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 me ters and earthquake bursts in Miyake-jima were observed over a 40-day perio d. These tilt steps and earthquake bursts seem to have a diurnal or semi-di urnal periodicity. Earthquake bursts were high around tilt steps. The corre lation of tilt steps and calculated shear strain components of tidal force was examined. If we consider some scatter of direction, shear strain compon ents of tidal deformation show good coincidence with tilt steps. The coinci dence is best in the N45 degreesE-N70 degreesE directions. A model to expla in 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 subunit. The shear movements in the N45 de greesE-N70 degreesE directions might effectively act to open gaps between c onduit and capped-rock and/or existing cracks. Vapor, sulfur dioxide, and/o r carbon dioxide are candidates for the pressure source in the volcano cond uit. If the tidal effect on volcanism in Miyake-jima can be applied to othe r cases, many similar observations can be explained.