations, where continuous measurements had been made, underwent a change about two months prior to the shock. The change was very similar in time sequence to that in the electric self-potential observed near one of the above two stations. Amplitudes of short-period geomagnetic variations also changed about two months before the shock. The result of repeated surveys disclosed a striking pattern of spatial distribution of presumably coseismic changes which is quite opposite to that obtained during a post-earthquake period. About two months prior to the M 5.0 earthquake which occurred several kilometers east of another station for continuous measurements, the total intensity remarkably decreased by 5 nT or so at the station. The field abruptly recovered after its occurrence; a coseismic change amounted to about 5 nT. In the case of the M 5.4 event of 1978, no coseismic change was detected at a temporary station located a few kilometers west of the aftershock area. Changes in amplitude of short-period geomagnetic variations also appeared before this M 5.4 earthquake. © 1980, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.