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 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 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

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Hiroshi YAKIWARA, Shuichiro HIRANO, Hiroki MIYAMACHI, Tetsuro TAKAYAMA,
Tomoya YAMAZAKI, Takeshi TAMEGURI and Masato IGUCHI

Fig. 1. Location of seismic stations. The locations ofocean bottom seismometers (OBSs) in the first andsecond observation periods are shown by crosses anddiamonds, respectively. A star indicates the position of
the Kagoshima Tidal Gauge. Thin and thick gray lines
are the isobaths every 10 m and 50 m, respectively.
Dotted curves illustrate the rims of calderas. Circles
show the epicenters of volcanic earthquakes that
occurred in the observation periods.

volcanic earthquakes, and submarine hydrothermal activity.
Excluding tectonic earthquakes and non-volcanic deep
tremors, several case studies of periodicity in volcanic orsubmarine hydrothermal activity were summarized in theirreview. Nasu et al. (1931) compared changes in waterlevels and the hourly numbers of earthquakes in andaround the east Izu Peninsula for 6 days in 1930. They
pointed out that times of low water corresponded with highearthquake frequency, and concluded that withdrawals of
oceanic tide loading were the trigger actions that set inmotions the hourly numbers of earthquakes. Rinehart(1972) investigated changes in the intervals between
ejections of three geysers in the United States for about 6
months in 1967, and showed that short-term tidal forces
and small earthquakes influenced periodic fluctuations in
the intervals. Kasahara (1984) deployed an OBS at 3.5
days after the beginning of the eruptions of Miyakejima
Volcano in 1983. They observed that the hourly numbersof earthquakes observed by the OBS increased at low-tidelevels in the first stage of the observation period and highand low-tide levels in the last stage.
Excepting Rinehart (1972), these case studies have compared only hourly numbers of events and tidal changes.
However, Nishizawa et al. (1995) used an ocean bottom
seismometer with a hydrophone (OBSH) at the TransAtlantic Geotraverse (TAG) hydrothermal field on the

Mid-Atlantic Ridge to record many pulse-shaped signals
with periods of more than one minute. The seismometerrecorded no ground motions corresponding to the signals.
They also compared the average amplitudes of the hydrophone outputs and theoretical water levels (ocean tides)
visually in the time domain over a period of one week andconcluded that the averages showed obvious increases at
low-water levels. Similar pulse-shaped events have also
been detected in the South Mariana Trough (Sato et al.,1995) and Middle Okinawa Trough (Kasahara et al., 1995).
In both these cases, the events were recorded only on thehydrophone channel, similar to Nishizawa et al. (1995).
Sato et al. (1995) demonstrated that period of high activity
corresponded to high-water levels over a period of eightdays. Kasahara et al. (1995) compared event activity withsolid earth and ocean tides for five days. Larger amplitudeactivity was observed just before earth tide minima, withsome exceptions. The coincidence among the solid earth
and ocean tides and the activity observed at the Okinawa
and South Mariana troughs, however, were less obvious
than that seen at the TAG.
After these periodicity studies, two studies showed thecomparison between volcano activity and changes in tidal
gravities (accelerations) in the frequency domain. Neuberg(2000) performed spectrum analysis and found that theperiodicities of seismic noises observed at volcanoes(Ruapehu, Merapi and Batur volcanoes) had diurnal andsemi-diurnal frequency peaks. The report also compared
the observed peaks and the theoretical tidal gravities in thefrequency domain, and concluded that the observed frequency peaks were distinguishable from the most prominent frequency components of tidal gravities, and that theperiodicity was not caused by tidal gravities. In the figuresof Neuberg (2000), we cannot recognize a correspondence
between changes in volcanic activity and the theoreticaltide gravities in the time domain. As candidates for theorigin of the periodicity, Neuberg (2000) proposed othernatural sources (temperature-induced stress fields, heavy
rain, atmospheric pressure, and melting glaciers). Following Neuberg (2000), however, Hagerty and Sherburn(2002) rejected the periodicity observed at Ruapehu volcano as being a natural phenomenon. On the other hand,
Custodio et al. (2003) presented an analysis of tidal periodicity of seismic noise and volcanic tremor observed at
Fogo Volcano, Cape Verde. The seismic periodicity wasstrong enough to be detected by simple visual inspection ofa long record of the raw signal (Custodio et al., 2003).
The most dominant spectrum peak, at 1.93 cycles per day,
was obviously analyzed. A comparison of the seismic
spectrum and the theoretical tidal gravities showed that the
spectrum peak, with 1.93 cycles per day, agreed well withthe most prominent component in the tidal gravities causedby the Moon. Unfortunately, the seismic signals were notcompared to changes in tidal gravities in the time domain
in the study.

