Journal of Natural Disaster Science, Volume 38，Number 1，2017，pp133-144

Vertical ground deformation related to the 2014 and 2015 eruptions at
Kuchierabujima Volcano, Japan detected by repeated precise levelingsurveys
Keigo Yamamoto*, Takahiro Ohkura**, Akihiko Yokoo**, Takeshi Tameguri*, Tadaomi
Sonoda*, Hiroyuki Inoue*** Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University** Aso Volcanological Laboratory, Graduate School of Science, Kyoto University,（Received : Feb. 7, 2017

Accepted : June 2, 2017）

Abstract
We conducted precise leveling surveys in Kuchierabujima Volcano, southwest Japan seven times after
the occurrence of the 3 August 2014 eruption, in order to detect the vertical ground deformation
associated with the eruptions that occurred in 2014 and 2015. The first survey data obtained in August
2014 suggest that vertical displacements associated with the 3 August 2014 eruption were notremarkable on the leveling route, which is located > 2.3 km away from Shindake crater. On the other
hand, the obvious ground uplift toward the central part of the volcanic edifice was detected during theperiod between August 2014 and March 2015 surveys. The results of the pressure source analysis
suggest that the rapid magma input occurred at a relatively large depth (optimally 7.0 km) and led to thelarge eruption on May 29, 2015. The ground uplift remained unchanged until at least October 2015,
even after the eruptions on 29 May and 18 June 2015. During the period between June and September
2016 surveys, a significant ground subsidence was observed, suggesting that most of the stored magma
was removed during this period.
Keywords：Kuchierabujima Volcano, eruption, repeated precise leveling surveys, vertical grounddeformation

1. Introduction
Kuchierabujima Volcano, located south of Kyushu Island, southwest Japan, is an active island volcano(Fig. 1). The volcano has repeated phreatomagmatic or phreatic eruptions at a summit crater of Shindakeor at a fissure on the east of the crater at a few year to a few decade intervals since 1841, before which
no historic eruptions have been recorded. Although no eruptions had occurred after September 1980, thevolcano erupted at Shindake crater on August 3, 2014 after 34 years of dormancy (Iguchi et al., 2017,
in this issue; Tameguri et al., 2016).
Before the 3 August 2014 eruption, the increase of activity of volcanic earthquakes had been
repeatedly observed, especially after the increase in July 1999. Most of the hypocenters were located at
a depth shallower than 500 m beneath Shindake crater (Yamamoto et al., 1997; Iguchi et al., 2007;
Triastuty et al., 2009). GPS campaign observations that had been conducted since 1995 detected
inflations of the ground around Shindake crater, and the pressure sources were located initially at a depth
around sea level beneath the crater, which became shallower over time (Iguchi et al., 2002, 2007).
Furthermore, the continuous GPS observation started in 2004 revealed that the ground inflations
occurred in association with the increase of activity of volcanic earthquakes (Saito and Iguchi, 2006).
Kanda et al. (2010) observed the total intensity of the geomagnetic field at the summit area since 2000.
They indicated that the estimated volcanomagnetic field variations were caused by the thermal
demagnetization that occurred in the shallow part beneath Shindake crater and changed in concordancewith the increase in seismicity. Thus the 3 August 2014 eruption occurred after various long-term
precursory activities that had been gradually progressing (Iguchi et al., 2017, in this issue; Tameguri etal., 2016).

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Fig. 1. (a) Location map of Kuchierabujima Volcano. Solid circles with a broken line denote thelocations of GNSS stations (Nanakama station in Kuchierabujima operated by Japan Meteorological
Agency and Kamiyaku2 station in Yakushima Island operated by Geospatial Information Authorityof Japan) and the baseline whose length changes are discussed in the text. (b) Leveling benchmarks(solid circles) and leveling route (solid curve) of Kuchierabujima Volcano. The topographic contour
interval is 100 m.

