International Workshop on the mechanism of Phreatic Eruption

Precursors to the eruption at Kuchinoerabujima volcano and decision
making of evacuation
Masato Iguchi1
1Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University
E-mail: iguchi.masato.8m(at)kyoto-u.ac.jp

On August 3, 2014 and May 29, 2015, eruptions occurred at the Shindake summit crater ofKuchinoerabujima Volcano, which is located in the Ryukyu Islands in southwestern Japan.
The Japan Meteorological Agency upgraded the Volcanic Alert Level (VAL) to 3 (restrictedzone within 2 km from the summit) after the 2014 eruption and to 5 (evacuation) after the2015 eruption. The possibility of implementing early warnings for eruptions and forecasting
the area most likely to suffer damage from volcanic eruptions was examined based onmonitoring data and the disaster areas of historic eruptions. A long-term process spanning
15 years and a relatively short-term process that could have been implemented immediately
before the onset of the 2014 eruption were identified. The seismicity, which mostlyrepresented extremely shallow (less than 0.5 km) volcano-tectonic earthquakes beneath thesummit crater, increased from July to December 1999. After the first increase in seismicity,
several repeated bursts of seismicity occurred. Repeated ground inflation events around thecrater detected by continuous GNSS were associated with the increase in seismicity.
Increases in the activity were also accompanied by increases in geothermal and fumarolicactivity. An alert zone of 1 or 2 km (VAL 2 or 3) from Shindake crater was established
immediately after three bursts of volcanic earthquakes occurred during the precursory periodof the 2014 eruption. However, there were no definite indications to trigger an upgrade to theVAL before the 2014 eruption because the relatively high volcanic activity had been stable
and the precursory tilt change began only a short time before the eruption. The phenomenaprior to the 2015 eruption were more intense than those prior to the 2014 eruption, asdemonstrated by the seismicity, which included a felt earthquake six days before the eruption;
the larger ground deformation; the higher rate of discharge of SO2 gas; and the higher
temperature. Among these precursors, the felt earthquake could be considered anappropriate indication to trigger an upgrade of the VAL from 3 to 5 for evacuation because
felt earthquakes suddenly increase the seismic energy, whereas other parameters showed
gradual progress. In historical cases, a few felt earthquakes were reported prior to theeruptions in 1931 and 1966. The felt earthquake before the 2015 eruption was induced by
the accumulation of other eruption-related parameters, such as an increase in the SO2

International Workshop on the mechanism of Phreatic Eruption

802015 eruption
Daily number of volcanic earthquake

28860
2014 eruption

40
200(m) 0.01

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May

0.00
-0.01

( ton/day)

-0.02
5000
4000

Slope distance change
SO 2 discharge rate

3000
2000
1000
0Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May
2014
2015

Fig. 1 Daily numbers of volcanic earthquakes (top), change in the slope distance(middle), and the discharge rate of SO2 gas (bottom) during the period from January 1,
2014, to May 31, 2015. The daily numbers were obtained by JMA. The slope distance
was measured between station KG1in Kuchinoerabujima, and GEONET station
Kamiyaku2 (960727) in Yakushima. The SO2 discharge rate was measured using thedifferential optical absorption spectroscopy (DOAS) traverse method on the volcano orby boat (Mori et al., 2017).
discharge rate, the inflation of the volcano, and the appearance of volcanic glow at thesummit. Decreases in the seismicity, SO2 gas discharge rate, and geothermal activity led toan initial reduction of the alert zone radius to less than 2.5 km in October 2015, and a further
reduction to less than 2 km (VAL 3) was later implemented based on the deflation around thesummit area in June 2016.
Main References
Mori, T., Morita, M., Iguchi, M., Fukuoka Regional Headquarters, 2017, Sulfur dioxide flux
monitoring using a public ferry after the 2014 eruption of Kuchinoerabujima Volcano,Japan, Jour. Nat. Disast. Sci., Vol. 38, 105-118.