Analysis of the Variation of Geomagnetic Total Force at Kusatsu-Shirane Volcano: The Remarkable Changes in the Geomagnetic Total Force in 1990 and the Estimated Thermal Demagnetization Model
Yamazaki, A., Churei, M., Tsunomura, S. & Nakajima, S.
Abstract
From 1976, repeat measurement of geomagnetic total force at Kusatsu-Shirane Volcano using proton-precession magnetometer has been carried out nearly every year by Kakioka Magnetic Observatory. At the measurement in September, 1990, remarkable changes in the geomagnetic total force were observed. The maximum change value during one year amounted to -11.1nT at No.5 observation point located in the southeast from Yugama. Increases of total force were observed at northern area of Yugama, and decreases at southern area. Such changes can be explained by the demagnetization of the material under the volcano. On the other hand, volcanic earthquakes and volcanic tremors have occurred frequently since February, 1990. Further, geochemical data have indicated high level activities of the volcano. It is inferred that the geomagnetic total force changes are caused by the thermal demagnetization of the rocks under near Yugama. By the use of least squares method, we have proposed two thermal demagnetization models and found that the estimated demagnetized area almost coincided with the focuses of volcanic earthquakes. These results imply that geomagnetic observation is a useful method for understanding volcanic activities at Kusatsu-Shirane Volcano and many other andesitic volcanoes.
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