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

To our knowledge, there is very little published literature that draws a parallel between signal amplitudes of
volcanic or hydrothermal activities and changes in tides inthe time domain. Most previous studies have been focusedon either time sequences for numbers of earthquakes orspectrum peaks. Also, there are no reported periodicities
of hydrothermal activity on shallow seafloors because observations using OBSs targeting seafloor hydrothermal
activities have been carried out only in the deep-sea bottoms of troughs or mid-ocean ridges. This study presents aclear and steady semi-diurnal periodicity in time and frequency domain observed very close to the shallow-seafloor hydrothermal fumaroles of Wakamiko Caldera anddiscusses the cause of the periodicity, comparing the observed periodicity, changes in tide levels and solid earth
tides.
2．Wakamiko caldera
Wakamiko Caldera (Kuwashiro, 1964; Shimomura, 1960)
is a submarine caldera having a WNW-ESW elongatedellipsoidal depression. It is located in the northeastern quarter of Aira Caldera (Matumoto, 1943). Wakamiko Caldera(6.5 km×5 km) has flat bottom 200 m under the sea surface (Aramaki, 1984). Aira Caldera caused a huge eruption
about 29,000 yrs B.P. (Aramaki, 1984; Nagaoka, 1988;
Okuno, 2002). The eruption produced the Osumi pumice
fall, the Tsuyama pyroclastic flow, and the Ito pyroclastic
flow (Aramaki, 1984; Nagaoka, 1988). The total volumeof its products has been estimated at about 150 km3(Machida, 2003). The vent of the largest eruption has
been estimated to lie in the Wakamiko Caldera, based on
the distribution of the dense breccia that erupted at an
initial stage of the Ito pyroclastic flow (Nagaoka, 1988).
Fishermen have seen bubbles just below the sea surface
in several areas in the extent of the caldera for many years.
The bubbling phenomenon, called “Tagiri” by the local
residents, provides visual evidence that volcanic activity of
the caldera is still active. Chujo and Murakami (1976)
conducted the first oceanographic survey of the area with a
sub-bottom profiler in 1975. They found sea bottom fumaroles located in several areas on the submarine caldera
floor. The profiler illustrated that the fumaroles generated
many bubbles at the sea bottom, and the bubbles floated tothe sea surface. Additionally, Ossaka et al. (1992) investigated the submarine fumaroles in 1976, 1977, 1991 and1992 using a submersible. They found over 30 areas withactive fumaroles and collected samples of the volcanic
gasses they emitted. They also measured temperatures justabove the hydrothermal vents. The maximum temperature
was 215° C. Yamanaka et al. (2001) did a geochemical
study of Wakamiko Calderaʼs seafloor hydrothermal system.
They investigated the fumaroles in 1998, 1999, and 2001
with a submersible to sample the shimmering fluids andbottom water around the fumaroles. Based on chemical
analysis, their results suggest that hydrothermal circulation

