Journal of Natural Disaster Science, Volume 38，Number 1，2017，pp119-131

Seismic Observations using Ocean Bottom Seismometers aroundKuchierabujima Volcano
Hiroshi Yakiwara1, Shuichiro Hirano1, Yusuke Yamashita2, Hiroshi Shimizu3, Kazunari Uchida3,
Kodo Umakoshi4, Kazuo Nakahigashi5,6, Hiroki Miyamachi1, Mitsuharu Yagi7, Hisao Kanehara7
and Shigeru Nakao1
1Graduate School of Science and Engineering, Kagoshima University
2
Disaster Prevention Research Institute, Kyoto University
3
Graduate School of Science, Kyushu University
4
Graduate School of Fisheries and Environmental Studies, Nagasaki University
5
Graduate School of Science, Kobe University
6
Now, School of Marine Resources and Environment, Tokyo University of Marine Science and Technology
7
Faculty of Fisheries, Nagasaki University(Received: Sep.27, 2016

Accepted: Jan.11, 2017)

Abstract
The remarkable eruption on Kuchierabujima, which occurred on May 29, 2015, had the potential to disrupt
land seismic observation if the volcanic activities had expanded. Since relatively deep-volcano-tectonic(DVT) earthquakes under the volcano may have extended into the sea region around the island, we deployed
four ocean bottom seismometers (OBSs) on the seafloor at distances at approximately 6.5 km from the crater.
During the OBS observation period from July 3 to October 9, 2015, 303 volcanic earthquakes were detected.
The daily numbers and the temporal amplitude changes indicate that seismic activities declined in the middleof August. However, only nine of the events were DVT earthquakes, and we were unable to identify any
clear differences between the hypocenter distribution characteristics of the DVT events discussed inprevious studies and those of our observations. However, since the temporal tendency of daily earthquake
numbers derived from our OBS data resembled that compiled by the Japan Meteorological Agency using
real-time land data, when eruption expansions occur on a small-scale remote island such as Kuchierabujima,
OBS observations can provide a useful backup to efforts aimed at grasping the seismicity.
Keywords: Kuchierabujima Volcano, Volcano-tectonic earthquake, Ocean bottom seismometer
1. INTRODUCTION
Kuchierabujima is one of the most active volcanoes along the Kyushu-Ryukyu arc of southwestern Japan(Fig. 1). Based on tephra stratigraphy, Geshi and Kobayashi (2006) constructed a volcanological history of
Kuchierabujima volcanic activities covering the last 30,000 years. In their study, they identified two major
pumice and scoria eruptions between 1,500 and 1,100 years ago, the formation of a volcanic edifice, lava
flow emissions, and repetitive Vulcanian-type eruptions and phreatomagmatic explosions over the last
10,000 years. Historical eruptions of the volcano since 1841 have been Vulcanian-type eruptions andphreatic explosions (Geshi and Kobayashi, 2006).
The Sakurajima Volcano Research Center, Disaster Prevention Research Institute (DPRI) of Kyoto
University has conducted continuous seismic observations of the volcano since December 1991 (e.g.,
Triastuty et al., 2009). Additionally, the Geological Survey of Japan, which is operated by the National
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K.NAKAHIGASHI, H,MIYAMACHI, M.YAGI, H.KANEHARA, S.NAKAO

Institute of Advanced Industrial Science and Technology (AIST), has performed ground deformation
observations using Global Positioning System (GPS) data in cooperation with DPRI since 2004 (Saito andIguchi, 2006).

Fig. 1: Medium- and long-term seismicity of crustal earthquakes around the volcano.
(a) Overview of volcano location. (b) Epicenter distribution of crustal earthquakes overthe last 16 years. The Nansei-Toko Observatory for Earthquakes and Volcanoes (NOEV),
Kagoshima University, identified these earthquakes using data derived from regional
real-time seismic stations. The gray curves are isograms of residual gravity anomalies(after Iguchi et al., 2007). “L” indicates a low-residual gravity anomaly. (c) Earthquake
spatiotemporal distribution. (d) Time-depth relationship of these earthquakes. The
epicenters of (b) and (c) are plotted on a plane rectangular coordinate system of Japan.

