Aeromagnetic Anomalies over the Area of Unzendake Volcano.

An aeromagnetic survey flight over the area of Unzendake volcano was conducted in August, 1991, after the commencement of this time eruption, and a geomagnetic anomaly map was derived from collected data. The comparison of magnetic anomalies with the ones in 1981 by the New Energy Development Organization was attempted in order to judge whether there was a significant change of magnetic anomaly pattern for these ten years. However, no large variations with wavelengths more than 6 km were detected. The magnetic anomalies of the area revealed a close correlation with the topography. Average magnetization of normally magnetized terrain was estimated to 2.9 A/m from statistical analysis, and the effect of topographic relief was removed. The residuals were interpreted in terms of a graben structure filled with magnetic volcanics and shallower lateral inhomogeneity of magnetization. And the possibility was pointed out that an old volcanic unit unknown around Sarubayama might have considerable extension under the surface.