Phreatic explosion at Motoshiranesan cone on 23 January 2018 results in one fatality and several injuries

The large Kusatsu-Shiranesan volcanic complex comprises three overlapping pyroclastic cones and numerous summit craters; it is located about 150 km NW of Tokyo in the Gunma Prefecture of central Japan. Intermittent short-lived historic activity has been reported from the northernmost Shiranesan cone since the beginning of the 19th century. An explosion at the southernmost Motoshiranesan cone in January 2018 resulted in one fatality and several injuries. Information about the event was gathered from the Japan Meteorological Agency (JMA) and various news sources.

Summary of activity during 1976-2014. Small phreatic explosions in the Mizugama and Yugama craters at the northernmost part of the Kusatsu-Shiranesan volcanic complex occurred in 1976, 1982, and 1983 (figure 14). Larger ash-bearing explosions in November and December 1983 sent tephra 30-40 km to communities downwind to the SE from the Yugama and adjacent Karagama craters on the Shiranesan cone. Intermittent increases in seismic activity near the Yugama crater coincided with water discoloration in the crater lake, and possible ejections of debris from hydrothermal activity in 1989 and 1996. Increased hydrothermal activity was noted on the N flank of Yugama during 2013-2014. Seismic swarms, deformation, thermal, and fumarolic activity increased briefly during early June 2014 in the area around the Yugama crater lake, but no eruption was observed. In late June 2014, JMA reported dying vegetation in a forested area 3 km SW of the Motoshiranesan summit area.

Figure 14. Subfeatures of the Kusatsu-Shiranesan volcanic complex as seen in Google Earth imagery, looking N. The northernmost bleached area includes the historically active Yugama, Mizugama, and Karagama craters, part of the Shiranesan cone. In the center of the complex is the Ainomine cone which has a ski area on the S flank. The southernmost edifice is the Motoshiranesan cone which has multiple craters at its summit, including Kagamiike or "Mirror pond". The explosions of 23 January 2018 occurred at Kagamiike and the adjacent crater to the N, in area referred to by JMA as Honkonoyama. Courtesy of Google Earth.

Activity during 2014-2017. Seismicity remained elevated from March to mid-August 2014 around the Yugama crater area. Ground deformation data suggested inflation between March 2014 and April 2015 in that area. Field surveys conducted on 4-5 and 10-11 November 2014 indicated fumarolic areas on the N and NE flanks of the Mizugama crater, but no other significant activity. Short-lived increases in seismicity were observed during January-February 2015. A field survey in May 2015 confirmed ongoing thermal activity on the N and NE wall of the Yugama crater, and the N and NE flank of the Mizugama crater. A small-amplitude, 2-minute-long tremor during late June 2015 was the first since January 2013; it was not accompanied by eruptive activity. The fumarolic activity on the N wall of the Yugama Crater was higher during a field survey in October 2015 than in had been the previous May.

Thermal activity was ongoing at Yugama and Mizugama craters during 2015-2017 along with intermittent fumarolic activity in the same general area, but no significant seismicity was reported. By June 2017 the decrease in the concentration of components derived from high-temperature volcanic gas in the lake, and the stable low-level seismicity in the area, led JMA to lower the warning level from 2 to 1 (on a 5 level scale) on 7 June 2017; they noted that the thermal activity continued around the Yugama crater throughout the rest of the year (figures 15-17).

Figure 15. A minor thermal anomaly persisted inside the NE crater wall at Yagama Crater at Kusatsu-Shiranesan throughout 2015-2017. Both visual (upper) and thermal (lower) images were taken during an overflight on 1 November 2017. View is to the north. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, November 2017).

Figure 16. Thermal anomalies persisted on the N and NE flank of the Mizugama crater at Kusatsu-Shiranesan during 2015-2017. These visual (upper) and thermal (lower) images were captured on 1 November 2017. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, November 2017).

Figure 17. Daily earthquake frequency at Kusatsu-Shiranesan during 1 January 2011-30 November 2017. Although earthquake counts temporarily increased during March-August 2014 and in January and February 2015, no eruptive activity was reported. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, November 2017).

Activity during January-March 2018. JMA reported that at 0959 on 23 January 2018 an eruption began at Kusatsu-Shiranesan coincident with the onset of volcanic tremor which prompted JMA to raise the Alert Level to 3 (on a scale of 1-5); there had been no prior indications of an impending eruption. Skiers at the popular Kusatsu Kokusai ski resort, located on the Ainomine cone, took video showing a plume of tephra and ejected bombs rising from vents around the Kagamiiki and adjacent crater at the summit of the Motoshiranesan cone (see Information Contacts for Mainichi for video link). Motoshiranesan is immediately adjacent S of the Ainomine cone and about 2 km SSE of the Yagama Crater on the Shiranesan cone where all previous historical activity had been reported (figures 14 and 18).

