Analysis of the Variation of Geomagnetic Total Force at the Kusatsu-Shirane Volcano: The Remarkable Changes in the Geomagnetic Total Force in 1990 and the Estimated Thermal Demagnetization Model
Akira Yamazaki, Masaaki Churei, Satoru Tsunomura and Shinzaburo Nakajima
Abstract
Repeat measurements of the geomagnetic total force at the Kusatsu-Shirane volcano using a proton-precession magnetometer have been carried out nearly every year since 1976 by the Kakioka Magnetic Observatory. During a measurement in September, 1990, remarkable changes in the geomagnetic total force were observed. The maximum change value during one year amounted to -11.1 nT at the No. 5 observation point located southeast from Yugama. Increases of total force were observed at the northern area of Yugama, and decreases at the southern area. Such changes can be explained by a 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 a high level of volcanic activities. It is inferred that the geomagnetic total force changes are caused by the thermal demagnetization of the rocks under the vicinity of Yugama. By the use of the 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 the Kusatsu-Shirane volcano and many other andesitic volcanoes.