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Before the August 3, 2014 eruption, four clear ground deformations accompanied by an obvious
increase of shallow earthquakes were observed in the volcano (Geological Survey of Japan and Sakurajima
Volcano Research Center, 2016). Triastuty et al. (2009) studied the activities of shallow volcano tectonic(SVT) earthquakes close to Mt. Shindake crater and concluded that the SVT earthquake seismicity resulted
from tensional stress accumulation caused by the intrusion of hydrothermal fluid.
However, few studies have been carried out on the hypocenter distribution and seismicity of volcano
tectonic (VT) earthquakes occurring at deeper levels under and around the volcano, even though Tameguri
and Iguchi (2007) reported on the isolated occurrence of relatively deep volcano-tectonic (DVT)earthquakes with depths of several kilometers, and Iguchi (2007) reported that DVT earthquakes had
occurred off the northeastern coast of the island in 1996 and 1999. That study concluded that the hypocentersof these DVT earthquakes were located in the vicinity of the eastern and southern rim of a low-gravity
anomaly region (Komazawa et al., 2007) where a caldera structure was presumed to exist (Fig. 1(b)).
However, there are few available details about the seismicity of DVT earthquakes because such earthquakes
have been rare in recent years.
On May 29, 2015, a remarkable eruption took place at the crater of Mt. Shindake, which is the active
crater of the volcano. Pyroclastic flow and surge descended from the eruption plume, and reached thewestern coast of the island. In response, the Japan Meteorological Agency (JMA) issued a Volcanic Alert
Level 5: ‘Evacuate’ and local residents and sojourners were required to depart from the island within theday. This evacuation proved troublesome for all island infrastructure requiring electric power, including theobservation systems for volcanic activity and their related wireless communication networks, which use
electricity provided by the island’s heavy oil power generation plant.
If the expansion of eruption activities had forced a halt to the delivery of heavy oil supplies to theisland, ground-based seismic monitoring and geodetic observations would have been forced to cease
operations as well. DVT earthquakes had been occasionally observed in areas under, adjacent, and outside
the volcano since 2000 (Fig. 1), and the expansion of volcanic activity could mean an increase in DVT
earthquakes in regions adjacent to the volcano.
If we were restricted to data collected from land seismic stations, it would be impossible to detect
small DVT earthquakes in the adjacent regions, and thus impossible to precisely determine the hypocentersof such earthquakes. With these points in mind, we decided to perform seismic observation at the seafloor
surrounding the volcano using ocean bottom seismometers (OBSs).
There have been no interruptions to the oil supply because volcanic activities gradually subsided
following the June 19, 2015 eruption, and the volcano has been quiescent since then, so the evacuation order
was called off on December 25, 2015. Although only a few DVT earthquakes (the observation of which is
our primary objective) were recorded, we were able to observe the process by which seismic activity
declined. Thus, in this paper, we report on the results of analyses conducted using seismic data collectedfrom our OBSs in conjunction with JMA land station data.
2. OBSERVATIONS AND DATA
As a part of the No. 417 educational cruise of the training ship Nagasaki Maru (842 gross tons) operated by
the Faculty of Fisheries of Nagasaki University, we deployed four pop-up-type OBSs onto the seafloor
around Kuchierabujima on July 3, 2015, 35 days after the May 29, 2015 eruption. These OBSs were released
via free-fall mode. Figure 2 shows the distribution of the seismic stations used in this investigation. The
OBSs were deployed on the seafloor at a horizontal distance of approximately 3.5 nautical miles (6.5 km)
from Mt. Shindake crater because of a JMA warning that the area within a 2.0 nautical mile (3.7 km) radiusof the crater may have been enlarged by volcanic activity expansion. The water depths of these OBS stations
were from 480 to 610 meters. The possibility of submarine eruptions declines if there is a certain amount ofwater depth.
Later, we used another multipurpose vessel, the No. 3 Aoi Maru (4.9 gross tons, Nagata Port,
Yakushima Island) to determine the positions of the OBSs on the seafloor via an acoustic survey. Throughout
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the voyages, the positions of both the Nagasaki Maru and No. 3 Aoi Maru were logged each second using
differential GPS (DGPS) systems. The positions of the OBSs on the seafloor were determined using bothacoustic distance measurements and the vessel's DGPS position data. As the vessel was not equipped withonboard navigation systems, our DGPS compass and a personal computer, in which electric navigation
charts (ENCs) issued by the Japan Coast Guard had been installed, were set up on the vessel during thecruises. By plotting the vessel’s position superimposed on the ENCs, we carefully guided the boat vessel tothe objective points. We successfully retrieved all OBSs via pop-up mode on October 30, 2015 using theNo. 3 Aoi Maru. However, in order to conserve battery power and to prevent physical damage to the OBS
sensors, we had timed the recording to end prior to their retrievals. As a result, the number of observationaldays was about 98, from July 3 through October 9.