After the 3 August 2014 eruption, various kinds of observation data showed a further increaseof volcanic activity (Iguchi et al., 2017, in this issue; Tameguri et al., 2016). These include theremarkable increase of SO2 emissions since December 2014 (Mori et al., 2017, in this issue), shorteningof the slope distance between the GNSS station in Kuchierabujima and that in Yakushima Island (Fig.
1a) during the period from December 2014 to January 2015, which is recognized as inflation of the
volcano (Iguchi et al., 2017, in this issue), volcanic glow above Shindake crater observed on and after
March 24, 2015 by JMA high-sensitivity monitoring camera (Japan Meteorological Agency, 2015),
increase in the activity of volcanic earthquakes especially since May 2015 and the occurrence of felt
earthquakes on January 24, 2015 and on May 23, 2015 (Iguchi et al., 2017, in this issue; Tameguri et al.,2016). These remarkable precursory activities led to the large eruption on May 29, 2015, when all theresidents in Kuchierabujima evacuated from the island (Iguchi et al., 2017, in this issue; Tameguri et al.,2016). Another eruption occurred on June 18, 2015 in which ash-fall was observed in Yakushima andTanegashima Islands (Fig. 1a) (Japan Meteorological Agency, 2015). However, the details of theeruption were unknown due to the bad weather and insufficient observation system due to the destruction
caused by the 3 August 2014 eruption.
A precise leveling survey is a classical technique, but the most accurate geodetic method inmeasuring the vertical displacements of the ground. Therefore, it has been applied to various kinds of
volcanic deformation studies (e.g. Dzurisin et al., 2002; Kimata et al., 2004; Yamamoto et al., 2013;
Murase et al., 2016). On Kuchierabujima Volcano, Disaster Prevention Research Institute (DPRI), Kyoto
University installed 20 leveling benchmarks around the northwestern foot of the volcano in May 1996,
along the route of about 7.5 km long (Fig. 1b). Since then, precise leveling surveys have been repeatedseven times before the 3 August 2014 eruption, including the May 1996, August 1996, September 1999,
November 2000, December 2001, December 2006 and December 2008 surveys. The results of these
repeated leveling surveys indicate that the vertical ground deformation was generally small, althoughrelatively large (within 5 mm) uplift or subsidence was occasionally observed along the southern halfof the leveling route between benchmarks KC103 and KUC7 (Fig. 1b), which are located relatively
close to Shindake crater (Iguchi et al., 2002; Yamamoto and Sonoda, 2014). Iguchi et al. (2002) pointed

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out that the vertical displacements expected from the shallow pressure source inferred from the GPS
campaign data were not inconsistent with the small displacements observed by the repeated levelingsurveys. Along the leveling route north of KC103, the observed vertical displacements were smaller
than about 2 mm (Yamamoto and Sonoda, 2014).
We conducted precise leveling surveys in Kuchierabujima Volcano repeatedly after theoccurrence of the 3 August 2014 eruption. In this paper, we present the results of these surveys anddiscuss the vertical ground deformation related to the 2014 and 2015 eruptions, in order to assess thecurrent status of the volcanic activity.
2. Observations
Table 1. List of benchmark coordinates used in
Just after the occurrence of the 3 August 2014
this study.
eruption, we conducted a precise leveling survey
Benchmark
LatitudeLongitude
along the above-mentioned leveling route during
Altitude
o
o
No.
(
N)
(
E)
(m)
the period of August 19-21, 2014, in order toKUC12N
30.47955
130.20028
28
detect the vertical ground deformation associatedKC117
30.47598
130.19708
26
with this event. In this survey, we measured only
KC116
30.47242
130.19605
10
the northern half of the leveling route (fromKC115
30.47123
130.19275
40
KUC12N to KC103 in Fig. 1b and in Table 1),
BM3N
30.46817
130.19183
47
since the southern part of the leveling route isKC114
30.46405
130.19378
18
located in the off-limits area assigned due to theKC101
30.46290
130.19673
36
eruption. Since then, we have repeated theKC103
30.46087
130.19808
62
leveling surveys only along the northern half ofthe leveling route during the periods of March 4-5, 2015, July 29-30, 2015 (after the 29 May 2015 andthe 18 June 2015 eruptions), October 7, 2015, March 24-25, 2016, June 22-23, 2016 and September 1415, 2016. As to the July and October 2015 surveys, we measured only the benchmarks between
KUC12N and KC101, due to the widening of the off-limits area associated with the 29 May 2015
eruption.
Table 2. Relative heights of benchmarks along the Kuchierabujima leveling route referring to thebenchmark KUC12N, which is located at the northern end of the leveling route.
BenchmarkNo.
KUC12N
KC117
KC116
KC115
BM3N
KC114
KC101
KC103
M.S.E.**

