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The 2011 eruptive activity of Shinmoedake volcano, Kirishimayama, Kyushu, Japan—Overview of activity and Volcanic Alert Level of the Japan Meteorological Agency—
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2013
Received: 9 November 2012
Accepted: 13 May 2013
Published: 8 July 2013
Following several years of small phreatic eruption events, Shinmoedake volcano in Kyushu, Japan, began a new phase of activity on 19 January 2011. The eruption commenced with a small phreatomagmatic event followed by subplinian events on 26 and 27 January. Lava emerged as a dome in the summit crater on 27 January and filled the crater until the beginning of February. During the subplinian events and lava dome growth, deflation of the deep magma chamber was observed by tiltmeters and GPS. Eruptive activity began to decrease in March 2011 and no eruptive event occurred after the 7 September 2011 event. In response to the eruption, the Japan Meteorological Agency (JMA) issued warnings repeatedly to prepare local people and infrastructures around Shinmoedake. JMA did not raise the Volcanic Alert Level before the first subplinian event, because of a lack of clear precursory signals. We conclude that a large amount of magma moved in the shallow Shinmoedake plumbing system for a short time, based on a number of geophysical observations. It is important to detect magma movement and understand eruptive phenomena quickly in order to mitigate risks of volcanic hazards.
After repeated phreatic eruption events in 2008 and 2010, Shinmoedake began magmatic eruptions in January 2011. Following a small phreatomagmatic event on 19 January 2011, the volcanic activity began with a subplinian event on 26 January. Three subplinian events, effusion of lava inside the summit crater and frequent vulcanian events took place. Here, we define “vulcanian event” as an eruption event accompanied by a strong shock wave and an explosion earthquake. During the subplinian events and effusion of lava, deflation of the deep magma chamber was observed by tiltmeters and GPS.
Over the course of the 2011 eruption, falling of a large amount of tephra and shockwaves damaged people’s lives and houses around Shinmoedake. To manage risks from volcanic eruption events of the 2011 eruption at Shin-moedake, the Japan Meteorological Agency (JMA) issued “volcanic information” to announce risks and restrictions on approaching to the volcanic areas for the public and others. JMA adopted a system of “Volcanic Alert Level” for active volcanoes in December 2007 (Ikeuchi and Yokota, 2007). This system was applied also to Shinmoedake in 2007, and JMA tried the risk management during the 2011 eruption by operating this system.
In this paper, we provide an overview of the 2011 eruption with the geophysical monitoring results mainly, and discuss the effect and problem of Volcanic Alert Level in the 2011 eruption at Shinmoedake.
2. Volcano Monitoring at Shinmoedake Volcano
3. Prior to the 2011 Magmatic Eruption
Both campaign and continuous GPS observations by JMA detected local ground deformation changes at Shin-moedake summit area before the 22 August 2008 eruption event. Inflation of the summit area was observed during 2005 to 2007, while deflation of the same area began in 2007 and continued until 2009, and the pressure sources for the inflation and deflation were identical, about 500–600 m below the crater with the volume changes of the inflation of about 1 × 10−5 m−3 (Takagi et al., submitted). More extensive ground deformation observed by Geospatial Information Authority of Japan showed a source of increasing pressure at 6–10 km depth, several kilometers northwest of Shinmoedake from the end of 2009 (Imakiire and Oowaki, 2011). The volume change for this source from the end of 2009 to January 2011 was calculated as 1.5×10−7 m−3 (Imakiire and Oowaki, 2011). Only after 2011 Shinmoedake eruption started, was it realized that this extensive deformation was caused by magma accumulation in the magma chamber that was located not just under the Shinmoedake crater but northwest of Shinmoedake.
List of 2008–2012 main eruptions of Shinmoedake.
Max Seismic Amp. (μ/sec) (SSE)
Max Air wave Amp. (Pa) (YNN)
Subplinian 7000m⋇ (Shimbori et al. submitted)
Subplinian 7000m⋇ (Shimbori et al. submitted)
Subplinian 7000m⋇ (Shimbori et al. (submitted))
After the eruption event on 10 July 2010, the number of volcanic earthquakes gradually decreased. However, at the beginning of October 2010, seismicity increased slightly and the plume activity became somewhat more vigorous (Fig. 3).
We evaluated the relative magnitudes of individual phreatic eruption events by comparing seismic and infra-sonic signals. Table 1 summarizes the amplitudes of volcanic tremors and infrasonic signals for these events. The infrasonic signals associated with the eruption event in 2008 could not be identified as they were masked by wind noise. Their amplitudes are similar for all phreatic eruption events, and are about one order of magnitude smaller than those for the magmatic eruption events which started in January 2011.
