Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 103-114

ArticleThe Inflation-deflation History of Aira Caldera for the Last 120 Years andthe Possibility of a Forthcoming Large Eruption at Sakurajima Volcano
Kenʼichiro YAMASHINA＊(Received December 22, 2011 ; Accepted December 28, 2012)
In order to assess the potential for future eruptive activity at Sakurajima volcano, southern Japan, grounddeformation around northern Kagoshima Bay (i.e., Aira caldera), especially the height change between leveling stations
2474 and 2480, was re-analyzed. Prior to the large eruption in 1914 at Sakurajima volcano, a remarkable inflation was
confirmed at Aira caldera, based on re-surveyed data that were not referred to in previous investigations. Considering
those data, the upper limit of the magma storage at Aira caldera just before the 1914 eruption could be obtained.
Although the 1914 eruption accompanied a remarkable deflation at Aira caldera, magma started to accumulate again
and was likely to have exceeded the level observed in 1900 (14 years before the large eruption). Around the early
1970s, it approached the level before the 1914 eruption. After the ground uplift stopped and slightly reversed during aperiod of extremely frequent explosions at Sakurajima volcano in the 1970s and 1980s, the inflation seems to again beapproaching the inferred level before the 1914 eruption, suggesting the possibility of the next large eruption. In
addition, inconsistencies between the inferred amounts of magma supply and observed volumes of erupted materials
were discussed and left for further study.
Key words : Sakurajima volcano, Aira caldera, eruption, inflation-deflation, leveling survey

1．Introduction
In January 1914, a large eruption occurred at Sakurajima
volcano, located several kilometers from the harbor ofKagoshima City in Kyushu, southern Japan. During thisPlinian type eruption, large amounts of volcanic ash andpumice were ejected from the summit crater and thickly
covered the surrounding region (e.g., Omori, 1914; Koto,
1916). In addition, lava flowed down to the coast fromnewly formed small craters on the west and southeast
flanks of the volcano. After the occurrence of the eruption, sea water overflowed into the salt fields and thecoastal region along the northern part of Kagoshima Bay.
According to a subsequent leveling survey by the Land
Survey of Japan (1915), a remarkable subsidence of theland was confirmed to have occurred over an area several
tens of kilometers in diameter (Fig. 1). It was one of themost remarkable eruptive events in Japan during the last
100 years.
The depressed topography of the northern part ofKagoshima Bay was basically formed as a composite
caldera, i.e., Aira caldera (e.g., Aramaki, 1984), which wascaused by great eruptions around 29 thousand years before
present in calibrated radiocarbon age (e.g., Okuno, 2002).
After that, Sakurajima volcano was formed as a post
caldera cone at the southern rim of the caldera. As discussed later, it is interesting that Sakurajima volcano was＊

Earthquake Research Institute, University of Tokyo, 1-1-1
Yayoi, Bunkyoku, Tokyo 113-0032.

not located at the center of the subsidence accompanying
the 1914 eruption of the volcano. Instead, the amount ofsubsidence at the benchmarks increased towards the centerof Aira caldera, and similar movement was also found at
triangulation points directed towards the same center (e.g.,
Land Survey of Japan, 1915; Omori 1916b; Hashimoto andTada, 1992). The co-eruptive subsidence of the groundstrongly suggests that magma had accumulated graduallyat Aira caldera, and that it pushed its way to Sakurajima
volcano in 1914 (e.g., Omori, 1916b; Sassa, 1956; Mogi,
1957; 1958; Kamo, 1994).
In previous discussions, however, the remarkable evidence
of inflation prior to the 1914 eruption (Yamashina, 1997)
was not considered. In order to discuss this pre-eruptive
deformation and the inflation-deflation process at Airacaldera, the leveling survey data obtained by both the Land
Survey of Japan (up to 1946) and the Geographical Survey
Institute of Japan (1960 and later; recently named theGeospatial Information Authority of Japan) are reviewed
in Section 2.
As indicated in Section 3, the present paper focuses on
the height change at leveling station 2474 versus 2480
located 9.5 km north-northeast. Previously, however, theheight change has been discussed for station 2474 versus
2469 located 8.5 km southwest (e.g., Sassa, 1956; Kamo,
1994; Eto et al., 1997; Yamamoto et al., 2010). Here,
Corresponding author: Kenʼichiro Yamashina
e-mail: yama@eri.u-tokyo.ac.jp

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Kenʼichiro YAMASHINA

Fig. 1. Ground subsidence around northern Kagoshima Bay (i. e., Aira caldera) confirmed by leveling and partly
triangulation surveys (Land Survey of Japan, 1915; Omori, 1916b). Solid circles and triangles are benchmarks andtriangulation stations, respectively. Numerals along leveling routes represent the station numbers of benchmarks.
Contour curves with intervals of 10 cm are the difference between the 1890s and 1914 drawn by Yamashina (1997). A
cross marks the location of an inferred deflation source in 1914, and open circles at Sakurajima volcano are the summit
craters.