Figure 18. Locations and images of the active vents at Kusatsu-Shiranesan during the eruptive event of 23 January 2018. Upper left: View is looking W at the Motoshiranesan summit craters. The crater with the pond in Box 1 is Kagamiike (yellow Japanese characters). Boxes 1 and 2 in the upper left photo are enlarged in the lower photos. Upper right topographic map shows the locations in red of the three vents. The upper red line and dot correspond to the vents shown in the lower right box 2. The lower red bar on the topographic map (near the small pond) corresponds to the vent shown in the lower left image as box 1. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, January 2018).

Photos and video posted in news articles showed tephra shooting tens of meters into the air, drifting E, and blanketing the nearby hillside (figure 19); JMA noted ashfall in Nakanojo-machi, in the Gunma Prefecture, about 8 km E. Tephra hit a gondola, shattering glass and injuring four skiers (figure 20). Material fell through the roof of a lodge, where about 100 people had already been evacuated. Ground Self-Defense Force troops were engaging in ski training at the time of the event; one member died from the impact of large tephra blocks, and seven others were injured.

Figure 19. Tephra from Mount Kusatsu-Shiranesan covers the N flank of the Motoshiranesan cone and much of the Ainomine cone in this view to the W taken on 23 January 2018. Photo by Suo Takeuma, AP, courtesy of CNN (Japanese man killed by falling rocks after volcano erupts at ski resort, 23 January 2018).

Figure 20. Fist-sized tephra blocks and ash ejected from the eruption of Mount Kusatsu-Shiranesan cover the floor of a damaged gondola at the Kusatsu Kokusai Ski Resort on 24 January 2018, courtesy of The Mainichi Japan (Damaged ski resort gondolas show the power of Gunma Pref. volcanic eruption, 25 January 2018).

The following day, on 24 January 2018, JMA noted that volcanic earthquakes were numerous but decreasing in number, and two 2-3-minute-long periods of volcanic tremor were detected at 1015 and 1049. Minor but elevated seismicity continued through 30 January, punctuated by periods of tremor. The largest fissure where the eruption occurred was oriented E-W, located just inside the N rim of the northernmost crater at the Motoshiranesan summit (figure 21). Kenji Nogami, a professor at the Tokyo Institute of Technology, confirmed that the event appeared to have been "a typical phreatic eruption" (Japan Times).

Figure 21. The largest fissure vent active in the 23 January 2018 explosion at Kusatsu-Shiranesan was still surrounded by ash and tephra when photographed during an overflight on 28 January 2018. The summit ropeway station of the ski area is at the image top just NW of the explosion vent. Courtesy of The Mainichi (Visitor traffic plunges in Kusatsu hot spring resort after deadly eruptions, 30 January 2018).

The Tokyo Volcanic Ash Advisory Center issued a single volcanic ash advisory on 23 January indicating a possible eruption, but it was not identifiable from satellite data. Observations made on 14 February 2018 confirmed the presence of the vents in the Kagamiike and adjacent crater, but there was no evidence of thermal activity and little fumarolic activity in the area (figure 22). Seismicity decreased steadily after the explosion on 23 January 2018 through the end of March 2018 and no further activity was reported (figure 23).

Figure 22. Vents from the 23 January 2018 eruption at Kusatsu-Shiranesan were still visible at the craters on 14 February 2014 during a helicopter overflight by JMA. The upper image, looking W, shows the large vent at the N side of the crater immediately N of the Kagamiike crater, as well as a smaller vent located to the W on the E flank of the adjacent slope. The lower image shows two smaller vents on the inner wall of the adjacent Kagamiike crater. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, February 2018).

Figure 23. Seismicity decreased steadily at Kusatsu-Shiranesan after the explosion on 23 January 2018. Graph shows the number of daily seismic events during 1 January-31 March 2018. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, March 2018).

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); The Mainichi (URL: http://mainichi.jp/english/, eruption video URL-https://mainichi.jp/movie/video/?id=121708141#cxrecs_s); The Japan Times (URL: https://www.japantimes.co.jp/); Cable News Network (CNN), Turner Broadcasting System, Inc. (URL: http://www.cnn.com/).