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occurs at the seafloor. Ishibashi et al. (2008) collected
sediment and pore fluid samples from the seafloor with asubmersible in 2003 and 2005 for geochemical analysis.
They estimated the temperature of the geothermal reservoir to be 205° C using a silica geothermometer.
Hidayati et al. (2007) investigated the hypocenters andfocal mechanisms of the volcano-tectonic earthquake
activity of Sakurajima Volcano. They proposed a modelthat described the plumbing system of the magma beneath
Aira Caldera and Sakurajima Volcano based on the regional stress field inferred from focal mechanism solutions
and geodetic pressure sources obtained by GPS observation (Kriswati and Iguchi, 2003) and first-order levelingsurveys (Eto et al., 1998). A major magma reservoir
related to the activity of the volcano probably exists
between Sakurajima Volcano and Wakamiko Caldera at a
depth of about 10 km. Because no other clear pressuresource beneath the caldera was identified by the series ofleveling surveys (e.g. Eto et al., 1998), this reservoir may
supply magma and volatiles to Sakurajima Volcano andseafloor fumaroles existing at Wakamiko Caldera.
3．OBS observation
In 2007 two pop-up type OBSs were used to conduct
seismic observations during two monitoring periods. The
OBS is composed of a three-component 4.5 Hz geophone(Mark Products, L28-BL) mounted on a gimbal, a datalogger (Katsujima, KDR-224K) equipped with a highaccuracy timing module, and alkaline batteries in a glass
sphere. The bearings of each horizontal component were
unknown. The sampling rate was 64 Hz and the amplification factor for all channels of the recorder with 16-bit
A/D converters was 40 dB. The digitized data was recorded
using digital audiotape. The OBS can record continuous
data for about two months, limited by the length of the tapeand the total capacity of alkaline batteries.
In the first period of the observation, from August 30
through October 11 (42 days), an OBS was deployed on
the seafloor of Wakamiko Caldera (Fig. 1, N1). Another
OBS was installed on the sea bottom in the south of the
volcano (Fig. 1, S1). As soon as the OBS at N1 wasthrown overboard, we confirmed visually bubbles ascending toward the sea surface below the release point. We,
therefore, expected that the OBS at station N1 was deployed very close to seafloor fumaroles. In the second
period, from October 26 through December 14 (48 days),
we deployed the same OBS units again. As the OBS
deployed at N1 had recorded ground motions of which theamplitudes were relatively large compared to those recorded at S1, station N2 was designed to be situated about
430 m east of station N1 (Fig. 1, N2). The OBS at station
S2, on the other hand, was planned to be deployed as close
as possible to the coordinates of S1 using the real-time
position of the vessels and navigation tools. The operation
was able to place the OBS deployed at S2 only 13.8 m

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Hiroshi YAKIWARA, Shuichiro HIRANO, Hiroki MIYAMACHI, Tetsuro TAKAYAMA,
Tomoya YAMAZAKI, Takeshi TAMEGURI and Masato IGUCHI

Fig. 2. Time changes, for 30 days (one month), in the unfiltered root-mean squares of velocity amplitudes in one minutewindows (RMSAs) for each component for the ocean-bottom seismometers; UD is the vertical component; H1 and H2
are horizontal components.

horizontally from the position of S1.
The internal clock of each OBS was calibrated justbefore deployment and after retrieval by comparing it to a
GPS clock. The internal clock data were adjusted using
the two time-calibration data points, assuming that the drift
rate was constant during the period of observation. Afterthe time calibrations were performed, we found that theinternal clock of the OBS deployed at N1 and N2 fluctuated several times within a few seconds, possibly being
stepwise drifts. Although we could not use the data recorded at N1 and N2 for hypocenter determinations ofearthquakes, the slight fluctuation did not affect the resultsof the present study.
The deployments and retrievals of the OBSs were carried out by a working vessel of 15 gross tons. Because thevessel had no navigation tool, a DGPS antenna and receiver system (SOKKIA, R80D) and the laptop computerthat the electronic navigation charts (ENCs) issued by theJapan Coast Guard were installed, were used to guide thevessel to the objective survey positions. The computerʼsnavigation software (PC Studio Alpha, AlphaMap, Lite
edition) displays the real time position of the vessel, ENCs,
bathymetric maps, and the objective approach points
simultaneously. Heading data issued by a GPS unit
(Germin, eTrex series, Vista) were also used to navigate