from stations 2474 to 2469, the locations approach
Sakurajima volcano. At this point, a secondary disturbance due to ground deformation originating at Sakurajima
volcano might be included in the observed data at 2469.
Another problem might be that a magnitude 7 earthquake
occurred in this area several hours after the commencement of the 1914 eruption (e.g., Omori, 1914). Although
the exact location of the earthquake is not clear, it wasprobably not very far from the city of Kagoshima in which
station 2469 is located (e. g., Omori, 1922; Abe, 1981;
Hashimoto and Tada, 1992). If so, coseismic deformation
might also affect the height of 2469 and adjacent stations.
Considering these possibilities, the observed data at 2480
may be preferable as a reference for discussing the heightchange at 2474, as well as the inflation-deflation process
occurring at Aira caldera.
Owing to the present results, these problems are foundto have not been serious because the patterns of thetemporal height changes were essentially similar between
2474-2480 and 2474-2469 at least in the period after
1914. However, it is useful to examine the height change
at 2474 versus 2480 because the data at 2480 witness theprevious assumption that station 2474 would have uplifted

gradually before the 1914 eruption with almost the samerate observed after the eruption. The data also suggest that
Sakurajima volcano could now very likely be in a preparatory stage for a forthcoming large event.
Subsequently, the absolute height change at each location, the co-eruptive deflation in 1914, and the balance ofmagma supply and erupted materials for 120 years areanalyzed in Sections 4 and 5. Section 6 will demonstratethat a large problem regarding a quantitative balance
between magma storage and discharge at Aira caldera and
Sakurajima volcano remains for further study.
2．Leveling data along Kagoshima Bay
2-1 First survey in 1892
In the Meiji era, more than 120 years ago, according to anational plan to establish a precise topographic map of thewhole country, the Land Survey of Japan was initiated tosurvey the large number of leveling and triangulation
points all over Japan. According to reports from the Land
Survey of Japan (1915) and Omori (1916b), a levelingsurvey between the benchmarks at stations 2455-2488,
2797 (located between 2488 and 2489), and 2489-2512
located in Kagoshima Prefecture was carried out inThe Inflation-deflation History of Aira Caldera for the Last 120 Years andthe Possibility of a Forthcoming Large Eruption at Sakurajima Volcano

February, April, and May 1892.
This first survey in the Kagoshima area is sometimes
referred to as having been carried out in 1891. This isprobably because a report that collected the results of oldleveling surveys (Geographical Survey Institute of Japan,1975) indicated that the whole route from Kumamoto to
Kagoshima Prefectures was surveyed in 1891. Although
the survey of the route started in September 1891 at thesouthwestern part of Kumamoto Prefecture, it continued
into the next year to complete the operation up to thesoutheastern end of the route on the southeastern coast ofKagoshima Bay.
2-2 Re-survey in 1900
Referring to the reports from the Land Survey of Japan(1915) and Omori (1916b), the heights of several levelingstations along the northwestern coast of Kagoshima Bay
were not surveyed in 1892 but in 1900, without any
explanation. This situation became clear from old documents left by the Land Survey of Japan (1891 and later,
1898) and also from the database compiled by the
Geospatial Information Authority of Japan: After the firstsurvey in 1892, the stone-pillar benchmarks buried atseveral leveling stations (i.e., 2475, 2476, 2478, and 2479)
were found to be damaged or lost. In order to restore thedamaged stations, they were re-installed and surveyedagain in March 1900 along the route from stations 2472 to
2483. Consequently, the difference between 1892 and 1900
was also found at the stations not experiencing trouble
over this period.
2-3 Leveling survey in 1897-1898 and later by
Kyushu Railways
Referring to Omori (1916b), prior to the re-survey in
1900, it was reported that the benchmarks along thenorthwestern coast of Kagoshima Bay (stations 24702481, excepting 2478) were surveyed during September
1897 and May 1898 by Kyushu Railways to construct a
new railway line connecting cities in Kumamoto andKagoshima Prefectures. The leveling survey was also
extended southwestwards along the leveling route including stations 2455, 2456, and 2469 during July 1912
and July 1913. After the 1914 eruption, they were all resurveyed in April and May 1915. However, the survey
must have been less accurate than those by the Land
Survey of Japan. In fact, for example, a systematic
southward-dipping trend of error might be included in the
results for 1897-1898. Therefore, in the present paper, theresults obtained by Kyushu Railways are not considered
but are left for further discussion.
Incidentally, the relative heights of stations 2475, 2476,
and 2479 in 1897-1898 were fairly different from theresults in 1892, but rather close to those in 1900. Since thestone pillars were reported to be re-installed in 1900 atthese stations (Land Survey of Japan, 1891 and later), thedate of the survey or that of the re-installation itself may
not be exact.

