ABSTRACT
Gamma-ray surveys at ten points in the Iwo-jima Island were carried out using a Nal(Tl) detector. The gamma-ray spectrum was obtained with a multichannel pulse height analyzer. Strong intensity of the gamma-rays 214Bi, an offshoot of inert gas 222Rn, was observed at the northern part of the island as expected, which is active area. The data of gamma-ray survey indicated the existence of fluid transportation at the northern part of the island. It was proven that the gamma-ray survey method were useful for estimating the activity of a volcano.
The stable isotope ratios of hydrogen and oxygen, tritium concentration, and concentration of chlorine ion in the volcanic water from the island were measured. The heavy δ D,δ 18O, low tritium and chlorine ion concentration were obtained. The data obtained indicated that the origin of the volcanic thermal waters from the Iwo-jima Island should be ascribed to the deep seawater, and that many micro-earthquakes occurring at the Iwo-jima Island are caused by the flow, expansion, or vibration of fluid body in the vapor-liquid separation process of seawater which is being repeated at the depth of underground.
