1st March, 2018 eruption of Shinmoedake

website launched: 19th March,2018
last updated:26th March, 2018

Shinmoedake amongst the Kirishima volcano group in Kyushu, Japan, erupted on 1st March, 2018.

Volcanic tremor beneath Shinmoedake of the Kirishima volcano complex.

We analyzed the tremor activity from October 3 2017 to the eruption in March 2018 mainly using the data recorded at stations operated by ERI close to the active crater of Shinmoedake.

Below are the findings :
(1)    After the eruption on October 11 2017, ground oscillation amplitude did not get back to the level it had before the eruption but gradually increased.
(2)    Tremor source locations that were resolved by the analysis concentrated around the sea level beneath the crater of Shinmoedake.
(3)    The tremor source moved between 1.5km below sea level in the northwest and just above sea level near the center of Shinmoedake, which occurred several times.
(4)    After the eruption on March 1 2018, the tremor source locations shifted to beneath the northeastern rim of the Shinmoedake crater, which is consistent with the location where lava outflew in the crater (NIED Daichi2. 1st report on terrain change inside Shinmoedake crater).

Since March 10 2018 when Vulcanian activity began, the lower limit of the ground motion amplitude nearly returned to the level before the October 2017 eruption.

Fig.1 Seismic stations (ERI, JMA, NIED) used for analyzing the tremor. Data from stations written in red were used to analyze the tremor source locations between October 3 2017 and March 3 2018.

※Method for analyzing the tremor source (Ichihara & Matsumoto, 2017, GRL）
The root mean square (RMS) of the ground velocity (three components) in the frequency range of 3.5—7 Hz was calculated in every 10 seconds. Then the median value in a 5-minute-long time window shifted every minute is taken as the amplitude value. We selected time windows in which the standard deviation of the amplitude ratio between each pair of the seismic stations in 10 min is smaller than a threshold value. Site corrections for the stations and estimation of average Q value of the area were done using amplitude distribution when stable tremor was clearly seen (between 21:30, October 10 to 01:00, October 11, 2017). The Q value was determined so as to be equivalent with that in the previous study (Ichihara & Matusmoto, 2017).

Fig.2 Amplitude change of the ground motion. Values shown here are the root mean squares of three-component ground velocity in the frequency range of 1-7 Hz with site corrections. The tremor once declined after the October 2017 eruption, but continued and gradually grew toward the 2018 eruption.
Fig. 3 Tremor source location. From above: altitude above the sea level (z), north-south distance from the center of the crater (y), east-west distance (x), and the fitting residual with the estimated tremor source location based on the amplitude ratios (RESmin). Points with fitting residuals smaller than 0.15 and 0.2 are marked with red and blue colors, respectively. The tremor source locations are plotted only if they are determined with sufficiently small fitting residuals so that a blank period does not indicate tremor quiescence.
Tremor sources in the periods marked by the colored arrows are indicated in the corresponding colors on the topographic map and the east-west cross section beneath the crater.
Fig.4 Comparison of the amplitude distribution of October 8 2017 before the October 2017 eruption and that of March 10 2018 in the late stage of the March 2018 eruption. The lower limit of the amplitude has declined to a low level for the first time in the last 5 months.
Fig.5. Migration of tremor source during the eruption in October 2017. Results estimated using different sets of selected seismic stations from 13 available stations around Shinmoedake are compared. The estimations with the 8 seismic stations used for the long term analysis in Fig. 3 are indicated in red closed markers (*). They are consistent with the other results including those obtained with a larger number of stations, which confirms the robustness of the estimation. No result of * is gained after the evening of October 14 due to missing data at SMW.

(Volcano Research Center: Mie Ichihara /Takao Ohminato )

15th March, 2018

Movie taken on 14th March, 2018

(Volcano Research Center: Fukashi Maeno)

12th March, 2018

Overview:

  • Lava kept growing inside the crater as a form of lava dome. The crater was filled with lava dome by midnight 8th, and stranded on the northwestern rim which is the lower part.
  • The state of lava dome is clearly captured by satellite SAR analysis, and it seems that lava supply stopped almost by 9th (citation 1, 2)
  •  Explosive eruption began in the afternoon of 9th, at the center of the lava dome that stopped growing.
  • The lava that stranded on northwestern rim flew slowly down the slope by its own weight.
  • The process of lava outflow and successive explosive eruptions resembles the process of eruption in January-February 2011.
[Below websites are only in Japanese] *NIED Website for Shinmoedake eruption in March 2018: http://gisapps.bosai.go.jp/nied-crs/2018-0003/index.html 

*GSI’s correspondence to the Shinmoedake eruption in March 2018: http://www.gsi.go. jp/BOUSAI/h30kirishima-index. htm

【Magma composition of Shinmoedake 2018 eruption】

The bulk chemical composition of pumice fragments and volcanic ash collected by evening March 7, 2018 was determined. (collected location: Near Shin-yu three-way street, collected by Fukashi Maeno on March 7)

*The magma of this eruption is andesite which is almost the same as of the magma in 2011 eruption.

* Like how it was in 2011 eruption, it is clear that the composition of volcanic ash from preceded phreatic to phreatomagmatic eruptions, had turned magmatic over time. This is a reflect of gradual decay in amount of altered materials within the volcanic ash. (Note that, as used samples are coarse ash and several mm-size pumice fragments, it may be a little different from the true composition of magma including phenocryst.)

Fig.1 Chemical component of Shinmoedake eruption in 2018 and 2011.

【Aerial observation】

We observed the summit crater of Shinmoedake from the air. Summary is as follows:

  •  Concentric growth patterns are well developed on the surface of lava dome accumulated at the summit crater of Shinmoedake.
  •  The central part of lava dome is almost flat or recessed, and covered with brownish ash. It is considered that the central part is the explosion point and also was the magma supply location. (fig.2)
  •  When observed on 9th March 5PM, lava stranded on northwestern rim, with steam smoke rising from its tip. At 3:30PM on the following day, the tip headed down the slope and its surface was rusty due to oxidation. (fig.3, 4)
  • It seems that the tip of the lava has advanced several tens of meters in a day, without any large collapse.
  • About 10 minutes before the explosion around 10:15AM on March 10, volcanic ash started to emit weakly from the ash-covered central part, and then a bulge appeared forming a circular cone, leading to eruption (Fig. 5).

(Chartered Cessna and helicopters from KYT and UMK were used for observation)

Fig.2. Northwestern rim of Shinmoedake summit crater on the afternoon of 9th March.
Fig.3. Northwestern rim of Shinmoedake summit crater on the afternoon of 10th March.
Fig.4. Close up image of fig. 3.
Fig.5 Bulging of the central part of the lava dome, about 5min before the explosive eruption at 10:15 AM, 10th March.

(Volcano Research Center: Setsuya Nakada)