2008.12*(m)-2.4964-17.8994
12.1495
19.1579-9.6960
7.5017±0.36

2014.08(m)
0.0000-2.4964-17.9001
12.1489
19.1568-9.6972
7.500934.3405±0.21

2015.03(m)
0.0000-2.4962-17.8996
12.1501
19.1578-9.6949
7.502634.3430±0.16

2015.07(m)
0.0000-2.4969-17.9001
12.1500
19.1585-9.6960
7.50302015.10(m)
0.0000-2.4970-17.8998
12.150219.1585-9.6952
7.5028

±0.28

±0.12

2016.03(m)
0.0000-2.4972-17.9003
12.149319.1575-9.6960
7.502434.3424±0.22

2016.06(m)
0.0000-2.4966-17.8993
12.1501
19.1587-9.69447.504034.3434±0.20

2016.09(m)
0.0000-2.4973-17.9006
12.1487
19.1566-9.69687.500934.3404±0.14* The results of the December 2008 survey are taken from Takayama et al. (personal communications) inorder to calculate the relative vertical displacements during the period between the December 2008 andAugust 2014 surveys. Since KUC12N is replaced between December 2008 and August 2014, the relative
heights of benchmarks in the December 2008 survey are calculated referring to benchmark KC117 andthe height of KC117 in the December 2008 survey is assumed as that in the August 2014 survey.
** Mean square error (mm/km)

The leveling instruments used in these surveys were a digital level (Leica DNA03) and Invar
bar-coded leveling staffs (Leica GPCL3). We conducted precise leveling surveys, so as to keep themisclosure of the round-trip survey below the allowable limit of 2.5×L1/2 mm according to the firstorder leveling survey regulation defined by Geospatial Information Authority of Japan, where L is the
distance in kilometers along the leveling route between the two measured benchmarks. The horizontalityof the line of sight of the level and circular level of the leveling staffs were checked and adjusted before
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each survey, in order to decrease the measurement errors. As a result of these preparatory calibrations
and careful measurements, mean square errors of the surveys were achieved with good accuracy ranging
from ±0.12 to ±0.28 mm/km (Table 2).
3. Results of the leveling surveys
From the data obtained in each leveling survey, we calculated the relative height of each benchmark
referring to benchmark KUC12N, which is located at the northern end of the leveling route and is thefarthest benchmark from Shindake crater (Table 2). The calculated relative height of each benchmark
was then compared with that of the previous survey, resulting in the relative vertical displacement of thebenchmark between the successive surveys (Fig. 2). The error bar in Fig. 2 denotes the error of thevertical displacement of each benchmark calculated as the root mean square of the relative height errorsof the benchmark in the two surveys concerned, where the relative height error of each benchmark is
estimated from the misclosures of round trip surveys by accumulating from the reference benchmark(Kimata et al., 2004; Murase et al., 2016). In each survey, we measured the air temperatures and madea correction for the thermal expansion of our Invar bar-coded leveling staffs. Therefore, the measured
data are basically free from such effects. If the expansion coefficient of the leveling staff has an
uncertainty of 4.0×10-8 /℃ (in the case of our leveling staffs), the scale error in the maximum height
difference of our benchmarks (about 60 m in Fig. 2h) is estimated to be 0.024 mm, where the temperature
difference is assumed to be 10 ℃. This means that the scale error is negligible in our measured data.
The resultant displacements during the period from December 2008 to August 19-21, 2014 (justafter the occurrence of the 3 August 2014 eruption) show a slight ground subsidence toward the south(i.e. toward the central part of the volcanic edifice including Shindake crater) (Fig. 2a). However, theamounts of the subsidence are less than 1.2 mm and are generally within the estimated errors, implyingthat the ground deformation associated with the 3 August 2014 eruption is not remarkable along thenorthern half of the leveling route.
The displacements during the period from August 2014 to March 2015 (Fig. 2b) indicate theobvious ground uplift, the amount of which is increasing toward the south or toward the central part ofthe volcanic edifice. The amount of uplift at the southern end of the leveling route (KC103) reaches 2.5
mm referring to KUC12N, which is much larger than the estimated errors. Since the leveling route wasestablished, it is the first time that obvious ground deformation was observed along the northern half ofthe leveling route, which is located more than 2.3 km farther apart from Shindake crater. It suggests that
subsurface inflation at a relatively large depth beneath the volcanic edifice was progressing in the
preparation period of the 29 May 2015 eruption.
During the period from March 2015 to July 2015 in which the 29 May 2015 and the 8 June 2015
eruptions occurred (Fig. 2c), we cannot find clear ground deformation. This suggests that the ground
uplift observed in the previous period (Fig. 2b) was not affected by these two eruptions. Somebenchmarks seem to deform locally in this period, although the displacements are generally small. Thisis supposed to reflect the circumstantial changes around the benchmarks caused by the small landslides
etc. because the land was not maintained due to the evacuation of all the residents from Kuchierabujima.
During the following period between July 2015 and October 2015, the vertical displacements indicate
no notable deformation (Fig. 2d). Thus the uplift was preserved.
During the period between October 2015 and March 2016, a subtle (less than 1 mm) groundsubsidence toward the south is obtained (Fig. 2e). On the contrary, the ground uplift was measured in
the following period of March-June 2016 (Fig. 2f). In the most recent period between June 2016 and
September 2016, we observed significant ground subsidence toward the south, where the amount of
maximum subsidence was as much as 3.1 mm at KC101.
Figure 3 represents the cumulative vertical displacements of the benchmarks referring tobenchmark KUC12N since the 19-21 August 2014 survey. It is clearly displayed that the ground upliftobserved during the period between August 2014 and March 2015 (solid squares) was preserved withrelatively minor fluctuations in the following periods from March 2015 to June 2016. Then the groundreturned to almost the same level in the 19-21 August 2014 survey during the period between June 2016
and September 2016.