4. Overview of the 2011 Eruption—Time Sequence
At 01:27 on 19 January 2011 (JST used throughout this paper), a small eruption event occurred. Although an ash plume of this eruption was not witnessed due to bad weather, ash fall was observed at Miyakonojo and Nichinan cities, Miyazaki Prefecture, located about 60 km southeast of Shinmoedake. The ash emission continued for about 2 hours. The amplitude of volcanic tremor and infrasonic wave for this event were similar to those of phreatic eruption events in 2008 and 2010 (Table 1). However, according to Geshi et al. (2011) and Suzuki et al. (2013), the ash of this eruption event contained pumiceous particles about 10% in volume. Therefore, it is considered that this eruption was phreatomagmatic and new magma had reached to a shallow level under the Shinmoedake crater.
During the lava dome growth, a tilt change showing down of the NWW side at TCG station (see Fig. 2) was observed, suggesting the deflation of the deep magma chamber. The total volume of deflation associated with subplinian events and subsequent lava effusion was estimated as about 107 m3 (Kozono et al., 2013). This volume roughly equals to the ejecta volume in magmatic eruption during 26–31 January. It may suggest that magma moved from the deep magma chamber northwest of Shinmoedake directly to the crater without stopping beneath the crater.
A-type: volcano-tectonic earthquake with clear P and S phases and the dominant frequency is relatively high. BH-type: low frequency earthquake with unclear S phase and the dominant frequency more than about 3 Hz.
BL-type: low frequency earthquake with unclear S phase and the dominant frequency less than about 3 Hz.
EX-type: explosion-quake associated with vulcanian event.
BP-type: harmonic quake with unclear P and S phases, characterized by regular peaks of spectra, and composed of fundamental frequency and its overtone.
At 15:41 on 27 January, the first vulcanian event occurred between the second and third subplinian events. Since then, 13 vulcanian events occurred until 1 March 2011 (Table 1). The vulcanian event at 07:54, 1 February was most energetic, accompanied by the largest shockwave (Table 1) whose maximum amplitude was 458 Pa at YNN station (see Fig. 2), about 3 km away from the Shinmoedake crater. The shockwave broke glass windows in houses at Kirishima City, located several kilometers from the crater. Ballistic projectiles reached up to 3.2 km from the crater (Maeno et al., 2013).
From the beginning of February 2011, eruptive activity including vulcanian events occurred intermittently (Fig. 8). A vulcanian event on 14 February emitted pumice fragments with diameter of 1.5–5 cm that fell about 16 km NE from Shinmoedake, which broke windshields there. In the 13 March vulcanian event, the ash plume rose as high as 4,000 m above the carter and pumice fragments with 1–4 cm in diameter fell about 9 km SE from Shinmoedake. Further eruption events which occurred after February 2011 sent pumiceous lapilli several kilometers downwind, although the frequency and magnitude of eruption events gradually had decreased with time. No eruption event occurred after the 7 September 2011 event. The number of volcanic earthquakes then decreased, and the seismicity declined by May 2012, returning to the level before the 2011 eruptive activity.
In addition to seismic and geodetic monitoring, JMA periodically measured SO2 flux from the crater using a compact UV spectrometer (COMPUSS) (Mori et al., 2007). During the subplinian events, SO2 flux exceeded 10,000 tons/day (Fig. 8). On 28 January when lava was found inside the crater, the flux was more than 40,000 tons/day (Mori et al., 2011). The flux decreased to the levels of several hundred tons/day in mid-February when eruption events were repeated, under 100 tons/day in March 2012, and few tons/day after May 2012.
5. Magnitude and Precursors of the 2011 Shin-moedake Eruption Events
6. Volcanic Alert Level of JMA
Time sequence of Shinmoedake Volcanic Alert Level.
Volcanic Alert Level (Applied area)
Factors of level changes
Strat of Volcanic Alert Level system in Japan
Small phreatic event occurred.
Volcanic activity became calm.
Small phreatic event occurred.
Volcanic activity became calm.
Volcanic earthquakes increased.
Level 3 (2 km)
Subplinian event occurred.
Level 3 (3 km)
Vigorous growth of lava dome, increasing possibility of pyroclastic flow by its blowoff.
Level 3 (4 km)
A vulcanian event occurred; ballistic bombs reached 3.2 km from the crater.
Level 3 (3 km)
Frequency and magnitude of eruptive events became lower than those during January–February.
