Geomagnetic Observation at Aso Volcano(Ⅰ)
Kato, Y.
Abstract
In order to make the fundamental reserch for prediction of volcanic eruptions, geomagnetic observations at Aso volcano started from 1979. The geomagnetic total field is continuously measured at two stations. One of these stations, AHK (Asosanroku) is situated at a distance of about 5 km to the north of Nakadake crater and the other, ASJ (Aso sanjyo) at a distance of about 0.8 km to the north-west-by-west of Nakadake crater. In addition geomagnetic surveys have been carried out twice a year. Total field is observed at 6 sites by the portable proton-magnetometer. Component fields are observed at 5 sites by the magnetometer theodolite GSI and the portable proton-magnetometer.
Mutual differences in the total field at night (00h-02h JST) between AHK, ASJ and Kanoya are calculated. They appear to include the noise due to the local difference of time-varying field of external origin. For the purpose of reducing the noise, correction for the difference of declination and inclination between compared stations and the reference station and for the locality of Dst field was made. As the result, the standard deviation of the monthly total field difference was reduced to about one-third of the value before the correction was applied.
Some examples in the total field differences between AHK and ASJ that might be related to the volcanic activity was observed. The typical anomalous variations were observed before and after explosion occurred on Sept. 6, 1979. Before the explosion there was a rapid increase in the total field difference, but no remarkable time change of the total field difference was observed after the explosion.