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Fig. 2. Vertical displacements of the benchmarks in Kuchierabujima Volcano referring to benchmark
KUC12N, which is located at the northern end of the leveling route during the periods between (a)December 2008 and August 2014, (b) August 2014 and March 2015, (c) March 2015 and July 2015,(d) July 2015 and October 2015, (e) October 2015 and March 2016, (f) March 2016 and June 2016
and (g) June 2016 and September 2016. The results of the December 2008 survey are taken from
Takayama et al. (personal communications) in order to calculate the vertical displacements during
the period between the December 2008 and August 2014 surveys (see footnotes of Table 2). Error
bars denote the errors of the vertical displacements of the benchmarks (see text). (h) Relative
elevations of the benchmarks referring to benchmark KUC12N.
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Fig. 3. Cumulative vertical displacements of the benchmarks in Kuchierabujima Volcano
since the August 2014 survey referring to benchmark KUC12N, which is located at thenorthern end of the leveling route.

4. Discussion
4.1 Secular change of vertical ground deformation since 1996
Since May 1996 when the leveling benchmarks were newly installed, some benchmarks wereaccidentally broken or became useless, whereas the benchmarks of KC117, KC116, KC115, KC114 andKC101 in the northern half of the leveling route have been continuously measured until the latest
leveling survey. Using the data observed at these benchmarks, we can obtain the secular changes of therelative heights of the benchmarks throughout the observation periods. In Fig. 4, we show the secularchanges in the relative heights of benchmark KC101 (Fig. 4a) and KC114 (Fig. 4b) referring tobenchmarks KC117, KC116 and KC115 resulting from repeated leveling surveys since May 1996.
During the period from May 1996 to August 2014 surveys, the relative heights change gradually.
The relative heights of KC101 and KC114 referring to KC116 or KC115 (open triangles and circles in
Figs. 4a and 4b) show a tendency of long-term ground uplift, while those referring to KC117 (opensquares in Figs. 4a and 4b) show a tendency of long-term ground subsidence, with some shorter upliftand subsidence fluctuations. One of the probable causes of these long-term height changes is the secularheight changes of the reference benchmarks, and further longer data acquisition will be needed toconfirm the cause of the long-term deformation.
Compared with the gradual height changes before the August 2014 survey, the relative heightchanges during the period between August 2014 and March 2015 surveys (gray shaded in Fig. 4) indicate
the significantly rapid and large ground uplift toward the central part of the volcanic edifice. During theperiod between August 2014 and March 2015, other observation data indicated remarkable signals
related to the increase of volcanic activity, including the significant increase of SO2 emission rate,
shortening of the slope distance between the GNSS stations in Kuchierabujima and in Yakushima Island(ground inflation of the volcano), increase in the activity of volcanic earthquakes and the occurrence offelt earthquakes (Iguchi et al., 2017, in this issue; Mori et al., 2017, in this issue; Tameguri et al., 2016).
It is inferred that the rapid magma input to the volcanic edifice occurred before the large eruption on
May 29, 2015. Such rapid and large height changes during the period between August 2014 and March
2015 are distinguishable from the other changes measured before the August 2014 survey, suggesting
the possibility of detecting the precursory ground deformation of the next large eruption if a similar

