Semi-Diurnal Tidal Periodicity Observed by an Ocean Bottom Seismometer Deployed at a Location Very Close to Seafloor Fumaroles in Wakamiko Caldera, Northeast of Sakurajima Volcano(＜Special Section＞Sakurajima Special Issue)

An ocean bottom seismometer (OBS) recorded obvious semi-diurnal periodicity of the average velocity amplitudes of ground motions on the seafloor of Wakamiko Caldera (an active submarine volcano) northeast of Sakurajima Volcano in southwestern Kyushu, Japan. The ground motions were probably generated by the activity of the calderaʼs seafloor fumaroles, because we found bubbles ascending from those fumaroles just after deployment of the OBS. We compared changes in root-mean-square ground-velocity amplitudes in one-minute windows (RMSAs), tidal gravities (accelerations), and water levels during the observation period to obtain the characteristics of the periodicity. Those characteristics are summarized as follows: 1) We observed clear semi-diurnal periodicity of the RMSAs throughout September, 2007, though sometimes the periodicity was less obvious. 2) The timing of maxima RMSAs corresponded to maximum tidal gravities in the time domain. 3) The frequencies of four peaks seen in the power spectra of the changes in RMSAs were identical with those of the four major tidal components. 4) In detail, changes in RMSAs show saw-tooth shapes, and are irregular in periods of diurnal inequality. 5) Long-term or irregular changes in fumarole activity are possibly dominant in the period. The activity of hydrothermal fluids ascending from the deeper portion toward the seafloor fumaroles, which make up part of the circulation of a hydrothermal system, could be advanced as increasing upward tidal gravities (accelerations).