2-4

105

Repeated surveys in the post-eruptive period
from 1914 to 1918
After the large eruption in 1914, the Land Survey ofJapan began to re-survey the benchmarks around northern
Kagoshima Bay and adjacent areas (i. e., stations from2457 to 2512 and from 2794 to 2797). The results
obtained during June and August 1914 proved aremarkable subsidence of the bay-side area (Land Surveyof Japan, 1915; Omori, 1916b). In order to confirm theextent of the subsided areas, a re-survey was also carriedout from December 1914 to April 1915 along the levelingroutes towards the northwest in Kumamoto Prefecture andthe northeast in Miyazaki Prefecture.
In this post-eruptive period, the benchmarks aroundnorthern Kagoshima Bay were again re-surveyed in
February 1915 (stations 2468-2488 and 2793-2797;
Omori, 1916b) and November-December 1918 (stations
2456-2512 and 2789-2797; Omori, 1920).
2-5 Repeated surveys from 1932 to 2006 and estimation of the missing data
Leveling surveys were repeated along northern
Kagoshima Bay by the Land Survey of Japan in 1932 and1946, and by the Geographical Survey Institute of Japan in
1960, 1962-1963, 1968, 1975, 1981, 1987, 1997, and2006. The observed data for these years are available in
the annual reports and special volumes published by the
Geographical Survey Institute of Japan (1955-2008, 1974
and 1975). In this period, there is a large problem inwhich unexpected trouble occurred in many benchmarks,
and it became necessary to interrupt the continuous
observation of a possible uplift or subsidence at eachstation. Such a lack of data is, if necessary, compensatedusing the following assumptions.
First, the benchmark at station 2474 is considered. It is
located on a little cape named Oosakinohana, penetrating
northern Kagoshima Bay, and is known to have subsided
the most in 1914 in the land area. In spite of its importance, troubles occurred there in 1932-1946, 19461960, 1963-1968, and 1968-1975. Fortunately, however,there seems to be a linear trend between the amounts ofdeformation at the respective stations in sections both
2470-2474 and 2474-2478. Therefore, any missing observation at 2474 could be estimated from the data at stations
2470 to 2478 using the method of least-square fitting tostraight lines. In order to determine the estimation error,
the same process is applied to the periods for which thedeformation at 2474 is known. The results indicate thatthe difference between the calculation and observation isusually within several millimeters as seen in Fig. 2.
The present paper also focuses on the deformation at
2480. This point is close to the northwestern-most part ofKagoshima Bay and had the least subsidence in this region
along the leveling route at the time of the 1914 eruption.
The station was fairly stable but was lost once during 1946
and 1960. Since the deformations at stations 2478, 2479,

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Kenʼichiro YAMASHINA

and 2480 are similar for almost every period, the missing
value can be estimated from those at 2478 and/or 2479.
The estimation error also seems to be within several millimeters as seen in Fig. 3.
2-6 Surveys by university members
During and after the 1970s, leveling surveys along theroutes within Sakurajima volcano and around northern
Kagoshima Bay were repeated by the Sakurajima Volcanological Observatory of the Kyoto University, theEarthquake Research Institute of the University of Tokyoand other cooperating organizations. Based on thesesurveys, detailed temporal changes in the ground wereobserved with respect to the eruptive activity at Sakurajima volcano (e.g., Eto, 1989b; Miyazaki, 1990; Eto et al.,
1997; Yamamoto et al., 2010). These surveys, however,
did not include the section focused on by the present paper,

Fig. 2. Difference of the height of station 2474 between
the observation and calculation obtained from the data
at adjacent stations from 2470 to 2473 and/or from2475 to 2478.

i. e., the stations from 2474 to 2480. Therefore, theirresults are not referred to in the following discussions.
3．120-year history of the relative height change between 2474 and 2480
3-1 Inflation prior to the large eruption in 1914
The extrapolation discussed in Section 2 enables plotting the progress of the height change at 2474 versus 2480
during almost 120 years, as seen in Fig. 4. The present
result supports the previous ones obtained for station 2474
compared with 2469 (e.g., Sassa, 1956; Kamo, 1994; Etoet al., 1997; Yamamoto et al., 2010). In those previous
results, however, the pre-eruptive uplift at 2474 from the
1890s to 1914 was not proved directly by the evidence ofleveling survey, but inferred from the analogy of the upliftobserved during 1918 and the 1960s, and probably the tidal

Fig. 3. Difference of the heights between stations 2480
and 2478 and/or 2479, representing an estimation errorof missing data at 2480.

Fig. 4. 120-year history of the height of station 2474 compared with 2480 located 9.5 km north-northeast.
Open circles are extrapolated from the data at adjacent stations.