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deformation is observed along the northern half of the leveling route. In Fig. 4, we can also identify
rapid and large ground subsidence during the period of June-September 2016 (Figs. 2g and 3).

Fig. 4. (a) Secular changes of relative heights of benchmark KC101 referring to benchmarks
KC117 (open squares), KC116 (open triangles) and KC115 (open circles), respectively, since
May 1996. Gray shaded zones denote the period between August 2014 and March 2015 surveys.
The three vertical broken lines indicate the timing of the 3 August 2014, the 29 May 2015 andthe 18 June 2015 eruptions. (b) Similar to (a) but for the secular changes of relative heights ofbenchmark KC114.

4.2 Pressure source analysis based on Mogi's model
The relative vertical displacements during the period from August 2014 to March 2015 (Fig. 2b) thatindicate the obvious ground uplift increasing toward the central part of the volcanic edifice wereanalyzed based on a spherical source model (Mogi, 1958). In the calculation, the effects of topography
along the leveling route were considered by replacing the source depth with source depth plus
benchmark elevation (Williams and Wadge, 1998). Our leveling data suggest that the inflation source
was located south of the leveling route. However, it is difficult to determine the horizontal location of
the source exactly, because the geometry of our leveling route was a nearly straight line and in addition,
our leveling data were available only along the northern half of the leveling route that is located apartfrom the center of the volcanic edifice, the candidate area of the source location. Thus we assumed the139

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horizontal source location to be beneath the active Shindake crater, and searched for the optimal source
depth so as to minimize the residual sum of squares. The searched source depth ranged from 0 to 30 km
below sea level, with a depth step of 0.1 km. Given the depth, the volume change at the pressure source
was calculated by the least square fit of the model to the observed data, so the volume change was notsearched.

Fig. 5. (a) Comparison between measured vertical displacements during the period from August
2014 to March 2015 (solid arrows) and theoretical vertical displacements calculated by using the
optimal pressure source (open arrows), respectively, where upward arrows indicate the ground
uplifts. The large solid circle denotes the assumed horizontal location of the pressure source. (b)The contour plot of the root mean square (RMS) misfit in millimeters as a function of the depth
and volume change of the pressure source. The large solid circle denotes the location of the optimal
source parameters. The shaded zone is the region where the RMS misfit is less than 0.41 mm,
representing the 68% confidence region estimated from the F-test (Arnadottir and Segall, 1994).

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The optimal depth of the inflation source was determined to be 7.0 km below sea level. The
volume increase of this pressure source was estimated to be about 2.6×106 m3 (Table 3). The theoreticalvertical displacements deduced from the optimal source accounted for the observed data fairly well (Fig.
5a). According to the F-test (Arnadottir and Segall, 1994), the 68% confidence intervals for the source
parameters were estimated to be less than 0.41 mm (root mean square (RMS) misfits) in our case. Figure
5b is a contour plot of RMS misfit in millimeters calculated as a function of depth and volume changeof the pressure source. Here, we can see an approximate view of the RMS misfit changes corresponding
to variations of the volume change of the pressure source, although the volume change was not searchedin our pressure source analysis. The large solid circle denotes the location of the optimal source
parameters and the shaded zone shows the region where the RMS misfit was less than 0.41 mm. The
shaded zone has an elongated shape as the volume change enlarges according to the depth increase. It
extends from a depth of 3.5 km (volume increase of 4.7×105 m3) to a depth of 24.8 km (volume increaseof 2.5×108 m3 : not shown in Fig. 5b). Figure 5 indicates that the optimal source is a plausible modelthat sufficiently explains the observed data, although the source depth is practically unconstrained by
the data.
Table 3. Estimated volume changes of the pressure source located at 7.0 km
depth beneath Shindake crater and root mean square (RMS) misfits.
Leveling survey
interval
Aug.2014 ~ Mar.2015
Mar.2015 ~ Jul.2015
Jul.2015 ~ Oct.2015
Oct.2015 ~ Mar.2016
Mar.2016 ~ Jun.2016
Jun.2016 ~ Sep.2016