Level 3 (2 km)
Seismic returned into the activity level before 2011.
JMA raised the Volcanic Alert Level from 1 to 2 on 30 March 2010, when a small phreatic eruption event occurred. In response, local governments around Shinmoedake set the areas of access restriction near the crater. After that, JMA kept the Volcanic Alert Level in 2, because small eruptive events occurred intermittently until July 2010 and the seismic activity remained in a high level.
The first subplinian event occurred at 14:49 on 26 January 2011. In response to this event, JMA raised the Volcanic Alert Level from 2 to 3 and expanded the target area with the distance from 1 to 2 km from the crater rim at 18:00 on 26 January. Local governments around Shinmoedake took emergency response measures such as setting up emergency operation centers.
On 30 January when the diameter of lava dome was confirmed to be about 500 m using a SAR image, analyzed by the Meteorological Research Institute of JMA, a Volcanic Warning was issued at 01:35 on 31 January 2011, and the target area enlarged from 2 to 3 km from the crater while keeping the Volcanic Alert Level at 3. This was because of the possibility of pyroclastic flows and surges caused by demolition of the lava dome if an eruption event were to occur. In response, local governments prohibited access to the volcano in the area with about 3 km from the crater. Small resorts of hot spring and prefectural roads within the area 3 km from the crater were closed by local government. One of local governments, Takaharu Town in Miyazaki Prefecture, which is so close to the crater that residents were able to listen to terrible rumbling and to have received deposition of a large amount of tephra, issued the evacuation advisory on their own judgment. About 600 residents who lived within the area up to 9 km east of the crater were moved to the evacuation center located about 10 km from the crater.
At 07:54 on 1 February, a vulcanian event, the fourth event (counting from the first event on 27 January) occurred and ballistic bombs reached about 3 km west from the crater. JMA issued the Volcanic Warning at 11:20, February 2011, and enlarged the target area from 3 to 4 km from the crater, keeping the Volcanic Alert Level at 3. Soon after, local government expanded the restricted area to about 4 km from the crater, and the prefectural road Route 1, connecting Miyazaki and Kagoshima prefectures, was closed.
On late March 2011, the frequency and magnitude of the eruption events decreased. JMA reduced the target area from 4 to 3 km from the crater on 22 March 2011, keeping the Volcanic Alert Level at 3. The prefectural road Route 1 was opened by the local government, reflecting reduction of the target area.
Seismicity returned to the level before 2011 and the geodetic monitoring by tiltmeters indicated no additional intrusion of magma beneath Shinmoedake in May 2012. JMA reduced the target area from 3 to 2 km from the crater on 26 June 2012, keeping the Volcanic Alert Level at 3. In response, the local governments made the area of 3 km from the crater accessible and hot spring resorts resumed their business.
7. Discussion and Interpretation of Shinmoedake Magmatic System
During the lava dome growth in the 2011 eruption, many BL-type earthquakes, harmonic earthquakes and tremors were observed (Fig. 8). BL-type earthquakes which are common at andesitic volcanoes (e.g. Iguchi and Ishihara, 1990) were accompanied by small infrasonic pulses. In the case of Sakurajima volcano, BL-type earthquake swarms, being accompanied by harmonic tremor, were often observed before explosive eruptions (Maryanto et al., 2005). At Shinmoedake, harmonic tremor episodes occurred from 30 January to the beginning of February. The durations of tremor episodes were several minutes to several tens of minutes. The tremor had regular peaks of spectra, composed of a fundamental frequency (~1 Hz) and its overtones. Like BL-type earthquakes, harmonic tremor is often observed at andesite volcanoes, such as Asama and Sakurajima (e.g. Tameguri et al., 2005). With regard to Sakurajima, Tameguri et al. (2005) suggested that harmonic tremors were related to periodic volume changes caused by the resonance of a gas pocket formed at the top of magma-filled and pressurized conduit. We suggest a similar interpretation of BL-type earthquake and harmonic tremor at Shinmoedake.
