IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1A1_2F-O17

Room B2

Date/Time: July 21 9:45-10:00

A precursory process of the 1914 eruption of the Sakurajima volcano inferred fromexperience of residents
Masato Iguchi
Kyoto University, Japan
E-mail: iguchi@svo.dpri.kyoto-u.ac.jp

The 1914 eruption of the Sakurajima volcano was the largest eruption in Japan in 20th century. The eruption wascommenced by plinean eruption at west and east flanks and followed by effusion of lava. Prior to the beginning of
the eruption at 10 AM on January 12, abnormal phenomena were recognized by residents. Here, I try to interpretthe abnormal phenomena based on recent knowledge obtained by volcano observations. 1) Discharge of CO2 .
In July 1913, two people died in a valley of southern part of Sakurajima and it was recognized that the deaths
were caused by lack in oxygen due to dense CO2 . Distribution of large discharge rate of CO2 from soil gas andquite dense concentration in boreholes suggest a large gas reservoir. Concentration of CO2 in gas from a hot spring
increases when inflation of the volcano is detected by a strainmeter in present activity of Sakurajima. The discharge
rate of CO2 increased rapidly due to inflation of the volcano half year before the eruption. 2) Uplift of Sakurajima.
In December 1913, water withered at a well, where the water had never withered even in low tides of spring tides.
The water wells were located near the sea coast and the water level was balanced with tide level. Descend of waterlevel was a relative displacement to uplift of Sakurajima. It is estimated that the ground was uplifted by 0.5 m andvolume increase of a source was 1.4×108 m3, assuming the source at a depth of 6km beneath central cones ofSakurajima. 3) Intense seismicity. Volcanic earthquake including felt one began to swarm at 3 AM on January 11
and the number of earthquakes increased until the beginning of eruption. 4) Acceleration of inflation. Sea bottom
exposed at SW coast at 2 AM on the day of the eruption. An underwater rock, which had never appeared even in low
tides of spring tides, was uplifted above sea water at least 60 cm. Taking theoretical tide level change into account,
it is estimated that the rock was uplifted by 80 cm and the uplift was caused by volume increase 1.7×108 m3 of asource beneath the central cones. The rapid inflation was associated with the intense seismicity. 5) Pressurizationof underground water. Around 6-7 AM, it was found that water overflowed from wells and hot water was effusedfrom sea coast. It is interpreted that the underground water beneath the volcano was pressurized due to ascent ofmagma to quite shallow parts at east and west flanks. It is estimated that intrusion rate of magma reached an orderof 108 m3/day immediately before eruption. On the other hand, intrusion rates of magma were 105 m3/day during
the period of frequent vulcanian eruptions at a crater of Minamidake from 1974 to 1992 and an order of 104 m3/day
during the present activity at the Showa crater. Intrusion rate of magma is an important parameter to forecast scaleof eruption.

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