The Inflation-deflation History of Aira Caldera for the Last 120 Years andthe Possibility of a Forthcoming Large Eruption at Sakurajima Volcano

107

Table 1. Inferred heights of benchmarks around northern Kagoshima Bay in February-April 1892, March 1900, and JuneJuly 1914 obtained in the present paper based on the results of the leveling survey by the Land Survey of Japan (referring
mainly to the database stored in the Geospatial Information Authority of Japan). Height changes during 1892-1900,
1900-1914, and 1892-1914 are also shown. However, the absolute values of heights and height changes at each station
are not so accurate and errors of several millimeters or more might be included. Here, it is assumed that the height of
2797 was 88.2230 m in 1914, and that the height change at station 2751 was negligible between 1892 and 1914.

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Kenʼichiro YAMASHINA

levels observed tentatively at Kagoshima harbor quay
close to station 2469. In fact, Omori (1916b) concluded
that the tidal level at the quay rose markedly from 19031905 to 1914-1916, suggesting the subsidence of the quay.
However the change of, for example, monthly mean sealevel fluctuated from 39 to 68 cm between the average in
1903-1905 and the same months in 1914-1916 (Omori,
1916a). Considering that there are various factors to disturb
the tidal levels, it seems difficult to discuss the quantity ofa reliable pre-eruptive uplift of the quay.
As seen in Fig. 4 and Table 1, the relative height ofstation 2474 versus 2480 increased 6. 22 cm from 1892
through 1900, with an average rate of 0. 79 cm/year.
Similar uplifts also occurred in 1918-1932 and 1946-1960
with almost the same average rate, i. e., 0. 83 and 0. 82
cm/year, respectively. This strongly suggests that, in theless active period at Sakurajima volcano, the relative uplift
at 2474 occurred nearly continuously at an almost stable
rate of about 0.8 cm/year up to at least the 1960s.
As a result, if it is possible to assume that the uplift
continued up to the time of the 1914 eruption, the total
relative uplift would amount to about 17 cm from 1892 to
the beginning of 1914. This is about 0.6 times the value
obtained from the difference in the survey between 1892
and 1914, indicating that the co-eruptive deformation in1914 cannot be obtained by taking only the difference inthe observed values in 1892 and 1914. If the location of
the source of the pre-eruptive uplift was similar to that of
the co-eruptive subsidence, the total co-eruptive subsidence at any station around northern Kagoshima Bay
would be roughly about 1.6 times the observed difference
between 1892 and 1914.
3-2 The 1946 eruption and a possible deflation
In 1946, a significant eruption occurred again at
Sakurajima volcano. In this year, there were repeatedexplosions with ash ejection at one of the craters at thesummit area, i.e., the Showa crater, and lava flowed down
to the southeastern mountain foot from March to May.
As seen in Fig. 4, the surveyed data before and after thiseruption, i.e., July to August 1932 and August 1946, weresimilar. However, this does not necessarily suggest thatthe ground surface was almost fixed during this period.
The actual process was probably a progressive gradualuplift and a rapid subsidence in 1946, as discussed before(e.g., Sassa, 1956; Kamo, 1994). The inferred process isindicated by dotted lines in Fig. 4.
A similar process might have occurred on a small scale
during December 1960 and February 1963. In these years,
explosions occurred intermittently at the summit area. In
March 1961, there were intense events in which thewindow glasses of many houses were broken at the foot of
the mountain (e.g., Japan Meteorological Agency, 1961).
It is not clear, but one or more of these explosions may
have contributed to interrupting the uplift.
Prior to this period, explosions occurred occasionallyfollowing a somewhat significant event in 1955. Including
the 1955 event, the correlation between the explosions andthe height change at 2474 is not known, because theinterval of the leveling survey was too long to detect asmall temporal difference.
3-3 Retreat of uplift during the 1970s and 1980s
In the 1970s and 1980s, the ground deformation
changed entirely. The uplift at 2474 slowed down aroundthe 1970s and gradually reversed a little around the 1980s.
During nearly the last 120 years, the 1970s and 1980s
were unusual in that explosions at the summit area of
Sakurajima volcano occurred again and again quite
frequently, and large amounts of volcanic ash fell downaround the volcano every year (e. g., Eto, 1984, 1989a;
Nakamura, 2002). Although individual explosions were
not large, it is evident that the different progress of theground deformation during this period was caused by theanomalous repetition of explosive activity.
Adding to this well-known understanding, it seems to beimportant that, around the early 1970s, the relative heightof 2474 compared with 2480 exceeded the level observedin 1900, and approached the inferred level just before the1914 eruption. This might suggest that the magma
reservoir at Aira caldera inflated almost to its upper limit
during this period. If the mechanical state of the conduit
connecting to Sakurajima volcano was the same as in
1914, a large eruption similar to the 1914 event might haveoccurred. During this period, however, the actual process
was quite different, i.e., there was an anomalous repetitionof explosive activities. Considering the occasional
repetition of explosions especially since 1955, this might
have affected the conduit to allow the gradualtransportation of magma to Sakurajima volcano.
3-4 Progress of uplift in the 1990s and 2000s
Ejection of volcanic ash at Sakurajima volcano has
decreased since around 1993 (e.g., Nakamura, 2002). Thischange soon resulted in the uplift at 2474. In the survey of1997, a slight uplift was obtained. In 2006, the uplift was
confirmed once again, exceeding the levels in 1900 and
1975. The average rate of the uplift was 0. 35 cm/year
from 1997 to 2006. This rate was still smaller than thatobtained before the 1970s, but approaching again theinferred level before the 1914 eruption. Assuming thesame uplift rate, the subsidence occurred in 1914 has
probably been recovered to about 92 % in 2012. This may
be a warning of the possibility of the next large-scale
eruption in the near future.
4．Altitude of the benchmarks
Leveling surveys observe the relative heights between
benchmarks. Therefore, a height change relative to acertain station can be obtained fairly precisely. However,
the absolute altitude or the true height change at eachlocation is not obtained directly. It is also problematic thatit takes many days to complete a survey of a long leveling