Information is preliminary and subject to change. All times are local (unless otherwise noted)

March 1976 (GV 1975)

Small eruption from Mizugama Crater

[JMA notes that a small eruption from Mizugama Crater occurred at about 1800 on 2 March 1976. A new crater 50 m in diameter and 10 m deep was formed in the NE part of Mizugama Crater.]

Information Contacts: Japan Meteorological Agency, Tokyo.

September 1976 (NSEB 01:12) Cite this Report

Three people killed by hydrogen sulfide

[On 3 August] a high school teacher and two students were overcome and killed by H₂S, which is emitted in considerable quantities [from fumaroles on the NW side of Motoshirane-san, 2 km SSW of Yugama Crater]. The gas had concentrated in a bowl-shaped area during a light rain on a nearly windless day.

Further Reference. Ossaka, J., and others, 1980, Variation of chemical composition in volcanic gases and waters at Kusatsu-Shirane volcano and its activity in 1976: BV, v. 43.

Information Contacts: JMA, Tokyo.

October 1982 (SEAN 07:10) Cite this Report

Phreatic explosion; volcanic tremor

A brief phreatic explosion from three vents on 26 October ejected a dark column that rose 100 m above the lakes in two summit craters, Yugama and Karagama (figure 1). Onset time of the explosion was uncertain, but the seismograph at the Maebashi District Meteorological Observatory, 1.1 km NE of Yugama, began to record volcanic tremor with an amplitude of 0.2 µm at 0855. Gray plumes were recognized at 0905. The tremor gradually increased in amplitude and peaked about an hour after onset.

Figure 1. Map of the summit region of Kusatsu-Shirane. Small circles indicate fumaroles; the other marks represent vents active since the 27 October 1982 eruption. An 'x' indicates an emission point of the 26 October 1982 eruption plume. The large open circle is Pit No. 6; the concentric circles are Pit No. 7. The solid circle is a new pit that formed on 13 November. Pits No. 2 and 3 (not shown) are on the W inner wall of Yugama. Courtesy of JMA.

Activity at Karagama ended at 0920, only 15 minutes after it was first recognized, and at 0930 activity at Yugama was reduced to white vapor emission (which ended the next day). Volcanic tremor, however, continued at peak levels for a few hours then gradually weakened, ceasing at 0124 on 30 October. A swarm of volcanic earthquakes lasting 5 hours was recorded on 26-27 October, and there was another minor swarm on [27] October.

JMA reported that a strong wind carried ash from as far as 3 km to the SE; ash was 1 mm thick on the crater rim. Tokiko Tiba reported a maximum of 3 cm of ash in the summit area. The total volume of ejecta was estimated to be 2,800-3,400 tons. No damage was reported. As of 2 November no further eruptions had been reported, and fumarolic activity in Yugama had declined. . . .

Information Contacts: T. Tiba, National Science Museum, Tokyo; JMA, Tokyo.

November 1982 (SEAN 07:11) Cite this Report

Lake temperatures measured after explosion

Temperatures measured in the lake in Yugama Crater for 6 days following the 26 October explosion [were: 14°C on 26 October, 46°C on 27 October, 55°C on 28 October, 56°C on 29 October, 48°C on 30 October, and 39°C on 31 October.]

Information Contacts: Y. Sawada, Meteorological Research Institute, Ibaragi; D. Shackelford, CA.

December 1982 (SEAN 07:12) Cite this Report

Volcanic tremor, phreatic explosion

At 0538 on 29 December, after 64 days of quiescence, a phreatic explosion from the pit formed in the 26 October explosion ejected a gray plume that rose about 300 m above the rim of Yugama Crater. The plume was 700 m above the rim by 1300, then suddenly declined about 1500. The seismograph at the Maebashi District Meteorological Observatory, 1.1 km NE of Yugama, began to record volcanic tremor 46 minutes before the explosion. Maximum amplitude was 2.1 µm. Tremor ended at 1538, but resumed at 1900 and continued until 1500 the next day. Ash traveled as far as 4 km, mainly NE. Scattered blocks and ejecta carried by spraying water were observed around the pit, including clay material on 30 December. No damage was reported. As of 5 January, no further explosions had been reported, but tremor had been continuous since 2 January.

Information Contacts: JMA, Tokyo.

April 1983 (SEAN 08:04) Cite this Report

Small phreatic explosion; harmonic tremor

. . . Local seismicity had declined after the December activity, but increased in early January (see table 1), when 91 volcanic earthquakes and 44 tremor events were recorded. After mid-January no tremor events were recorded, but the number of volcanic earthquakes was greater than background level in early March.

