Volcanomagnetic effect observed during the 1986 eruption of Izu-Oshima Volcano.

Abstract

Remarkable changes in the total geomagnetic intensity were observed associated with the 1986 eruption of the Izu-Oshima Volcano. They are summarized as follows: (1) Long-term precursor: decrease in the total intensity at one station on the southern slope of the central cone, Miharayama, continued since 1981, with a constant rate of -5nT/yr. (2) Middle-term precursor: the decrease there accelerated beginning in April, 1986. (3) Short-term precursor: in October, 1986, an increase in the total intensity was observed at two stations on the north side of Miharayama, while readings at two southern stations decreased. (4) At the first stage of the Phase I eruption, step-like changes were observed at the two southern stations. (5) A decrease in the total intensity was observed at two other stations outside the caldera at the time of the fissure eruption on November 21, and a few survey points and one station showed significant changes. (6) For a few months after the eruption, no remarkable magnetic variations were found outside the caldera.(1), (2) and (3) are interpreted as thermal demagnetization of vent materials due to high-temperature volatiles dispatched from ascending magma. (4) is most likely due to rotation of vent materials. (5) is attributed to the piezomagnetic effect, together with the thermal demagnetization, due to intrusive dykes. In the 1950 eruption a demagnetized area with a 2km radius was formed at a depth of 5km (RIKITAKE, 1951a). In contrast, during the Phase II activity on November 21 and 22, 1986, an intrusive event is suggested to have occurred beneath the southeastern region of the volcano, as inferred from the observed total field changes.