The Inflation-deflation History of Aira Caldera for the Last 120 Years andthe Possibility of a Forthcoming Large Eruption at Sakurajima Volcano

route. In the case of the present region, the altitudes of thebenchmarks for the first survey in the 1890s was obtainedby connecting the leveling route with the tidal station at
Hososhima located on the east coast of middle Kyushu,
roughly 120 km northeast of station 2797.
The benchmarks along northern Kagoshima Bay weresurveyed in 1892 including 2797 located near the northeast
corner of Kagoshima Bay. Nearly three years later, theleveling route between 2797 and Hososhima was surveyedfrom October 1894 to February 1895. Here, the junction
point 2797 was surveyed twice in February 1892 and in
January 1895. In order to connect the results of thedifferent leveling routes, it was assumed that there was no
vertical ground deformation at 2797 during this period (e.
g., Land Survey of Japan, 1915; Omori, 1916b). However
such an assumption is not necessarily reasonable in thepresent area, so the true height change is discussed below.
As mentioned in Section 3-1, during the pre-eruptive
period from 1892 to 1914, the relative height between
stations 2474 and 2480 probably uplifted by 0.6 times theobserved subsidence between 1892 and after the 1914
eruption. Based on the surveys in 1892, 1900, and 1914,
similar ratios are obtained for the relative heights of2474-2472, 2474-2477, 2477-2480, and 2482-2480, i.e.,
0.4, 0.6, 0.7, and 0.7, respectively.
Using an averaged ratio of 0. 6, annual uplifts aretentatively estimated as 2. 3, 1. 6, and 0. 93 cm/year atstations 2474 (−84.32 cm), 2480 (−56.70 cm), and 2797(− 33. 66 cm), respectively. Here, the values in parentheses represent the difference in height between the 1890sand 1914, neglecting the problem just discussed. These
values, however, differ slightly from those presented by
the Land Survey of Japan (1915) and Omori (1916b). Thisis because the present values were obtained under theassumption that station 2751 in Miyazaki City, locatedabout 60 km ENE of station 2797, was fixed, and that theold values obtained in the 1890s by the Land Survey ofJapan were corrected to eliminate the mis-closure of theleveling route, along which the survey took several years
to complete the loop. The heights also differ from thoseby the Geographical Survey Institute of Japan (1975) andHashimoto and Tada (1992) who assumed the fixed point
at 2736 and 2751-1, respectively. In the present paper, thefixed point is tentatively moved to 2751 in order to avoid
an apparent discrepancy in the height difference between
2751-1 and 2751, i.e., there is a slight inconsistency of
about 1 mm between the data reported by the Land Surveyof Japan (1915), the Geographical Survey Institute ofJapan (1975), and the database stored in the Geospatial
Information Authority of Japan.
According to the value stated above, i.e., 0.93 cm/year,
station 2797 is tentatively suggested to have uplifted about
2. 7 cm during the three-year interruption of the survey.
Considering this, all of the heights of the benchmarks
along northern Kagoshima Bay should be lowered by 2.7