incorporating in the software.
The coordinates and depths of each OBS station were
determined as follows. The vessel was navigated to four
points. One was the OBS release position, and the otherthree form a triangle surrounding that point. The horizontal distances between the centroid and each vertex were
nearly equal to the depth of the OBS estimated using thebathymetric map. At each point, we measured direct
distances with acoustic signals between the vessel and thedeployed OBS several times. The vessel coordinates were
logged using the DGPS. The distance data and vessel
coordinates at the moment of the each measurement wereinverted to calculate the coordinate and depth of the OBS.
4．Observed periodicity of ground motion velocity amplitudes
4-1 Changes in root-mean-squares amplitude
Root-mean-squares amplitudes in one-minute time
windows (herein termed RMSAs) were calculated from thecontinuous ground velocity data recorded by the OBSs inorder to compare them. Not only the continuous ground
velocity data but also the RMSAs in this study were notfiltered. Fig. 2 shows changes in RMSAs on three (onevertical and two horizontal) components at each station for
two 30-day periods in September and November, 2007.
The vertical and both horizontal RMSAs observed at N1

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

273

Fig. 3. RMSA changes for 3 days. UD, H1, and H2 are the same as in Fig. 2. The RMSA changes observed simultaneously
at the two seismic stations (JSU and AIRH) on land are also shown. The locations of the stations are plotted in the maps.

were remarkably larger than those at the other stations.
Although station N2 was only about 430 m eastward fromstation N1, the RMSAs at N1 were several times larger
than those at N2. At deployment, as soon as we released
the OBS into the sea above station N1, we saw bubbles
below the sea surface, so we presumed that the OBS on thesea bottom at N1 was deployed very close to highly active
seafloor fumaroles. On the contrary, it is possible that noseafloor fumaroles existed near station N2, because we
saw no bubbles on the sea surface above N2 at deployment
and retrieval of the OBS. As mentioned above, the OBS
deployed at N1 probably recorded ground motions related

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Hiroshi YAKIWARA, Shuichiro HIRANO, Hiroki MIYAMACHI, Tetsuro TAKAYAMA,
Tomoya YAMAZAKI, Takeshi TAMEGURI and Masato IGUCHI

Fig. 4. Comparison of time changes in RMSAs, water levels, and tidal gravities for three days. The top and bottom plots(scale at left) show the RMSA changes over time at N1 and S1, respectively. The second (dotted) curve (scale at right) isthe change over time of water levels observed at KG. The third (thick) curve (scale at right) shows changes in theoretical
tidal gravities (accelerations). Examples of times where maximum and minimum RMSAs occurred at N1 are labeled HA
and LA. HW and LW show the timing of the maximum and minimum water levels, respectively. Vertical dotted lines
correspond to the time of maximum RMSAs at N1.

to seafloor fumarole activity.
4-2 Characteristics of RMSA changes
In Fig. 2 the RMSAs on the horizontal components ofeach station are recognized as mostly quasi-regular. Noticeable RMSAs increases at S1 from September 11 to 20
were possibly excited by two strong typhoons that passed
near Kyushu Island. Additionally, the OBSs at S1 and S2
were set under well-used navigation routes and near anchorage positions in Kagoshima Harbor, so it sufferedfrom vessel noises. Therefore, episodic, irregular increases
are seen in the RMSAs at S1 and S2. On the other hand,the characteristics of the RMSAs at N1 are different fromthose of the other stations as described below.
Fig. 3 shows the changes in RMSAs on each component
for three consecutive days (September 25 through 27, 2007
at N1 and S1 and November 13 through 15, 2007 at N2 andS2). The semi-diurnal changes in the RMSAs at N1 areclearly observable and are independent of the days andnights. The RMSAs have maxima at about noon andmidnight, and minima in the morning and evening over thethree days. Because the OBS at N1 was probably deployedvery close to seafloor fumaroles, the semi-diurnal periodicity observed at N1 suggested that the amplitudes ofground motions generated by the high activity of the sea-