At 0459 on 9 March the seismograph at Maebashi District Meteorological Observatory recorded a swarm of volcanic earthquakes that lasted only a few minutes. JMA scientists noted that these earthquakes were similar in wave form to those recorded during previous eruptions. . . . In April, activity was limited to occasional white vapor ejections. On 10 April, one vapor plume rose to 60 m above Yugama Crater. Volcanic tremor, possibly accompanied by vapor ejections, was observed 9-12 and 19-20 April.

Information Contacts: JMA, Tokyo.

May 1983 (SEAN 08:05) Cite this Report

9 March activity was only seismic

Tokyo Institute of Technology personnel visited the volcano on 12 March to investigate a report of a small phreatic explosion from Pit No. 7 on 9 March [originally reported in 8:4]. A brief swarm of volcanic earthquakes had been recorded at 0459 that day at the Maebashi District Meteorological Observatory. The Institute team found no fresh ash. Although ash was reported to have fallen on the frozen crater lake, another observer said that the lake was not frozen on 9 March. The team concluded that the 9 March activity had been only seismic.

Information Contacts: JMA, Tokyo.

July 1983 (SEAN 08:07) Cite this Report

Small plume emitted; volcanic tremor; A-type events

On 26 July a small phreatic explosion occurred at the NW rim of Yugama Crater, at the volcano's summit. JMA personnel observed the eruption, at Pit No. 6, formed during the 26 October phreatic explosion and the site of a similar explosion on 29 December. Volcanic tremor (amplitude 0.2-0.3 µm) started at 1031. About 1110, the dominant frequency of the tremor decreased and white vapor was ejected. Volcanic rumbling intensified at 1140; about 1/3 of the crater lake was covered with ash by 1150. Accompanied by strong rumbling, a dark plume was ejected at [1204]; it had risen above the crater rim by [1212]. Volcanic tremor returned to a higher frequency at 1213, but shifted back to lower frequency about 20 minutes later. Emission of a white vapor plume that rose to about 100 m above the pit was continuous in the afternoon. Volcanic tremor ended at 1720. The plume weakened suddenly at 1730.

Local seismicity had been at a high level since last autumn. Seismographs recorded 209 volcanic earthquakes in June and 227 in July. On 22 July, swarms of A-type earthquakes occurred. After the 26 July eruptive episode, seismicity declined slightly but remained above background level.

Information Contacts: JMA, Tokyo; T. Tiba, National Science Museum, Tokyo.

October 1983 (SEAN 08:10) Cite this Report

Explosions eject large tephra

UPI reported that thunderous explosions ejected tephra on 13 November at 1144. [Blocks fell a few hundred meters from the vent; original press reports of a more distant fall of large tephra were incorrect.] Tephra [reached a town] (Nakanojo) 40 km SE of the volcano. A "secondary" eruption occurred 25 minutes later. No injuries were reported.

Information Contacts: UPI.

December 1983 (SEAN 08:12) Cite this Report

Summit explosions and seismicity

Summit explosions from Kusatsu-Shirane occurred on 13 November and 21 December. Precursory seismic activity began with a large-amplitude discrete event on 2 November. Tremor was continuous between 0700 and 2000 on the 10th. Stronger tremor was recorded at 1722 on the 12th. Its mean amplitude on the seismograph, initially 1.9 µm, gradually increased to more than 5.0 µm at 2000, when the wave form of the tremor became similar to one that had accompanied a mud at Aso Volcano. Tremor gradually declined during the early morning of 13 November.

Explosions occurred 13 November at 1144 and 1208 from small vents (Pits No. 6 and 7) on Yugama Crater's [N] wall and a new pit that appeared about 50 m E of Pit No. 6. [During a 22 December inspection] Tokyo Institute of Technology personnel found that a [45] cm-wide, 45 m-long fissure on the N wall of the adjoining Karagama Crater had also ejected ash. JMA noted that the line extending SW from Pit No. 6 across Karagama Crater's N wall has been active since the eruption of October 1982.

Lapilli were scattered as much as 600-700 m S of Yugama. Ash was carried 30 km downwind although most fell into the lake in Yugama Crater. The second explosion was accompanied by a ground shock of intensity III (JMA Scale) at the rest area 700 m S of Yugama. Signals from the seismograph at the Maebashi District Meteorological Observatory stopped immediately after this explosion. A field survey revealed that [blocks] had made many craters 1-3 m in diameter and 50 cm deep on the upper SE outer slope of Yugama. The buried seismograph cable seemed to have been cut by one of these [blocks].