Fig. 2: Distribution of seismic stations in this investigation. Crosses indicate the positionsof ocean bottom seismometers (OBSs) on the seafloor. Solid squares show the land seismic
stations operated by the JMA. The solid triangle shows Mt. Shindake crater. The contour
line and thick isobath intervals are set at 100 meters while thin isobaths are depicted every
20 meters. The inner and outer circle radii are 3.7 km and 6.5 km (2.0 and 3.5 nautical
miles), respectively.
These OBSs were equipped with a three-component velocity sensor consisting of a digitizer, a digitalrecorder, and an accurate internal clock with very small drift rate (0.02–0.05 s/day). The natural frequencies
of velocity sensors were 1 Hz (Station O1 and O3) or 4.5 Hz (Station O2 and O4). The analog/digital (A/D)
resolutions were set to 20 bits for Stations O1 and O3, 22 bits for Station O2, and O3 and 24 bits for Station
O4. The sampling frequency of all OBSs was set to 200 Hz. Since the internal clock of an OBS cannot becorrected during the seafloor deployment because radio waves cannot be transmitted through the water, wemeasured the time differences between the OBS clocks and a GPS timing module on the vessels just before
deployment and immediately after retrieval. Assuming constant drift rates for each OBS, we were able to122

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correct the OBS clock values throughout the observation period. These time calibrations were successful
for all OBSs.
The continuous waveform data obtained by the OBSs were then merged with the continuous
waveform data collected by four JMA land stations (Fig. 2), after which we plotted the merged continuous
waveform on paper charts in order to visually identify the volcanic earthquakes. Using the three-component
seismic waveform of the selected earthquakes, we then read out the arrival times of P- and S-waves at eachseismic station in order to determine hypocenter locations and magnitudes. However, it was difficult for the
OBSs to determine the hypocenters of very shallow micro-earthquakes close to Mt. Shindake crater with
sufficient accuracy because seismic waves attenuate along the propagation path, and because the high noiselevels present on the seafloor are larger than the weakened waveforms.

3. OBSERVED VOLCANIC EARTHQUAKES DURING THE OBS OBSERVATION PERIOD
The spatiotemporal distribution of regional crustal earthquakes that occurred during the period from 2000
to 2015 is shown in Fig. 1. The hypocenters were selected from a catalog of the Nansei-Toko Observatory
for Earthquakes and Volcanoes (NOEV, Kagoshima University), which meant that our OBS observation
period was included in the spatiotemporal distributions (Fig. 1). It should be noted that our OBSs
observations were carried out during a period when seismic activity was relatively low.

Fig. 3: Waveform examples for each earthquake type observed via OBS. The three component
waveforms of all OBSs are shown. UD refers to the vertical component waveform. H1 and H2 are thetwo horizontal components. (a) Relatively deep volcano-tectonic (DVT) earthquakes. (b) Shallow
high-frequency (SHF) earthquakes. (c) Shallow low-frequency (SLF) earthquakes. (d)Monochromatic-type (MT) earthquakes.
Based on the specifications of Triastuty et al. (2009), Fig. 3 shows examples of four volcanic
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K.NAKAHIGASHI, H,MIYAMACHI, M.YAGI, H.KANEHARA, S.NAKAO

