An anomaly of ELF band vertical magnetic flux as a precursor of dome formation at Unzen volcano and its model analysis

Abstract A peculiar environmental vertical magnetic flux anomaly was observed at the frequency of 223 Hz, which was 40 and 75 days before the 10th and 11th dome formations on Unzen volcano. The small but clear anomaly of order of 0.5 pT / Hz had appeared just on the vertical magnetic flux and never on the horizontal ones, and showed in many cases an intermittent oscillations of about a half- to 1-min period with almost the same size amplitudes. In this paper, we show a possibility that the vertical anomaly corresponds to the electromagnetic phenomena when magma passes the upper layer of aquifer under a volcano and give a simplified model which represents the phenomena and mechanism of generating the anomalous vertical magnetic flux. The anomaly lasted from the end of 1992 to September 10, 1994. After the termination of the appearance, the Unzen volcano weakened the activity and no events have been reported since then.