AN ANOMALY OF ELF BAND VERTICAL MAGNETIC-FLUX AS A PRECURSOR OF DOME FORMATION AT UNZEN VOLCANO AND ITS MODEL ANALYSIS

A peculiar environmental vertical magnetic flux anomaly was observed a t the frequency of 223 Hz, which was 40 and 75 days before the 10th an d 11th dome formations on Unzen volcano. The small but clear anomaly o f order of 0.5 pT/root Hz had appeared just on the vertical magnetic f lux and never on the horizontal ones, and showed in many cases an inte rmittent oscillations of about a half-to 1-mill period with almost the same size amplitudes. In this paper, we show a possibility that the v ertical anomaly corresponds to the electromagnetic phenomena when magm a passes the upper layer of aquifer under a volcano and give a simplif ied model which represents the phenomena and mechanism of generating t he anomalous vertical magnetic flux. The anomaly lasted from the end o f 1993 to September 10, 1994. After the termination of the appearance, the Unzen volcano weakened the activity and no events have been repor ted since then. (C) 1998 Elsevier Science B.V.