earthquake waveforms recorded by each OBS: DVT earthquakes, shallow high-frequency (SHF)
earthquakes, shallow low-frequency (SLF) earthquakes, and monochromatic-type (MT) earthquakes. SHF
earthquakes have a wide frequency range spectra of 6 to 25 Hz, while SLF earthquakes are dominated bylower frequencies of around 1 to 5 Hz (Triastuty et al., 2007). With the exception of the DVT earthquake,these volcanic events occurred at very shallow depths within several hundred meters around Mt. Shindakecrater (Triastuty et al., 2009). Of these volcanic earthquakes, only the DVT earthquake observations
obtained using OBSs were effective for improving the accuracy of our hypocenter determination process.
In Fig. 3, the initial motions of the observed waveforms are conspicuously attenuated at Station O3,
which led us to conclude that, unluckily, the OBS had landed on a thick unconsolidated seafloor deposit.
Furthermore, the 1 Hz sensor stations (O1 and O3) tended to record long-period waveforms generated byocean waves. The predominant DVT earthquake frequencies were from several to 10 Hz, which are
obviously high in comparison with those generated by ocean waves. Thus, using the above factors, we
calculated the amplitudes of each waveform by averaging the maximum vertical component amplitudes of
the two OBSs equipped with 4.5 Hz sensors (Stations O2 and O4). Ultimately, 303 events whose seismic
amplitudes were larger than or equal to 5.0 ´ 10-7 m/s were identified during the OBS observation period.

Fig. 4: Daily numbers and time changes of observed volcanic earthquake amplitudes. (a) Daily
numbers of volcanic earthquakes (Details of Volcanic Activity, JMA). (b) Daily numbers of earthquakesthat could be identified by maximum amplitude data collected at OBS Station O2 and O4. (c) Amplitude
time changes of earthquakes shown in (b). The colors show the earthquake types.
Figure 4(b) shows daily numbers of the observed volcanic earthquakes. In Fig. 4(c), the observed
amplitude of each earthquake was plotted against the time axis. This figure clearly shows that the daily
numbers of volcanic earthquakes decreased in the middle of August. The SLF earthquakes were prominent
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and clustered before the middle of August, while the SHF earthquakes increased during the first half of thatmonth. No clear cluster of SLF earthquakes was observed after the middle of August. Additionally, only a
few SHF earthquakes occurred after the end of August and the amplitudes of these above-mentioned
earthquake types showed a general tendency to decrease over time after the end of August. Meanwhile, MT
earthquakes showed isolated occurrences during the period from the end of August to the middle ofSeptember when SLF and SHF earthquake activities declined. Collectively, the volcanic earthquake activity
decline took place in the middle of August 2015.
4. HYPOCENTER DISTRIBUTION OBTAINED BY OBS
4.1. Seismic velocity model and estimation of station corrections
In general, P- and S-wave arrival times at an OBS are significantly later than would be expected based on
velocity model calculations, primarily owing to thick unconsolidated sediments beneath the seafloor.
Consequently, observed travel times from hypocenters to the seismic stations are much longer than thecalculated theoretical travel times. Since these time delays significantly influence the locus obtained via
hypocenter determination calculations, the estimation of correction values, called station correction, whichtake the P- and S-arrival delay times into consideration, is very important. The models shown in Fig. 5(a)were derived from the results of other observations, which combined OBSs with land seismic stations,performed in and around the Tokara and Nansei-Syoto Islands since 2014 (Yakiwara et al., personal comm.).
The P- and S-wave arrival time station corrections for each station were estimated as follows:

Fig. 5: One-dimensional (1D) P- and S-wave velocity models used in this investigation. (a) RegionalP- and S-velocity models. These models were derived from regional data obtained by island seismic
stations and OBSs observations in and around the Tokara Islands (Yakiwara et al., 2015, personalcomm.). Open circle velocities were estimated by inversion schemes. The velocities between the circles
were obtained by linear interpolation. (b) Local P- and S-velocity models used to locate the volcanic
earthquakes in and around Kuchierabujima. The P-wave velocity at a depth of 15 km (gray square) is
identical to that of (a). An open square (3.50 km/s) at a depth of 0 km was determined using a gridsearching method. Assuming Vp/Vs = 1.73 (constant ratio), we assigned the S velocities. The velocity
gradient above a depth of 0 km equals that between a depth of 0 and 15 km.