109cm from the reported values in 1892. However, correcting
the heights in 1892 results in changing the correction term
itself. After iterations of the same process, the annual
uplifts have been revised to 2.3, 1.5, and 0.86 cm/year atstations 2474, 2480, and 2797, respectively. The
correction term is now replaced to 2.51 cm, and thus all of
the heights of the benchmarks along northern Kagoshima
Bay are lowered by 2.51 cm as new inferred values for
1892. Consequently, (i) the differences in the heights
between 1892 and 1914 and (ii) the amounts of the actualsubsidence are inferred to be − 81. 8 and − 131 cm at
2474, −54.2 and −87 cm at 2480, and −31.2 and −50
cm at 2797, respectively, although errors of several
millimeters or more might be included. The inferredheights of stations 2455-2797 in 1892, 1900, and 1914 arelisted in Table 1.
Along the leveling route between 2751 and 2797, theinferred heights for 1892 are lowered by 0 at 2751 and 2.51
cm at 2797. For the other stations, the correction term isinterpolated in proportion to the distance from 2797.
5．Inflation-deflation models based on the leveling survey data
5-1 Formulation of displacements and volume
changes
If a subsurface inflation-deflation source is approximated by a spherical pressure source in a semi-infinite
elastic medium, vertical and horizontal ground displacements at an arbitrary place, i.e., uz and ur, respectively, areexpressed as follows, referring to the formulation by
Sezawa (1931) and later by, e. g., Soeda (1944) andYamakawa (1955).
u z=C 0DR 3,
u r=C 0rR 3.
Here,
C 0=

λ+2μa 3P 0,
2μλ+μ

R= D 2+r 2 .
D, R, and r are the depth, the three-dimensional
distance, and the horizontal distance of the center of thepressure source, respectively; λ and μ are Lameʼs elastic
constant; a3P0 is a coefficient to represent the intensity ofthe pressure source. Incidentally, P0 is the amount of the
expanding or contracting pressure change on a sphericalsurface with an arbitrary but small radius a.
In the same notation, the volumetric change of thepressure source, ΔV, is approximated as follows(Yamashina, 1986, 1992):
ΔV =πa 3P 0μ .
Otherwise, using the estimated vertical displacement uz

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Kenʼichiro YAMASHINA

at an arbitrary place,ΔV =32πλ+μ R
u z.
λ+2μ D

5-2 Total volume of deflation in the 1914 eruption
A plausible source of the deflation is analyzed with anassumption of λ=μ, based on the revised values ofsubsidence obtained in Section 4. The best model fitted at
first to the difference between 1892 and 1914 for thestations represented in Fig. 1 is 10.4 km in depth and 590
×106 m3 in volume contraction. In this calculation, theheight of the reference point, i.e., station 2751, is allowed
to change freely. As a result, in the best-fit model, 0.4 cmof the subsidence is added at all stations.
The present result is essentially the same as thoseobtained by Mogi (1958; 10 km in depth and about 660×106 m3 in volume contraction) and Hashimoto and Tada
(1992; 8.8 km in depth and 510×106 m3 in volume contraction). However, the progress of the pre-eruptive
inflation during 1892 and 1914 was not included in thosemodels. When this is taken into consideration in thepresent model, the co-eruptive contraction of the pressuresource becomes 1.6 times the value stated above, assumingthat the same inflation rate continued up to the time of the1914 eruption. That is, the volume of the contraction ismodified to 940×106m3 in the present model.
In 1914, Sakurajima volcano extruded a huge volume oflava estimated to be 1560×106m3 (Omori, 1914), 1140×106m3 (Koto, 1916), 900×106m3 (Harada, 1948), or more
precisely 1340×106 m3 (Ishihara et al., 1981). In addition, large amounts of ash and pumice were also ejected (e.
g., Omori, 1914; Koto, 1916). Considering the density ofthe materials, the volume, i.e. 620×106m3 (Omori, 1914),
would roughly correspond to 310×106 m3 in volume ofmagma.
In spite of these facts, previous deflation models for the1914 eruption estimated the subsurface contraction at Airacaldera to be far less than the total volume of magma
actually erupted. The present revised model decreases thisdifference to some extent. However, it is still interestingthat the contraction of the pressure source at Aira caldera is
not sufficient to explain the erupted materials even in thepresent model. The lack of magma, if it is actually correct,
might suggest the possibilities to be mentioned in Section
6.
5-3 Magma storage and discharge up to the 1960s
including the 1946 eruption
Before and after the 1914 eruption of Sakurajima
volcano, i.e., 1892-1900, 1918-1932, and 1946-1960, inflation at Aira caldera seems to have continued at nearly aconstant rate as seen in Fig. 4. Considering this, the same
inflation process at Aira caldera was assumed to continue
up to at least the 1960s. Consequently, since the fraction
0.6 out of 1.6 (cf. Section 3-1) is considered to be thecontribution of the 22-year accumulation prior to the 1914