floor fumaroles correlated with periodic phenomenon suchas ocean tide loadings. On the other hand, the changes in
RMSAs on the horizontal components at the other stations(N2, S1 and S2) illustrate “mountain-shaped” diurnalchanges; increases and decreases during the daytime andnighttime, respectively. Also, a sudden decrease in the
RMSAs occurred just after noon for about one hour. The
characteristics of the horizontal RMSAs at N2, S1 and S2
were probably generated by human activities around thebay. The mountain-shaped changes are not seen in vertical
RMSAs but are prominent only in horizontal RMSAs (Fig.
3, N2, S1 and S2), possibly because artificial noises fromland are incident from the almost horizontal direction. On
the other hand, the changes in the RMSAs on verticalcomponent of the ground motion observed at two permanent seismic stations on land (Fig. 3, JSU and AIRH) obviously draw mountain shape. On the contrary no mountainshaped change is shown in N1ʼs RMSAs. The relatively
large amplitudes of the ground motions observed at N1
probably hide the artificial noises.
4-3 Comparisons between RMSAs and tidal phenomena
Fig. 4 compares the RMSAs on vertical components ofground velocity at N1 and S1 to changes in water levels

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

275

Fig. 5. Examples of the raw waveforms (ground velocity,
vertical component) observed at the time of minimum
and maximum RMSAs. The times of LA and HA are
the same as in Fig. 4.

observed by the Japan Meteorological Agencyʼ s Kagoshima Tidal Gauge of (Fig. 1, KG) for three days from
September 25 through 27, 2007. The maxima and minimaof the RMSAs at N1 (Fig. 4, LA and HA) clearly precededat the times of the low- and high-water (Fig. 4, LW and
HW) by about one hour. The relation between the RMSAs
and the water-level peaks observed at KG was not seen at
S1. Aperiodic changes in the RMSAs at S1 did not correlate to the time sequence of the sea water level. Fig. 5
shows an example of the raw velocity waveforms (thevertical component of N1) observed at the times of LA andHA (Fig. 4). Visually, we can recognize that the amplitudes of the waveforms recorded at HA are slightly larger
than those at LA.
The ocean tide loading hardly explains why the maxima
and minima RMSAs precede at the times of the low- andhigh-water. Therefore, we used a source cord (Nakai,1979) to calculate the forces of the solid earth tide tocompare with the changes in RMSAs observed at N1. The
program computes, given any position on the earthʼs surface and a time, the tidal gravity (vertical derivative of
tidal potential) due to the relative astronomical positions of
the Earth, Moon and Sun. Fig. 4 also draws a parallel
between the RMSAs on the ground velocities observed at
N1 and the tidal gravities in the time domain. The maxima
and minima RMSAs at N1 are in sync well with maxima
and minima tidal gravities over the three days. The
vertical RMSAs observed at N1 increase as the tidal

Fig. 6. Comparison between RMSA changes over time at
N1 and theoretical tidal gravities (accelerations) for 30days (September, 2007). Dots and curves are the
RMSAs (scale at left) and the gravities (scale at right).

gravities become larger, and decrease as the tidal gravities
are reduced. Visual inspection of Fig. 4 shows clearly that
there is a strong positive correlation between changes in
RMSAs at N1 and tidal gravities, even without spectrum
analysis. Figure 6 compares changes in vertical RMSAs at
N1 and tidal gravities over 30 days in September, 2007.
With the exception of periods of diurnal inequality (i.e.
from September 19 through 22), the correlation is obvious
throughout the period. The obvious semi-diurnal periodicity observed at N1 is probably a quasi-steady phenomenon associated with the seafloor fumaroles. In addition,
irregular components like the step-like changes in RMSAs
at N1 may reflect changes in activity of the submarine
fumaroles. The long-term changes in the RMSAs shownin periods of diurnal inequality may correlate with changesof seafloor fumarole activity.
In order to find the prominent frequencies of the periodicity, we analyzed power spectra using the changes in
RMSAs at N1 over 30 days in the month. We thencompare the frequencies and the major tidal components ofharmonics. Fig. 7 shows the power spectra of the changes
in RMSAs and theoretical tidal gravities. It is possible thatfour peaks corresponding to the four major tidal