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K.NAKAHIGASHI, H,MIYAMACHI, M.YAGI, H.KANEHARA, S.NAKAO(1)
We selected 27 regional earthquakes that occurred around the Tokara Islands within the
OBS observation period.
(2)
Assuming the hypocenter locations were correct, we calculated travel time residuals of
the P- and S-wave arrivals at each station by using the P- and S-wave velocity model shown in Fig. 5(a).
(3)
We then subtracted the value of P-wave travel-time residuals at Station JKC (Fig. 2) fromthe values of each other station. The travel time residuals were adjusted so that the P-wave travel time
residuals at Station JKC became 0.00 s.
(4)
In order to estimate station corrections, the obtained relative travel-time residuals werethen averaged for each station.
Because in comparison with the distance from Kuchierabujima to the regional earthquakes
hypocenters around the Tokara Islands seismic stations were located in a sufficiently narrow area, thevelocity difference between the one-dimensional (1D) model and the true velocity resulted in seismic wave
delays that were nearly identical. Based on this consideration, we decided to accept the adjusted values of4) above as the station corrections.
Figure 6 shows the station corrections of P- and S-waves of each station. A positive value for thestation correction (red circle) indicates a seismic wave arrival delay. As for the OBSs, the station corrections
show large positive values that are caused by sediment deposits beneath the seafloor. The largest correction(estimated at Station O3) is consistent with conspicuous seismic wave attenuations. Contrastingly, theestimated corrections of the land seismic station were small.

Fig. 6: Station corrections at each OBS and land seismic station used in this investigation. Positive andnegative values (red and blue circles) show the delay and advance, respectively, relative to P arrivals at
Station JKC. In general, remarkably large and positive values were estimated on S arrivals at the OBSs due
to unconsolidated sediment deposits beneath the seafloor. Note, a correction of S arrivals at Station KER4
was not obtained in this analysis because no horizontal component data was available.
4.2. Hypocenter distribution during the OBS observation
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Seismic velocity models are also required when using P- and S-wave arrival times to determine thehypocenters of seismic events. Because the regional velocity models shown in Fig. 5(a) were too fast to be
applied to the shallow depth range of volcanic regions, we reconstructed the 1D P- and S-wave velocity
models shown in Fig. 5(b) as follows:(1)
The P-wave velocity at a depth of 15 km was identical to that of Fig. 5(a).
(2)
The P-wave velocity (3.50 km/s) at a depth of 0 km was determined by a grid searching
method in which we iteratively performed hypocenter determinations by changing the velocity from 2.0 to
4.0 km/s by 0.1 km/s.
(3)
Assuming Vp/Vs = 1.73 (constant ratio), we assigned the S velocities.
(4)
The velocity gradient above a depth of 0 km was assumed to equal that between depthsof 0 and 15 km.
For the 67 earthquakes observed during our observation period, we identified P- and S-wave arrival
times at each station using seismic waveforms displayed on computer monitors. Using the identified P- andS-arrivals and velocity models of Fig. 5(b), we then determined the hypocenter locations of the volcanic
events. Of these earthquakes, 62 volcanic earthquakes were located in and around Kuchierabujima Volcano,and were plotted in Fig. 7.