eruption, the annual volume of magma accumulation
would be roughly 0.6/1.6/22≃0.017 times the 1914 coeruptive contraction, i.e., 16×106m3/year.
A deflation process similar to the 1914 eruption is likely
to have occurred in 1946. Although the deflation associated with the eruption in March-May 1946 was not observed directly, the leveling survey in August 1946 indicated that the difference from the previous survey in Julyand August 1932 was fairly small. That is, the volume
accumulated between 1932 and 1946, i. e., a 14-year
period, may thus be considered to have been effectively
cancelled by the 1946 eruption. The accumulation during
14 years would be about 230×106m3.
The possible deflation calculated for 1946 is about 1/4
that for 1914. This corresponds to the scale of the 1946
eruption being smaller than the one in 1914. Quantitatively, the volume of lava erupted during March and May
1946 was previously estimated to be about 100×106 m3(Hagiwara et al., 1948) or 92×106m3 (Morimoto, 1948).
Subsequently, it was analyzed more precisely as 180×106
m3 (Ishihara et al., 1981). In addition, the volume of ash
was estimated to be 20×106m3 (Kobayashi and Ishihara,1988). Alternatively, based on the distribution map of
ash-fall deposits by Hagiwara et al. (1948), the volume ofash might be inferred to be roughly 130×106m3, if extrapolating the distribution function of ash-falls proposed by
Eto (1989a) into the area outside of the map. In either
case, including ash which might correspond to about
10-65×106m3 in volume of magma, the total volume ofmagma that erupted in 1946 would not be so muchdifferent from the estimated value of contraction, i. e.,
about 230×106m3.
Here, the same inflation-deflation source as that in 18921914 is assumed because the height change in that period
was large at every leveling station around Aira caldera, andthus it would result in the most reliable solution for themagma reservoir. In order to check this assumption,height changes at 2469-2458, 2485-2487, and 2501-2504
located at southwest, northeast, and southeast of Airacaldera, respectively, are presented in Fig. 5. All of theirtemporal changes were similar to those at 2474-2480 in
Fig. 4, although it was, of course, not exactly the same.
This suggests that the present assumption is supported atleast to some extent.
5-4 Consumption of magma in the 1970s and 1980s
In the 1970s and 1980s, small explosions occurred
frequently at Sakurajima volcano. During the same period,
Aira caldera almost stopped inflating or began to deflateslightly. Based on the elaborate observation of ash fall, it
was pointed out that, roughly speaking, when the quantityof ash-fall deposits exceeded 10×106 ton/year, inflation
stopped and deflation began at Aira caldera (e.g., Sakurajima Volcanological Observatory, 1995). This weight
may be 15-17 × 106 ton/year if the fine ash distributed
outside the analyzed area is considered using the dis-

The Inflation-deflation History of Aira Caldera for the Last 120 Years andthe Possibility of a Forthcoming Large Eruption at Sakurajima Volcano

Fig. 5. Height changes at stations 2469, 2485, and 2501
compared with 2458, 2487, and 2504, respectively.
Open circles are extrapolated from the data at adjacentstations.

tribution function of ash-falls proposed by Eto (1989a).
However, the weight corresponds to only about 5-9x106
m3 /year in volume of magma, far less than the probablesupply rate of magma between 1892 and the 1960s, i.e.,
16x106 m3 /year, excepting at least a few years after the1914 eruption.
6．Discussion on the balance of magma supply andconsumption
6-1 Models to explain the volumetric balance in
1914
The volume of extruded lava in 1914 might not necessarily correspond to that of contraction, if vacant spaces
existed in the magma reservoir. In addition, if the inflation
rate accelerated just before the eruption, the inferred
volume of the contraction would be increased. Althoughthese factors could decrease the missing volume mentioned in Section 5-2, the effect would be limited
quantitatively.
The possible inconsistency of the volumetric balance
could be explained if all of the erupted materials in 1914
were not supplied from the magma reservoir at about 10
km in depth at Aira caldera, but also from another source
inside Sakurajima volcano. It is clear that if part of theerupted material was previously stored in Sakurajima
volcano itself, the amount of the co-eruptive supply from111

the reservoir at Aira caldera would be less than the total
volume of the eruption. Quantitatively, however, it will
not be easy to compensate the large difference because adeflation was not dominant at Sakurajima volcano in 1914
(cf. Fig. 1).
Considering the difficulties discussed above, the following explanations might be possible. (i) First, a supplyof magma from a deeper source beneath Aira caldera
might explain the missing volume. In the pre-eruptive
period, magma must have come up to the reservoir of Airacaldera year by year from the deeper source. In case of thelarge-scale eruption, once a conduit has opened from thereservoir at Aira caldera to Sakurajima volcano, magma
might have been supplied promptly from the deeper source
to the reservoir, or directly to the surface craters of the
volcano. If so, the volume change of the magma reservoir
at Aira caldera does not necessarily coincide with thevolume of erupted materials.
(ii) Second, if the reservoir at Aira caldera and thecraters at Sakurajima volcano are in fact connected by aconduit, the upward slope should be, roughly speaking,
only 45 degrees or less. Considering this, another speculation might also be possible: Materials erupted at Sakurajima volcano might not have been supplied from thereservoir at Aira caldera but from another deeper source
located beneath the volcano. If this deeper source also
connected with the reservoir at Aira caldera, after the large
eruption at Sakurajima volcano, the magmatic pressuremust have decreased not only at this deeper source but also
at the reservoir of Aira caldera. This would result in a
possible withdrawal of magma and the volume contraction
at Aira caldera.
All possible explanations have not yet been established,
and thus detailed physical and petrological considerations
are left for further study, including an evaluation of thepresent calculation based on a simple inflation-deflation
model.
6-2 Balance of magma supply and erupted materials
since the 1970s
In the case of the eruptive activity at Sakurajima
volcano during the 1970s and 1980s, the volume of erupted
materials was reported to be far less than the inferred
amount of magma supply at Aira caldera, as mentioned in
Section 5-4. In this period, there were no observations foraccumulation of considerable volume of magma at Sakurajima volcano or any other shallow place (e.g., Ishihara andEto, 1978; Eto, 1989b; Eto et al., 1997; Iguchi et al., 2008;
Yamamoto et al., 2010). Although it would not be enoughin volume, an underestimation of the erupted materials, e.
g., fine ash ejected high into the sky might contribute to thedeflation of the reservoir during this period. A temporalchange in the magma supply rate at Aira caldera might beanother possibility. Observations might strongly suggest
this (e.g., Eto et al., 1997; Iguchi et al., 2008; Yamamoto
et al., 2010), but no particular reason for such a change has