Volume change(m3)+2.6×106-2.1×105+3.3×105-1.1×106+1.9×106-3.6×106

RMS misfit
(mm)
0.37
0.71
0.36
0.40
0.43
0.37

Before the 3 August 2014 eruption, the inflation sources had been located at shallower depths
than 100 m below sea level beneath Shindake crater by using the data of GPS observations (Iguchi etal., 2002, 2007; Saito and Iguchi, 2006). The volume increases at these sources were estimated as
ranging from 6.2×103 m3 to 1.7×105 m3. Although the source depth and the volume increase estimatedin this study were unconstrained, the depth of the pressure source and its volume increase during theperiod between August 2014 and March 2015 were significantly larger than those before the August
2014 eruption under the assumption that the source was located beneath Shindake crater. A deeper sourcewith a larger volume increase was originally supported by the fact that obvious ground uplift was
detected along the northern half of the leveling route located apart from Shindake crater.
In this study period, we also observed relatively remarkable ground uplift during the period ofMarch-June 2016 (Fig. 2f) and ground subsidence during the period of June-September 2016 (Fig. 2g).
Attempting to analyze the data of these two periods by the same pressure source calculation processused in the period between August 2014 and March 2015, we obtained the optimal source depths andvolume changes of 30.0 km and 4.0×108 m3 for March-June 2016 data and 14.3 km and -4.3×107 m3
for June-September 2016 data, respectively. These are much larger depths and volume changes thanthose obtained during the period between August 2014 and March 2015, and the source depths are also
unconstrained by the data. Therefore, in the following discussion, we would rather additionally assume
the source depth to be 7.0 km than determine the source depth variations, in order to compare the resultswith those during the period between August 2014 and March 2015. We only calculated the optimal
volume changes at the pressure source by the least square fit of the model to the observed data.
In Table 3, we show the estimated volume changes of the pressure source at 7.0 km depth beneath
Shindake crater of the two periods as well as those during the periods of March-July 2015, July-October
2015 and October 2015-March 2016, in which minor vertical displacements were observed. The RMS
misfits of these estimations were 0.36-0.71 mm (Table 3), similar to the amounts of the verticaldisplacement errors (Fig. 2), suggesting that these sources are among possible models that explain theobserved data.

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The cumulative volume changes since the August 2014 survey were calculated from Table 3 anddisplayed in Fig. 6. It was inferred that even after the 29 May and 18 June 2015 eruptions about 2-3×
106 m3 magma remained until October 2015. The amount of the ejected magma associated with the 29
May 2015 eruption was estimated as 66-110×104 tons (2.6-4.4×105 m3, where the density of magma
was assumed to be 2500 kg/m3) inferred from the volcanic plume heights (Meteorological ResearchInstitute of Japan, 2015). Removal of the order of 105 m3 magma from the pressure source at 7 km depth
does not conflict with our estimates in Fig. 6 (and with observed vertical displacements). It is notnecessary that the removal of magma from the source at 7 km depth was simultaneous with the eruption.
Considering that the volcanic glow was observed above Shindake crater on and after March 24, 2015
by a JMA high-sensitivity monitoring camera (Japan Meteorological Agency, 2015), part of the magma
was supposed to have moved from 7 km depth to the shallow part beneath Shindake crater in late March2015. In addition, we calculated the expected slope distance change on the baseline between the GNSS
stations in Kuchierabujima and Yakushima Islands (Fig. 1a) assuming some depths and volume changesof the Mogi sources beneath Shindake crater. It is suggested that a few millimeters slope distanceshortening observed during the period from December 2014 to January 2015 (Iguchi et al., 2017, in thisissue) was not produced by our optimal source parameters during the period between August 2014 andMarch 2015 (depth of 7 km and volume increase of 2.6×106 m3: this source produces about 0.2 mm
slope distance elongation), but was caused by another shallower inflation source. This result also
supports the magma movement to the shallow part beneath Shindake crater.

