Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 77-90

ArticleAn Interpretation on Secular Changes in Deformation Caused by
the 1914 Eruption of Sakurajima Volcano
Izumi YOKOYAMA＊(Received January 4, 2011 ; Accepted April 16, 2012)
The 1914 eruption of Sakurajima volcano was one of the largest ones in its history and first observed withscientific methods of those days. The eruption flowed out a large volume of lavas, and was accompanied withremarkable deformations on and around Aira caldera. The deformations consist of three stages: pre-eruptive, coeruptive and post-eruptive ones: The first one is determined by temporal tide-gauge observations at Kagoshima Port but,its origin is still unsettled. The second one is a quasi instant and elastic deformation, and therefore interpretable by asimple model. However, the location of center of depression is ambiguous due to the insufficient number ofobservations. Prior to the interpretation, it is re-examined considering the results of triangulations in the wider area, andthe location changed about 5 km S, on Sakurajima Island. The revised location of depression center can well interpretthe observed depression and compromises with seismological and petrological knowledge. The last one is a secular
recovery stage, and it has remained highly problematic whether its origin is endogenous or exogenous. The presentauthor interprets that the recovery changes are composed of two factors: In the period before around 1940,
viscoelasticity of the crust acted predominantly (endogenous) and later magma pressure accumulated in the reservoir
gradually joined to act (exogenous) and its contribution had become noticeable after 1955. The recovery of thedepression is interpreted as viscoelastic with retardation time of 16.6 years. Hence, viscosity of the earth crust around19
Aira caldera is determined at about 3 × 10 Pa· s on the assumption that the crust is Kelvin-type. The value iscomparable to those obtained with Miyake volcano and by laboratory measurement of a granite piece.
The other cases of viscoelastic deformations accompanied with volcanic eruptions are searched: the 1779 eruptionof Sakurajima volcano and the 1983 eruption of Miyake volcano are exemplified.
Key words : Sakurajima volcano, volcanogenetic deformation, recovery of deformation, viscoelasticity of the crust

1．Introduction
Sakurajima volcano stands on the S rim of Aira caldera
measuring about 20 km in diameter. The historical records
of eruptions on Sakurajima date back to 708 A.D. In its
eruption history, the 1471〜76, the 1779〜80 and the 1914
eruptions are the largest threes, all discharging considerable amounts of lavas from parasitic craters. At present,
the volcano is one of the most active volcanoes in theworld.
The 1914 eruption was fully described and studied
geophysically by Omori (1914-1922) and geologically byKotô (1916). The present discussion owes very much totheir works though it is limited to a study on deformations
caused by the eruption. The significant subsidence around
Aira caldera caused by the 1914 eruption were confirmedby repeated precise levelings and triangulations by theMilitary Survey as shown in Figs. 1 and 4 where the location of center of the subsidence is inevitably ambiguous
because the observations were limited on land and their
number was not enough. The secular recovery of the 1914＊

Higashi 1-17-7-1304, Kunitachi, Tokyo 186-0002, Japan.

subsidence has been pursued for about one century after
the eruption. During the period, the 1946 eruption issued
lava flows and the recovery was partly reversed. After
that, the recovery has proceeded further and attained theoriginal level around 1970.
Such remarkable crustal deformations and their secularchanges have held the earth-scientistʼs interest and roused
many arguments. Firstly Sassa (1956) mentioned possible
relationship between the secular recovery of the 1914
subsidence and the activities of Sakurajima volcano. And
Mogi (1958) discussed the deformation and achieved tointerpret it by a source of dilatational type. Later
Yokoyama (1986) studied the secular changes on the dataof those days, and further, extended his idea to a discussionof caldera structure in general (Yokoyama, 2005).
2．Deformation Related to the 1914 Eruption of Sakurajima Volcano
Networks of precise levels along the route circulating
Aira caldera were first established in 1892 by the Military
Corresponding author: Izumi Yokoyama
e-mail: yokoizu@aol.jp