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Hiroshi YAKIWARA, Shuichiro HIRANO, Hiroki MIYAMACHI, Tetsuro TAKAYAMA,
Tomoya YAMAZAKI, Takeshi TAMEGURI and Masato IGUCHI

steeper on the left of the vertex, and the absolute values of
the gradients of increasing RMSAs are somewhat steeper
than the decreasing ones. The RMSAs decrease slowly
after the maxima. An opposite relation between changes
in RMSAs and tidal gravities, however, were observed on
September 28 and 29 (Fig. 6). In Fig. 6, the timing ofseveral peaks of the tidal gravities is different fromchanges in the RMSAs. The relation between the RMSAs
and tidal gravity may not be a linear, but a non-linear
relation.

Fig. 7. Power spectra of the RMSAs at N1 very close to
seafloor fumaroles using data on 30 days in September,2007. Spectrum of the theoretical tidal gravities
calculated by Nakai (1977) is also shown. The names
and peak frequencies (cycles per day; c.p.d.) of thefour major tidal components are written in the bottom
spectrum.

components M2, S2, K1, and O1 are clearly seen. The
frequency of the largest peak is identical with that of theM2 (1.93 c.p.d.) component. Another semi-diurnal peak
(2.00 c.p.d.) can be recognized, and corresponds to thefrequency of component S2. Two diurnal peaks are also
obvious in the spectra and agree with components K1 (1.00
c.p.d) and O1 (0.93 c.p.d.), respectively. Additionally,
relative magnitudes on each peak of RMSAs also can be
explained qualitatively by the relation of the four tidal
components. As described above, a close relationshipbetween the RMSAs at N1 and the tidal gravities isapparent. Therefore, we concluded that the changes in
RMSAs at N1, which probably reflect ground motions
related to seafloor fumaroles, had a strong correlation totidal gravities in the time and frequency domain.
Strictly speaking, the changes in RMSAs at N1 do notprecisely correspond to the tidal gravities, because the
RMSAs at N1 in the period shown in Fig. 4 do not appear
as a sine curve but are saw-toothed and slightly asymmetrical. The changes in the RMSAs seem to be slightly

5．Discussion
We found semi-diurnal periodicity of the changes in
RMSAs observed at the sea bottom very close to active
shallow seafloor fumaroles in Wakamiko Caldera, although
the periodicity was sometimes not so obvious. There are
two important characteristics of the changes in RMSAs.
The first is a one month long quasi-steady phenomena that
correspond to tidal gravity in the time and frequency
domain. The second is that the changes in RMSAs show
saw-tooth shapes and are irregular in periods of diurnal
inequality. To our knowledge, the quasi-steady monthlong tidal-periodicity of amplitudes of ground motions has
not been reported by previous studies. The semi-diurnal
periodicity observed at the TAG (Nishizawa et al., 1995)
was not recorded by a seismometer but only a hydrophone.
The present semi-diurnal periodicity, therefore, differsfrom the phenomena observed at the TAG. The time
sequences in velocity amplitudes of ground motions
observed at Fogo volcano (Custodio et al., 2003) for 5.5
days were compared with tides not in the time domain but
only in the frequency domain. Thus we cannot make adetailed comparison between the Fogo volcanoʼs periodicity and the present one.
Most of the maxima RMSAs were observed around thetimes of maximum tidal gravities (accelerations). This
relationship may indicate that the upward tidal forces drive
the increased activity of the seafloor fumaroles. Also, thetime lags between the RMSAs and tidal accelerations arequite small or nonexistent. These characteristics would
suggest that the activity responds readily to variations inexternal forces. Ocean tide loadings may also affect changes
in RMSAs. As described in the previous section, theslightly asymmetrical changes in the RMSAs suggest thatthe relation between the RMSAs and tidal gravities may bea non-linear relation. We should therefore investigate why
the changes in RMSAs are not symmetrical but slightly
asymmetrical. Because tidal loadings reach maxima about
one hour late, the loadings may delay rises in the fumaroleʼs activity. Also, delay of the minima tidal loading by
about one hour may attenuate the decrease rate of theactivity of the fumaroles in initial of the declining stage.
Let us consider the tidal forces acting on the seafloor as asuperposition of the tidal gravities and the ocean tideloadings. The loadings per unit area are equal to the hydro-