Seismic activity increased again 18-19 December, when 32 and 31 discrete events were recorded. After a brief period of activity from 0954-1007 on the 21st, continuous tremor resumed at 1022. The tremor, accompanied by some large discrete events, saturated the seismograph from 1035-1105, then gradually declined, ending at 1220.

A Kusatsu town employee working S of Yugama first noticed the sound of vapor emission at 1010; 20 minutes later he observed a dark plume containing mud and lapilli rising 300 m above the crater rim. Ash traveled 400-500 m SE, darkening frozen Yugama Lake. Eruptive activity began to decline about 1100. Tokyo Institute of Technology personnel found that the eruption sites were Pits No. 2 and 3 on the W inner wall of Yugama. A small amount of ash was also ejected from the fissure in Karagama Crater that was active in November.

Information Contacts: JMA, Tokyo.

January 1984 (SEAN 09:01) Cite this Report

1983 activity summarized

1983 activity is summarized in table 1; locations are on figure 1.

Table 1. Seismic activity at Kusatsu-Shirane, 1983. Courtesy of JMA.

Further Reference. The 1982-1983 eruptions of Kusatsu-Shirane volcano, in XIX IUGG General Assembly, 1987, Report on volcanic activities and volcanological studies in Japan for the period from 1983 to 1986, p. 5-8.

Information Contacts: JMA, Tokyo.

June 1986 (SEAN 11:06) Cite this Report

Increase in microearthquakes since 1984

Monthly frequency of microearthquakes recorded at the volcano reached 96 in June, the highest since the active phase of 1982-84. A gradual increase in seismicity began last year (figure 2).

Figure 2. Monthly seismicity at Kusatsu-Shirane, 1978-86. Arrows indicate five eruptions in 1982 and 1983.

Information Contacts: JMA.

October 1987 (SEAN 12:10) Cite this Report

Earthquakes increase to five times monthly average

Many small volcanic earthquakes were recorded 14-19 October by the seismograph 1 km NE of Yugama, the largest of the three crater lakes. The monthly total of volcanic earthquakes reached 114, compared to the monthly average of less than 20. Seismic activity declined to a low level after 22 October. Epicenters were estimated to be several hundred meters E of Yugama. JMA's Mobile Volcanological Observation Team established two additional seismographs and began 10 days of observation on 18 October. No significant changes in fumarolic activity were observed during field surveys on 23 and 26 October.

A phreatic explosion from Yugama crater ejected 20-cm blocks to 600-700 m from the crater lake on 13 November 1983. Small phreatic explosions occurred from Yugama and Karegama (the E crater lake) on 26 July and 21 December 1983.

Information Contacts: JMA.

October 1988 (Ref 1989)

Seismicity during September-October 1988; tremor in January 1989 associated with phreatic emissions

[Ida and others (1989) report that swarm-like seismicity began at the end of September 1988 and continued through October, with most located events centered SE of Mizugama crater. Hypocenters remained in the same area during the following period, but focal depths shallowed and episodic volcanic tremor became more frequent. Continuous volcanic tremor began suddenly on 6 January 1989 and lasted for almost 24 hours. The tremor was associated with emission of steam or water from the NW part of Yugama Crater, which distributed a small amount of ash over the crater lake.]

Reference. Ida, Y., Osada, N., Sawada, M., Koyama, E., and Kagiyama, T., 1989, Seismological Study Based on Recently Installed Permanent Stations and a Small Eruptive Event on January 6, 1989 at Kusatsu-Shirane Volcano: Bulletin of the Earthquake Research Institute, University of Tokyo, v. 64, p. 325-345 (in Japanese with English abstract and figure captions).

Information Contacts:

February 1990 (BGVN 15:02) Cite this Report

Highest amplitude tremor since 1982-83 activity, but no eruption

Volcanic tremor began at about 1445 on 27 January and continued through 1 February. The largest amplitude was about 0.2 µm. A field survey on 2 February found no new ash deposition around the crater. Water discoloration in Yugama crater lake was observed as usual.

Volcanic tremor resumed at 0019 on 12 February and continued until 0417 the next day. Additional tremor episodes occurred on 18 and 24 February. Amplitudes of 1.3 µm on 18 January and 1.1 µm on 24 February were the highest recorded since the last eruption in 1982-83. The number of discrete volcanic earthquakes also increased between 17 February and 7 March.