Kenʼichiro YAMASHINA

112

been confirmed. Further discussions are desirable in the
future.
7．Conclusions
Ground deformation around northern Kagoshima Bay,especially the height change between stations 2474 and2480 was re-examined for the last 120 years. Here,northern Kagoshima Bay approximately overlaps the collapsed area of Aira caldera. With respect to the present
results, the following points have been clarified.
(1) Along the leveling route of special interest, several
stations including 2474 and 2480 were surveyed twice
before the 1914 large eruption at Sakurajima volcano,revealing a marked pre-eruptive inflation of a plausible
magma reservoir at Aira caldera.
(2) Around the early 1970s, the magma storage at Airacaldera might have exceeded the level observed in 1900
and approached the inferred level before the 1914 eruption
based on the history of the height change between 2474
and 2480. This could be the reason for the remarkablerepetition of explosive activity continuing at Sakurajima
volcano in the 1970s and 1980s. If magma was not supplied smoothly to Sakurajima volcano during that period, alarge eruption could have occurred.
(3) Explosive activity and ash ejection declined sincethe 1990s, and the height of 2474 compared with 2480, i.e.,
the inflation at Aira caldera, is probably again approaching
the inferred level before the 1914 eruption. If the inflation
continues, it may result in the next large-scale eruption in
the near future.
(4) A deflation model for the 1914 eruption was reanalyzed, i. e., the depth of the source and the volume
contraction are estimated to be 10.4 km and 940×106m3,
respectively. In addition, 16×106 m3 of magma is considered to have accumulated annually before and after theperiod of a large eruption at least up to the early 1970s.
Although several preliminary possibilities have been
discussed, a quantitative balance of magma accumulationand erupted materials has not yet been explained well. Fora better understanding of the volcanic process, detailed
discussion is left for further study, including investigating
the plausibility of the values obtained in the present paper.
Acknowledgements
The author would like to express his gratitude to Prof.
Masato Iguchi of the Kyoto University, Prof. Tetsuo
Kobayashi of the Kagoshima University, and referees fortheir valuable comments and suggestions. The author
would also like to thank the members of the Land Survey
and the Geospatial Information Authority of Japan(formerly the Geographical Survey Institute of Japan), fortheir elaborate precise leveling survey over many years.
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(Editorial handling Masato Iguchi)

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Kenʼichiro YAMASHINA

120 年間にわたる姶良カルデラの膨張収縮過程と桜島火山における大きな噴火の可能性
山科健一郎

桜島火山の噴火活動について見通しを得るため，北部鹿児島湾とほぼ位置が重なる姶良カルデラの周りの地殻変動について，特に水準点 2474 番と 2480 番の相対的な高さの変化に注目して検討を加えた．この区間
では，これまで注目されることがほとんどなかったが，1914 年の桜島火山の大きな噴火に先立って明瞭な膨張が生じていたであろうことが確かめられた．これにより，大きな噴火に先立って姶良カルデラ地下に蓄えられたマグマの 1914 年時点の限界量が推測される．1914 年の噴火によって姶良カルデラでは顕著な収縮が生じたが，その後膨張が進んで 1900 年（大噴火まで 14 年）の水準を超え，1970 年代初め頃には 1914 年の限界値にかなり近づいたと考えられる．桜島では，1970 年代から 80 年代にかけて爆発的活動が活発に続いた．この期間，姶良カルデラの膨張は停滞およびやや後退していたが，桜島の爆発的な活動が低下すると再び変動が進み始めた．近年，ここで推測された 1914 年の限界値にさらに近づきつつあり，大きな噴火の可能性があり得ることを示している．一方，姶良カルデラ地下のマグマ量の増減と桜島からの噴出物量の収支については課題があり，今後の検討に委ねられる．