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

277

Fig. 8. Results of a simple simulation assuming a linear combination of tidal gravities (accelerations) and ocean tideloadings. The RMSAs at N1 are also shown by dots (scale, left). The constant of proportionality, k, corresponds to therelative weight of the ocean tide loadings. The thin curves are the normalized tidal forces calculated with four values of
the constant. The constant increases with increasing relative weight.

static pressure, increasing in proportion to water depth.
Assuming that the RMSAs respond linearly to the combination, and in proportion to the tidal accelerations (forces
per unit bulk) and ocean tide loadings, a simple relation
with no time lag, because of the response instance, isderived as
RMSA(t)=c0[a･tg(t)−b･wl(t)](1)

Here, the terms tg(t) and wl(t) are the time changes of
the normalized tidal acceleration and water levels observed
at KG (Fig. 1) using maximum values of those, and a, b
and c0 are constants of proportionality. The second termin the square bracket of the right hand of Eq. (1), b･wl(t),relates to the ocean tide loadings. The negative sign of theterm assumes that the activity of the fumaroles is repressedas the hydrostatic pressure increases. The coefficients express the strengths of the tidal accelerations and theloadings. Moreover, generalizing the relation in Eq. (1),
we obtain the following simple model using coefficients c1
and k.
RMSA(t)=c1[tg(t)−k･wl(t)](2)

Here, c1=c0･a, and k=b/a. Coefficient, k, shows therelative strength (weight) of the tide loadings to theaccelerations; the weight of the loadings increases as thecoefficient increases. We provide several values of thecoefficient, k, to compare with the changes in RMSAs.

Fig. 8 shows the relative changes in RMSAs calculatedusing four values of k. Normalized curves are shown in
the figure. If the coefficient is large, the contribution of
the ocean tide loadings increases, and the peaks of thecalculated RMSAs are later than those observed. It
suggests that the changes in RMSA are mainly controlled
by tidal acceleration of solid earth tides. As a result of thesimple model, no values of k do the saw-tooth shapes of
the RMSAs appear. Thus, the superimposition of tidal
gravities and ocean tide loadings can hardly explain theslightly asymmetrical changes in RMSAs. Interestingly,
the left side of the RMSAs is relatively steep with more
gradual decreases during the decreasing tidal stage (i.e. on
September 13, Fig. 6). On the contrary, the right side isrelatively steep during increasing tidal stages (i. e. from
September 28 through 29, Fig. 6). Long-term changes inseafloor activity may cause asymmetric changes in
RMSAs. Ishibashi et al. (2008) suggested that area ofhydrothermal circulation of Wakamiko Caldera must be
horizontally as large as 5 km on the basis of an investigation of the isotropic composition of the hydrothermal
component of the caldera. The hydrothermal system isthought to be driven by a magmatic heat source beneaththe caldera (Ishibashi et al., 2008). The active fumaroles
on the seafloor play the role of discharging fluids in thehydrothermal circulation around Wakamiko Caldera. The
hydrothermal fluids ascending from the deeper portion

278

Hiroshi YAKIWARA, Shuichiro HIRANO, Hiroki MIYAMACHI, Tetsuro TAKAYAMA,
Tomoya YAMAZAKI, Takeshi TAMEGURI and Masato IGUCHI

