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 wasconfirmed 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. Inaddition, 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 survey1．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 evidenceof 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