During the 2011 activity of Shinmoedake, assignment of Volcanic Alert Levels by JMA was challenging because of the lack of clear precursors before the onset of subplinian events or other vulcanian events. On the other hand, after the beginning of February, tilt changes accompanying BH-type earthquake swarms were often observed up to 60 hours prior to the individual eruption events. This result suggests that the prediction of occurrence of eruption events may be possible for Shinmoedake eruption. At some volcanoes, similar tilt changes are observed just before eruptions, as follows. The inflation and deflation tilt patterns have been clearly detected for individual effusive events of the Kilauea volcano, which has a very large eruption rate (Johnson, 1987). At Asama volcano, inflation tilt changes were observed prior to the eruptions in 2004 (Churei and Katayama, 2006). Tilt and strain changes associated with a vulcanian event at Sakurajima volcano (Iguchi et al., 2008). Iguchi et al. (2008) explained the process of precursory signal and eruption as follows: (1) pressure and volume increase during inflation caused by intrusion of magma and accumulation of volcanic gas in the conduit, and (2) pressure and volume decrease during deflation as magma and gas are erupted. We suggest that similar process was occurred at Shinmoedake.
Though tilt changes prior to the subplinian event on January 26 were subtle to non-existent, small tilt changes on 18 and 26 January prior to magma eruption events were observed (Fig. 6). Kato and Fujiwara (2012) point out that the both tilt changes were similar to those observed prior to individual events that occurred after the beginning of February. The tilt change vectors on 18 and 26 January were also similar to those after the beginning of February. These observations suggest common deformation sources among those eruption events. Kato and Fujiwara (2012) estimated the total volume changes on 18 and 26 January in orders of 104 to 105 m3. The volume of deflation of the deep magma chamber during subplinian events and lava dome growth was estimated as about 107 m3 (Kozono et al., 2013). It is in two orders larger than the inflation volume prior to 18 and 26 January events, and is similar to the ejecta volume from 26 January to the beginning of February.
Figure 5 shows that the volcanic tremor, the infrasound and the tilt change coincided with the start of the subplinian event on 26 January. According to tiltmeter observation, deflation of the deep magma chamber is thought to have started at 16:00, preceded by the increase of seismic and air wave amplitudes at 14:49. These results may suggest that a large amount of magma started to move from the deep magma chamber to beneath the summit crater at around 16:00. The eruption between 14:49 and 16:00 were caused by the magma which once had intruded into the shallow region beneath Shinmoedake prior to the subplinian event. As little magma moved toward Shinmoedake until the sub-plinian event started, it was difficult to forecast the onset of subplinian event with a sufficient leading time. Therefore, JMA had to issue the Volcanic Alert Level based on the visual observation of the eruptive phenomenon. Despite the delay of Volcanic Alert Level, local governments and residents could prevent any loss of life or injury. To improve the accuracy and timeliness of future alerts at Shinmoedake, JMA installed additional observation stations in the northwestern part of Shinmoedake. Through this improvement, it is expected that JMA will have better sensitivity to discern changes in magma plumbing and movement that may lead to eruption; this should allow issuing of timely alerts. In addition, during the 2011 Shinmoedake eruptive activity, deflation of magma chamber, caused by the subplinian events and lava dome growth, was observed by tiltmeter (Fig. 5) and GPS (Kozono et al., 2013), which enabled us to evaluate the location and volume of magma and was useful to detect magma movement by geodetic observations.
During the 2011 eruption, a large amount of ash by the subplinian events damaged habitation areas around Shin-moedake. Windows of houses near Shinmoedake were broken by the strong shockwave from the vulcanian event on 1 February. Since then, the eruptive activity decreased with time, though it continued intermittently until September 2011. The seismic activity started to decline in March 2012, and reached the level before the eruption in May 2012.
In response to the 2011 Shinmoedake eruption, JMA issued several Volcanic Warnings and advised local governments to take critical mitigation measures. At the start of the eruptive activity, JMA had set the Volcanic Alert Level as 2 and the local governments prohibited the people to approach near the summit crater. JMA could not issue Volcanic Warnings before the onset of subplinian event due to a lack of clear precursory signals. The Volcanic Alert Level was upgraded by JMA, based on the observation results of eruption phenomena. Integration of geodetic, seismic, and gas flux monitoring made it possible to clarify migration of a large amount of magma from the deep magma chamber, located northwest of Shinmoedake, to the summit crater soon after 26 January subplinian event started. The 2011 eruption at Shinmoedake highlighted the importance of detection and interpretation of magma migration in order to issue timely warnings to local people and governments in order to mitigate risks from volcanic phenomena.
We are deeply grateful to the staff members of Kagoshima Local Meteorological Observatory and Fukuoka District Meteorological Observatory for continuous observations of Shinmoedake. We also received generous support from Dr. Setsuya Nakada, Dr. Alan Linde, Mr. Tetsuyuki Ueyama and Mr. Satoshi Harada who read the manuscript and gave valuable comments. Thanks are also to Dr. Christina Neal of AVO and an anonymous reviewer who gave helpful comments to revise this paper.
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