toward the seafloor fumaroles may advance with increasing upward tidal forces because of the permeable seabottom sediments. This idea is supported by the fact thatthe semi-diurnal periodicity disappears in periods ofdiurnal inequality, while activity variations are dominant
in the RMSAs changes. Inertia of the hydrothermal fluids
ascending may cause the slightly asymmetrical changes in
RMSAs observed at N1.
6．Conclusion
Using an ocean bottom seismometer, we observed obvious quasi-steady semi-diurnal periodicity of the averageground motion at the sea bottom very close to seafloor
fumaroles in Wakamiko Caldera, an active submarine
volcano, located in the northeastern quarter of Aira Caldera. We compared changes in root-mean-square groundvelocity amplitudes in one-minute windows (RMSAs) withtidal gravities (accelerations) and water levels to obtain thecharacteristics of the periodicity as follows.
1) The semi-diurnal periodicity in RMSAs observed at
Wakamiko Caldera was probably caused by fumarole activity.
2) The periodicity was clearly observed throughout September, 2007, though sometimes it was less obvious. The
timing of maxima RMSAs corresponded to the timing of
maxima tidal gravity in the time domain. Frequencies ofthe prominent four peaks seen in the power spectra are
identical to those of the four major tidal components (M2,S2, K1, and O1).
3) The changes in RMSAs show saw-tooth shapes, andare irregular in periods of diurnal inequality. Long-term or
irregular changes of activity of the fumaroles are possibly
dominant in the period.
We proposed a simple model that relates the change intidal force, as represented by a superimposition of tidal
gravities and ocean tide loading effects to changes in
RMSAs, without time lag, to reproduce the characteristicof the observed saw-tooth changes in RMSAs. Although
the model cannot explain the saw-tooth changes, itsuggests that changes in RMSAs are mainly controlled bytidal acceleration of solid earth tides. The activity ofhydrothermal fluids ascending from the deeper portion
toward the seafloor fumaroles can be advanced asincreasing upward tidal forces.
Acknowledgements
We thank the captain of the working vessel, Mr. K.
Kawabata, for skillful operating the vessel on the voyages
to deploy and retrieve the OBSs. We are grateful to thesecretary, Mr. S. Koyanagi, of the East Sakurajima fishermenʼ s cooperative association and the Japan Fisheries
cooperative of Kagoshima Prefecture for accepting ouroperation. We acknowledge Prof. T. Kobayashi of Kagoshima University for his useful suggestion about theeruption histories of Aira and Wakamiko Caldera. We

express our sincere thanks to Prof. H. Shimizu of Kyushu
University for special suggesting the relation among theobserved periodicity, the tidal gravities, and the ocean tideloadings. The suggestions of two anonymous referees also
led to significant improvements in this study. We
acknowledge the Japan Meteorological Agency (JMA) for
use of the data set of water levels observed at Kagoshima
Tidal Gauge, downloaded from the JMA homepage. We
used the continuous waveform data opened to the public
by National Research Institute for Earth Science andDisaster Prevention (NIED) and JMA. We thank theagencies for their technical assistances.
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(Editorial handling Takeshi Nishimura)

若尊カルデラ底の海底噴気孔の極近傍に設置した海底地震計で観測された
半日周潮的な周期性
八木原

279

寛・平野舟一郎・宮町宏樹・髙山鐵朗・山崎友也・為栗健・井口正人桜島火山の北東に位置する若尊カルデラの海底において，海底地震計 (OBS) で観測された地動速度の振幅変化に，明瞭で半日周潮的な周期性が認められた．OBS の投入直後に，海底噴気孔から海面に浮上する気泡を視認したため，OBS で観測された地動はカルデラ底の海底噴気孔の活動によって生じたと考えられる．
1 分毎の地動速度 RMS 振幅値 (RMSA) を求め，その変化と起潮力の理論値の変化，及び潮位計による水位の観測値の変化とを比較して得られた特徴は次のとおりである．1) 2007 年 9 月を通じて，RMSA に明瞭で半日周潮的な周期性が認められた．ただし，時々，不明瞭な期間があった．2) RMSA の極大の時刻が，起潮力の極大の時刻と時間領域で一致する．3) RMSA のパワースペクトルに認められる 4 つのピークの周波数
は，主要 4 分潮の周波数と同一である．4) 詳細にみると，RMSA 変化はのこぎり刃状を呈する．また，日潮不等の期間においては，RMSA 変化が不規則であった．5) RMSA の長周期，または不規則な変化は，日潮不等の期間において卓越する．地熱システムの循環の一部を形成する，深部からカルデラ底に向かって上昇する地熱流体の活動が，起潮力によって増加するのであろう．