CSAMT MEASUREMENTS ACROSS THE 1986 C-CRATERS OF IZU-OSHIMA ISLAND, JAPAN

Controlled-source audiofrequency magnetotelluric (CSAMT) measurements were carried out across the 1986 craters of Izu-Oshima Island one year after the November eruptions. After 2D analyses of the two profiles, excepting shallow resistivity variations, basically two-layered structures were found. A deep conductive layer below the resistive lava corresponds to the thermal water Shallow (<200 m) conductive bodies corresponding to an old 8th century vent and one of the 1986 craters were also located. These are probably fracture zones containing meteoric water. Away from the profiles, a distinctive three layered structure was discoverd with a freshwater lens occurring between an upper resistive lava and a lower sea water layer. It is thought that the two profiles lacked the fresh water lens due to mixing of fresh and saline water by thermal convection. -after Authors