Fig. 6. Cumulative volume change since August 2014 survey at the pressure source located at 7.0
km depth beneath Shindake crater. The three vertical broken lines indicate the timing of the 3August 2014, the 29 May 2015 and the 18 June 2015 eruptions.

After 1-2×106 m3 volume decrease and increase in the following periods from October 2015
through March 2016 to June 2016, most of the remaining magma appears to have been removed during
the period of June-September 2016. Since no magma ejection associated with the eruptions wasobserved during the period of June-September 2016, one of the possible causes of the volume decrease
was magma drain back to deeper than 7 km, although no other supporting evidence of the magma drainback was found. Contraction of the baseline length of GNSS stations had been observed across Shindakecrater since January 2016, and it continued at least until July 2016 after that the data were missed dueto the GNSS station problems (Japan Meteorological Agency, 2016). This might be a signal that themagma drain back began at a shallow part beneath Shindake crater in early 2016. In order to clarify thecause of the volume decrease, successive monitoring by repeated precise leveling surveys is needed.

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5. Conclusions
The vertical ground deformation related with the 2014 and 2015 eruptions at Kuchierabujima Volcano
was revealed by the precise leveling surveys conducted repeatedly after the occurrence of the 3 August
2014 eruption. In the surveys, we measured only the northern half of the leveling route, which was
located more than 2.3 km apart from Shindake crater, since the southern part of the leveling route wasin an off-limits area assigned due to the eruptions.
The results of the August 2014 survey suggest that the vertical displacements associated withthe 3 August 2014 eruption were not remarkable along the northern half of the leveling route. On theother hand, the obvious ground uplift toward the central part of the volcanic edifice was detected during
the period between August 2014 and March 2015 surveys. The ground uplift remained unchanged until
around October 2015, even after the eruptions on 29 May and 18 June 2015. After the following
relatively minor ground subsidence and uplift, significant ground subsidence toward the central part ofthe volcanic edifice was observed during the period between June and September 2016 and the groundreturned to almost the same level of that in the August 2014 survey.
From the analysis based on Mogi’s model assuming that the horizontal source location to bebeneath Shindake crater, the optimal depth of the inflation source was estimated to be 7.0 km below sea
level with a volume increase of 2.6×106 m3 by using the obvious uplift data during the period from
August 2014 to March 2015. The depth of the pressure source and its volume increase were significantly
larger than those before the August 2014 eruption inferred from the GPS data. During the period from
August 2014 to March 2015, other observation data, including the SO2 emission rate, GNSS andseismicity simultaneously indicated remarkable signals related to the increase of volcanic activity. It issuggested that the rapid magma input to the volcanic edifice occurred and led to the large eruption on
May 29, 2015. The results of the source volume change calculations in the following periods suggestthat most of the stored magma before the 29 May 2015 eruption appeared to be removed during theperiod of June-September 2016. Since no eruption was observed during the period of June-September
2016, one of the possible causes of the volume decrease was magma draining back to the depths.
Repeated precise leveling surveys are an important and useful technique for assessing theeruptive activity at Kuchierabujima Volcano. In order to evaluate the potential for the following eruption,
successive monitoring with repeated precise leveling surveys is needed.
Acknowledgements
We thank the staff of Sakurajima Volcano Research Center, Disaster Prevention Research Institute,
Kyoto University, for help and assistance with the observations. The leveling surveys were conductedwith the support of Yakushima town and Japan Meteorological Agency. We are also grateful to two
anonymous reviewers and a sub-editor for constructive and valuable comments and suggestions. Somefigures were drawn using GMT software (Wessel and Smith, 1995). This study was supported by MEXT
KAKENHI Grant Number 15H05794 and by the Ministry of Education, Culture, Sports, Science andTechnology (MEXT) of Japan, under its Earthquake and Volcano Hazards Observation and Research
Program.
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