Information Contacts: JMA

May 1990 (BGVN 15:05) Cite this Report

Seismicity remains high; high-amplitude tremor continues

Seismic activity remained at the highest levels since the 1982-83 eruptive period. The daily number of earthquakes has remained high since increased seismicity began in mid-February (figure 3). The number and amplitude of tremor episodes also remained high with one in January, six in February, two in April, and nine in May. Water discoloration in Yugama crater lake, first noted in February, was usually evident through May (during 13 of 19 visits January-May).

Figure 3. Daily number of recorded earthquakes at Kusatsu-Shirane, 1978-June 1990. Arrows mark eruptions. Courtesy of JMA.

Information Contacts: JMA

June 1990 (BGVN 15:06) Cite this Report

Continued vigorous seismicity

The number of volcanic earthquakes persisted at a high level as of early July. Six volcanic tremor episodes were recorded in June. No eruption occurred and fumarolic activity in and around the active crater rim has remained unchanged.

Information Contacts: JMA.

July 1990 (BGVN 15:07) Cite this Report

More frequent volcanic earthquakes and tremor

The number of volcanic earthquakes . . . remained at high levels as of 8 August, with 212 earthquakes in June and [243] in July. Twelve volcanic tremor episodes were recorded by the seismograph 1.1 km NE of Yugama Crater.

Information Contacts: JMA.

August 1990 (BGVN 15:08) Cite this Report

Seismicity remains strong

During August, 171 earthquakes . . . and 35 tremor episodes . . . were recorded. Tremor amplitude ranged from 0.0 to 1.3 [µm].

Information Contacts: JMA.

September 1990 (BGVN 15:09) Cite this Report

Seismicity remains strong

Seismicity has remained at high levels . . . . During September, 184 earthquakes . . . and 44 volcanic tremor episodes . . . were recorded. Tremor amplitude also decreased from as much as 1.3 µm in August to 0.1-0.5 µm in September.

Information Contacts: JMA.

October 1990 (BGVN 15:10) Cite this Report

Continued high seismicity but no surface changes

Seismicity has remained at high levels... (figure 4). During October, 213 earthquakes (up from 184 in September) and 29 tremor episodes... were recorded. Tremor amplitudes were similar to previous months. Earthquakes were centered 1 km E of... Yugama Crater. Seismicity remained similar as of 14 November. No changes in surface activity were observed.

Figure 4. Monthly number of earthquakes at Kusatsu-Shirane, January 1978-October 1990. Arrows at top of figure mark eruptions. Courtesy of JMA.

Information Contacts: JMA.

November 1990 (BGVN 15:11) Cite this Report

Strong seismicity continues but no surface changes

Seismicity has remained at high levels since February (figure 4). During November, 117 earthquakes and 27 volcanic tremor episodes were recorded . . . . Seismicity remained similar in early December. No changes in surface activity were observed.

Information Contacts: JMA.

December 1990 (BGVN 15:12) Cite this Report

Continued vigorous seismicity

Seismicity has remained at high levels since mid-February. During December, [101] earthquakes . . . and 11 tremor episodes . . . were recorded.

Information Contacts: JMA.

January 1991 (BGVN 16:01) Cite this Report

Continued seismicity but no change in surface activity

Seismicity has remained at high levels since February 1990. During January, 92 earthquakes and 24 volcanic tremor episodes were recorded . . . . No changes in surface activity were observed. Similar activity was noted in early February.

Information Contacts: JMA.

February 1991 (BGVN 16:02) Cite this Report

Continued seismicity but no surface changes

Earthquakes and tremor activity continued at levels similar to previous months. No changes in surface activity were observed.

Information Contacts: JMA.

March 1991 (BGVN 16:03) Cite this Report

Brief peak in seismicity

Seismicity continued at high levels through mid-April, after [131] recorded earthquakes and 11 tremor episodes in March. A sharp, brief increase in seismicity on 9 March (42 recorded earthquakes), prompted the JMA to issue a notice ("Extra Volcanic Information"), the first since August. No changes in surface activity were observed.

Information Contacts: JMA.

April 1991 (BGVN 16:04) Cite this Report

Continued seismicity

In April, seismicity remained similar to previous months, with a total of 110 earthquakes and one tremor episode recorded... (figure 5). No surface activity was observed.

Figure 5. Daily number of recorded earthquakes (top) and tremor episodes (bottom) at Kusatsu-Shirane, January 1989-April 1991. Courtesy of JMA.

