Interpretation of time changes in the apparent resistivity observed prior to the 1986 eruption of Izu–Oshima volcano, Japan

Time changes in electrical resistivity were observed before the 1986 eruption of Izu–Oshima volcano, Japan. The aim of this paper is to try to interpret these observed changes in terms of the evolution of a conducting body that simulates the ascending magma by finite difference numerical modeling. Before the time changes were examined, it had been shown that the present numerical model well reproduces the spatial characteristics of the observed apparent resistivities. After some trials involving forward calculation, the time changes, observed during a few months before the eruption, were found to correspond to the formation of a small magma reservoir several hundred meters below the summit. The volume of this reservoir was estimated to be 5×106 m3, which is in good agreement with the volume of magma drained back from the conduit after the eruption, as estimated from repeated gravity surveys. By comparing the modeling results and observations, the mean ascending velocity of the magma head was estimated to be about 100 m per month during the ten months before the eruption.