Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 269-279

ArticleSemi-Diurnal Tidal Periodicity Observed by an Ocean Bottom Seismometer
Deployed at a Location Very Close to Seafloor Fumaroles inWakamiko Caldera, Northeast of Sakurajima Volcano
Hiroshi YAKIWARA＊, Shuichiro HIRANO＊, Hiroki MIYAMACHI＊, Tetsuro TAKAYAMA＊＊,Tomoya YAMAZAKI＊＊,＊＊＊, Takeshi TAMEGURI＊＊ and Masato IGUCHI＊＊(Received January 4, 2011 ; Accepted December 26, 2012)
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 theOBS. 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 spectraof 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 asincreasing upward tidal gravities (accelerations).
Key words : semi-diurnal periodicity, tidal gravity, ocean tide loading, seafloor fumaroles, Wakamiko Caldera

1．Introduction
Sakurajima Volcano is located in the northern part ofKagoshima Bay in southwest Japan. Most of Sakurajima
Volcano is surrounded by the sea except its southeasternend (Fig. 1). Hypocenters of volcano-tectonic earthquakesthat occurred around the volcano (Hidayati et al., 2007) werelocated beneath the sea using data obtained by seismic
stations on land. Seismic observations made by ocean bottom seismometers (OBSs) would provide more precise
hypocenters and focal mechanisms for those earthquakes.
In 2007 we performed seismic observation using two OBSs
over two periods in order to obtain seismic data on theearthquakes that are generated beneath the bay. Although
no volcano-tectonic earthquakes beneath the bay occurred
during the monitoring periods, we found obvious semidiurnal changes in root-mean square ground-velocity
amplitudes in one-minute time windows observed by one＊Nansei-Toko Observatory for Earthquakes and Volcanoes,
Graduate School of Science and Engineering, Kagoshima
University, 10861 Yoshino, Kagoshima 892-0871, Japan.
＊＊
Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University, 1722-19 Sakurajima-Yokoyama, Kagoshima 891-1419, Japan.

OBS installed on the seafloor of Wakamiko Caldera, northeast of Sakurajima Volcano (see Fig. 1). The clear, quasisteady semi-diurnal periodicity agreed with the solid earth
tide in the time domain throughout the observation period,
which was over one month. Also, frequencies of fourpeaks derived from the power spectrum analysis for thechanges in root mean squares are good agreement with thefrequencies of the four major tidal components.
Tides can influence the periodicity of submarine hydrothermal activity (Glasby and Kasahara, 2001). The semidiurnal periodicity observed on the seafloor of Wakamiko
Caldera, therefore, suggests that changes in solid earth
tides and/or ocean tide loadings modulate submarine fumarole activity. Several researchers have investigated theperiodicities of tectonic earthquakes or volcanic activity.
Glasby and Kasahara (2001) reviewed the influence oftidal effects on the periodicity of tectonic earthquakes,＊＊＊

Disaster Prevention Research Institute, Kyoto University,
Gokasyo, Uji 611-0011, Japan.
Corresponding author: Hiroshi Yakiwara
e-mail: yakiwara@sci.kagoshima-u.ac.jp