Fig. 7: Hypocenter distribution during the OBS observation period. The circle colors correspond to the
hypocenter depths.
However, only nine earthquakes recorded during the OBSs observation period matched our targeted
DVT earthquake type, and the magnitudes of these DVT earthquakes were 1.1 or less. The depth range ofthese DVT earthquakes was from 4.7 km to 6.9 km, and no DVT earthquake was located deeper than 7 km
during the observation period. Of these DVT earthquakes, three were micro-earthquakes that occurred
beneath the east end of the island, were nearest to Mt. Shindake crater, and had magnitudes between 0.0 and
0.6. Three other DVT earthquakes had magnitudes between 0.9 and 1.1, and were located off the northeast
shore of the island. The final three DVT earthquakes were observed on the west side of the island. One had
a magnitude of 0.3 and was located beneath the west end of the island at a depth of about 6 km, while thefinal two earthquakes, whose magnitudes were 0.2 and 0.7, occurred off the west coast of the island. Thus,in terms of DVT earthquakes, the area in and around the volcano was mostly calm.
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In contrast, the remaining 53 earthquakes were SHF and SLF events that occurred in the area close
to Mt. Shindake crater. The depth range of these events was from the Earth’s surface to a depth of 2.1 km.
A seismicity gap from 2.1 km to 4.7 km in depth was plotted between the relatively deep earthquakes andshallow events close to the crater, even though very few DVT earthquakes were observed. However, theuncertainties related to identifying seismic wave arrival times at each station might have resulted in thevertically elongated hypocenter area for the SHF and SLF earthquakes. Additionally, the increased OBS
data uncertainty might have arisen from the longer epicenter distances, and from the high sea bottom noises
in comparison with those of land seismic stations.
Figure 8 shows the spatiotemporal distributions of the 62 hypocenters used in this analysis. As thefigure shows, the SHF earthquakes were dominant until the middle of August 2015. Conversely, no DVT
earthquakes occurred in the vicinity of the volcano until the beginning of August (Fig. 4). After the middleof August, the numbers of SHF earthquake hypocenters obviously decreased because the numbers of theevents decreased, and because few relatively large amplitude events occurred (Fig. 4). Three DVT
earthquakes were identified around the volcano in the middle of August, while the remaining DVT
earthquakes were identified after the middle of September. As for the SLF earthquakes, only three eventswere identified because the initial phases of the other events were too weak to precisely read out the arrival
times.
Fig. 8: Spatiotemporal distributions of the volcanic earthquakes during the OBS observation period. The
circle colors show earthquake types.
5. DISCUSSION
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Because temporal changes of hypocenters and focal mechanisms of VT earthquake are closely related tostress changes, the dearth of DVT earthquakes during our observation period implies that clear stress statechanges caused by the charge or discharge of magma did not occur in the deep portion, e.g., main magma
reservoir, of the volcano. In 2010 and 2011, the JMA began global navigation satellite system (GNSS)
observations at four stations on the volcano’s flank in order to facilitate monitoring of volcano crustal
deformations. Other GNSS stations operated by the National Research Institute for Earth Science andDisaster Resilience (NIED), AIST, and the Geospatial Information Authority of Japan (GSI) were also
installed on the volcano. Because of the eruption disasters that occurred in 2014 and 2015, among thebaselines between GNSS stations on the volcano, we could only identify one temporal baseline lengthchange (JMA station) and no clear baseline length changes were recorded during our OBS observation
period (Fukuoka Volcanic Observation and Information Center and Kagoshima Meteorological Observatory,2016), which is consistent with the scarcity of DVT earthquakes throughout the period. As for the volcano’s
main reservoir, Yamamoto et al. (2017) performed repeated precise leveling surveys in order to estimate
pressure sources. Assuming that the pressure source was located under Mt. Shindake crater, they estimatedthe source depth of 7.0 km. However, no DVT earthquakes occurred in the region neighboring the mainpressure source.
The DVT earthquake hypocenter regions determined during our observation period have also beenrecognized in previous studies covering periods when no eruptions were observed. Iguchi (2007) mentioned
the active regions of DVT earthquakes off the northeast coast of the island, which occurred in 1996 and1999. The hypocenters of these events were located on the east and south rim of a caldera, which wasestimated based on low-gravity anomalies (Iguchi, 2007; Komazawa et al., 2007). As the DVT earthquake
hypocenters observed in the present investigation were also located close to the southern rim of the lowgravity anomaly region, thereby implying the existence of a caldera structure, DVT earthquakes could be
expected to occur under the present geological structure in the same manner as those occurring during the1996 and 1999 activities. In addition, Tameguri and Iguchi (2007) pointed out isolated DVT earthquake
occurrences in the volcano’s west side region in 2005, which matches our observational results. Collectively,
there are no clear seismic activity differences between the results of our observation and these previous
study results.
Since there was the possibility that observations on the island could be obstructed by the expansionof eruption activities, we initiated OBS observations shortly after the May 29, 2015 eruption. Figure 4(a)shows the daily numbers of volcanic earthquakes along with other volcanic information contained in the“Details of Volcanic Activity” issued by JMA, including real-time seismic station data on the island. The
temporal daily number trends of Fig. 4(a) resemble those of Fig. 4(b), which are derived from our OBS data.
Although the OBS data was only available for post-processing (not real-time processing), the temporalchange of seismicity related to volcanic activity could also be observed.
If eruption activities continue expanding, all or several stations for ground-based seismic monitoring
may stop observations. Figure 9(a) shows the hypocenter locations determined by the use of data collectedfrom just four OBSs. A total of 59 hypocenters were also located from among the 62 earthquakes plotted inFig. 7. As for the remaining three earthquakes, we could not locate the hypocenters because less than fourP-arrivals were available. Although several of the epicenters shown in Fig. 9(a) shifted from 3 to 5 km,
almost all of the epicenters were located at nearly the same position when compared with the epicentersplotted in Fig. 7. However, the hypocenter distribution in N-S and E-W cross sections differed considerablyfrom that of Fig. 7. Furthermore, we were unable to estimate the hypocenters for SHF earthquakes with any
precision because no S-arrival times were observed for the earthquakes. In other words, only four P-arrival
times were available. In the case of the hypocenter determination using only OBSs data, it was difficult tolocate the hypocenters with sufficient levels of accuracy. Hence, these results and Fig. 4 suggest that theobservations taken using just four OBSs, without ground-based seismic data, could also be useful for
grasping epicenter distribution, daily earthquake numbers, and temporal amplitude changes of the volcanic
earthquakes.
On the other hand, Fig. 9(b) shows the hypocenter distribution that was determined using data
observed at the four OBSs and a land seismic station, JKC. The obtained hypocenter distribution of the 62
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earthquakes strongly resembles that of Fig. 7. Thus, the hypocenter determination was clearly improved
simply by adding data collected from a land seismic station to the OBSs data. Collectively, seismic
observations using OBSs can provide data efficiently at times when several seismic stations have beeninterrupted by the expansion of eruption activities.
Since the possibility of submarine eruptions declines if there is a certain amount of water depth, andsince there is little chance that the OBSs will suffer direct damage due to the eruptions, they can record
ground motions continuously related to volcanic activities. Hence, when eruption expansions occur in asmall-scale remote island like Kuchierabujima, the observation using OBSs can provide a useful backup toefforts aimed at grasping seismic activities related to the volcanic event.