Information Contacts: JMA.

February 1996 (BGVN 21:02) Cite this Report

Minor hydrothermal ejection in Yu-gama crater

According to Kusatsu-Shirane Volcano Observatory (Tokyo Institute of Technology), at 1044 on 7 February geophysical changes occurred. A hydrophone submerged in Yu-gama pond recorded large amplitude sound waves and a meter registered water-level changes. Observers on 14 and 24 February saw discolored water near the NW part of the pond's surface and pieces of broken ice, 20-30 cm in size, along the shore. Therefore, on 7 February, a small magnitude ejection might have occurred at the pond. When a similar phenomenon was last observed, 6 January 1989, it was ascribed to hydrothermal activity.

Information Contacts: Volcanological Division, Japan Meteorological Agency, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan

July 1996 (BGVN 21:07) Cite this Report

Seismic activity increases

According to the Kusatsu-Shirane Volcano Observatory, Tokyo Institute of Technology, many small earthquakes were detected near the vent on 8 July.

Information Contacts: Volcanological Division, Japan Meteorological Agency, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan

February 2013 (BGVN 38:02) Cite this Report

Minor tremor and small earthquakes during 2011-2012

On 7 February 1996, hydrophone data and water level changes suggested that a small hydrothermal ejection may have occurred at Kusatsu-Shirane (also known as Kusatsu-Shiranesan) at Yugama crater's pond (BGVN 21:02). Several months later, on 8 July, numerous small earthquakes were detected by the Kusatsu-Shirane Volcano Observatory (BGVN 21:07). The volcano is about 150 km NW of Tokyo (figures 6 and 7; also refer to the sketch map in figure 1, SEAN 07:10). This report summarizes seismicity between May 2011 and February 2013 based on available reports from the Japan Meteorological Agency (JMA).

Figure 6. A sketch map showing the location of Kusatsu-Shirane (Kusatsu-Shiranesan) in Honsho, Japan. Courtesy of JMA.

Figure 7. An aerial photo of Kusatsu-Shirane, as viewed from the S. The photo, taken on 29 May 2008, shows the overlapping pyroclastic cones and two of the three crater lakes. Courtesy of Flickr user rangaku1976.

On 27 May 2011, tremor was detected at Kusatsu-Shirane; no further information was provided. During 5-7 June 2011, an elevated number of microearthquakes with low amplitude occurred around Yugama crater (the main crater). No volcanic tremor or significant deformation was detected during this time. Thereafter, activity gradually diminished to background levels.

Field surveys during 27-29 June and 12-13 July 2011 revealed that elevated thermal anomalies persisted inside Yugama crater's N flank, the N fumarole area, and the slope located N to NE of Mizunuma crater. Ground temperatures around fumaroles remained high.

On 18 July 2011, a short period of tremor (duration 2.5 min) was detected. No change in fumarole activity was observed.

On 10 August 2011, an aerial survey was conducted in cooperation with Gunma prefecture. The survey found that the distribution of thermal anomalies and fumaroles in Yugama crater and the N fumarole area had not changed.

During 16-18 August, an elevated number of microearthquakes with low amplitude occurred near and to the S of Yugama crater. Significant deformation was not detected. Seismicity remained at background levels during the other days in August. High temperatures persisted on the N flank inside the main crater.

A field survey on 8 March 2012 found that the high temperatures on the N slope of Mizugama crater and the N fumarole area were the same as those found during a previous survey conducted during 27-29 June 2011. Very weak steam plumes at the N fumarole area of Yugama were sometimes observed by a camera at Okuyamada, though bad weather and mechanical trouble prevented their observation for long periods. The ground temperature in the fumarole area NE of Yugama crater remained elevated since its rapid rise in May 2009, despite occasional fluctuations.

According to JMA, the occurrence of small amplitude volcanic earthquakes occasionally increased during March 2012. The hypocenters were located just beneath the S part of Yugama crater. No tremor or significant crustal change was noted in GPS data.

During 1-2 April 2012, seismicity increased slightly, then subsided. No tremor, change in fumarole activity, or crustal change was observed, and no further reports have been issued on activity at Kusatsu-Shirane as of February 2013.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/); rangaku1976, Flickr (URL: http://www.flickr.com/photos/rangaku1976/).

