Tidal influence on the 2000 Miyake jima eruption and its implications for hydrothermal activity and volcanism

During the 2000 Miyake jima volcanic activities, forty-six steps on tilt meters and earthquake bursts in Miyake-jima were observed over a 40-day period. These tilt steps and earthquake bursts seem to have a diurnal or semi-diurnal periodicity. Earthquake bursts were high around tilt steps. The correlation of tilt steps and calculated shear strain components of tidal force was examined. If we consider some scatter of direction, shear strain components of tidal deformation show good coincidence with tilt steps. The coincidence is best in the N45°E-N70°E directions. A model to explain tilt steps by tidal triggering is proposed. According to this model, the tidal force triggered or accelerated the opening of gaps between volcanic conduit and capped-rock near the summit. The shear movements in the N45°E-N70°E directions might effectively act to open gaps between conduit and capped-rock and/or existing cracks. Vapor, sulfur dioxide, and/or carbon dioxide are candidates for the pressure source in the volcano conduit. If the tidal effect on volcanism in Miyake-jima can be applied to other cases, many similar observations can be explained.