Fig. 9: Hypocenter distribution in the OBS observation period. (a) Hypocenter distribution determinedusing only OBSs data. (b) Hypocenter distribution determined using data collected from OBSs and a land
seismic station (JKC).
6.

CONCLUSION

In response to the powerful eruption of Kuchierabujima Volcano on May 29, 2015, we deployed four OBSs
onto the seafloor around the island on July 3, 2015 and retrieved them on October 30, 2015. Corrections to
all the devices’ internal clocks were completed successfully. Since the OBS timers were set to stop recording
prior to the retrieve, the observation period was from the deployment day to October 9, 2015 (about 98 days).
The continuous waveform data obtained by the OBSs were then merged with the continuous waveform datacollected by four land stations operated by the JMA.
After determining the maximum amplitude using the waveform data of each recorded earthquake,
303 events were identified during the OBS observation period. The daily numbers and temporal changes ofthese earthquakes indicate the decline in volcanic earthquake activity took place in the middle of August
2015. After estimating travel time corrections for each station, hypocenter determinations were performed.
The results showed that 62 volcanic earthquakes were located in and around the volcano, only nine of whichwere DVT earthquakes, the primary focus of our observations.
The depth range of these DVT earthquakes was from 4.7 to 6.9 km. Additionally, a seismicity gap
from a depth of 2.1 to 4.7 km existed between the DVT earthquakes and shallow events close to the crater.
Finally, we were unable to identify any hypocenter distribution characteristic differences in our DVT
earthquake observations and those of previous studies.
Although few DVT earthquakes were observed, we were able to examine the process in which
seismic activity declined and found that the temporal tendency of daily volcanic events derived from OBS
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data closely resembled that obtained by JMA using real-time land seismic station data. Thus, when rapid
eruption expansions occur on a small-scale remote island like Kuchierabujima, OBS observations canprovide a useful backup to efforts aimed at grasping the seismicity related to volcanic activities, even though
pop-up-type OBS data are not available in real-time.

Acknowledgements
We wish to thank all crewmembers and trainees of the training ship Nagasaki Maru (Nagasaki University)
for assisting in the deployment of the OBSs. We would also like to thank the captain of No. 3 Aoi Maru,
Kazunao Shiba, for his assistance with the surveys and the OBS retrieval. We acknowledge the assistanceof the Dean of the Faculty of Fisheries, as well as the executives and the president of Nagasaki University.
The suggestions of anonymous referees and the subeditor led to significant improvements in this paper. We
would also like to thank the JMA for allowing us to use waveform data collected by their permanent stationsin the vicinity of Kuchierabujima Volcano.
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H.YAKIWARA, S.HIRANO, Y.YAMASHITA, H.SHIMIZU, K.UCHIDA, K.UMAKOSHI,
K.NAKAHIGASHI, H,MIYAMACHI, M.YAGI, H.KANEHARA, S.NAKAO

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