June 2014 (BGVN 39:06) Cite this Report

Seismicity, deformation, and hydrothermal emissions during 2013-2014

Although no eruption ensued, the Japan Meteorological Agency (JMA) reported that the Alert Level for Kusatsu-Shirane was increased on 3 June 2014 from Level 1 (Normal) to Level 2 (Near-crater warning). Concern rose owing to elevated seismicity including shallow earthquakes within a few kilometers of the surface, deformation, elevated temperatures, and more. Many of the anomalies centered at or near Yugama, the largest of the three crater lakes within the volcanic complex (figures 8 and 9). Snow melted associated with fumaroles adjacent Yugama. Yugama lake was the scene of discolored water and floating sulfur on at least two occasions during 2013-2014. Seismicity rose in March 2014; ground deformation, in April 2014; and ground temperatures, in May 2014. Volcanic tremor was absent during this reporting period of February 2013-July 2014.

Figure 8. Annotated topographic map covering Kusatsu-Shirane and the surrounding area. Circled areas correspond to three fumarolic fields: 1, N-flank fumaroles (see figure 9); 2, NE-flank fumaroles (where typically the highest temperatures were measured from the three sites); and 3, the NNE shoreline of Yugama crater lake. Area 3 was also monitored by a web camera (maintained by Tokyo Institute of Technology). The Kusatsu-Shirane visitor center is located near the red letter P. Courtesy of JMA.

Figure 9. The N-flank fumarolic region (labeled 1 in figure 8) of Kusatsu-Shirane seen at 1058 on 5 December 2013 looking S. This is an aerial photo taken from ~2,440 m altitude. Snow had melted in some areas within the dashed red ellipse. The crater lake in the middle ground is Yugama. Courtesy of JMA.

Activity during February-December 2013. JMA maintained Alert Level 1 through 2013 and fumarolic activity continued at a low level from the three zones (see figure 8). Temperatures measured with an infrared camera were typically less than 40°C. On 17 March 2013, JMA reported an increase in the number of small volcanic earthquakes with epicenters near the Yugama crater lake. Seismicity dropped back down to normal levels during April-September, and thermal activity continued at low levels.

JMA reported that seismicity was temporarily elevated in October 2013 in the area between Yugama crater lake and the Yamada-toge Pass (figure 10). Hypocentral depths were within 4 km of the surface, relatively shallow compared with previous months.

Figure 10. Map of earthquakes located during November 2012-October 2013. Earthquakes from October 2013 are highlighted in blue, indicating that seismicity was located between the crater lake named Yugama (labled with red symbols) and a pass named Yamada-toge (~2 km NW of Yugama crater lake). Earthquake depths were within 4 km of the surface. Courtesy of JMA.

Aerial observations on 5 December 2013 determined that Yugama crater lake had two areas of cloudy water and, in one location, floating sulfur was visible on the lake surface. Figure 11 shows a photo of the lake with areas sketched in to show the approximate extent and location of where cloudy water and floating sulfur were found. Similar observations were made later, on 22 April 2014, when a cloudy area was noted during an overflight.

Figure 11. Locations of where discoloration was observed within Kusatsu-Shirane's crater lake, Yugama, are marked on this aerial photo taken on 5 December 2013. A cloudy area as well as sulfur debris was noted. Courtesy of JMA.

Elevated seismicity during March-July 2014. During January-February 2014, up to 7 earthquakes occurred per day. An abrupt increase in seismicity was observed on 6 and 7 March 2014 when 10 and ~40 earthquakes per day respectively were recorded (figure 12). These earthquakes were centered within the area of Yugama. The note at the bottom explains the symbols refering to lake Yugama and Mount Ainomine to the lake's S. Earthquake depths were within 5 km of the surface and very clustered with few outliers (~2 earthquakes) located slightly deeper and further S. Seismicity subsided on 8 and 9 March, but during the following days, seismicity increased again and thereafter persisted for a few months. No other changes were detected from the area; diffuse white plumes continued to rise from the fumarolic zones.

Figure 12. Map and cross sections showing seismicity at Kusatsu-Shirane during 1 July 2013 through 31 July 2014. Courtesy of JMA.

Beginning in April, the GPS network registered an inflation trend across the Yugama crater region. Small-amplitude seismicity increased periodically during April-May. Ongoing magnetic surveys determined an increase in regional temperatures in May, but JMA cautioned that this characteristic was likely independent of the thermal emissions monitored at the fumarolic sites. Temperatures from the fumarolic sites had not changed significantly.

On 3 June, JMA raised the Alert Level from 1 to 2 (on a scale of 1-5) due to the ongoing seismicity, anomalous temperatures from magnetic surveys, and deformation. Gas emissions from the fumarolic sites, monitored by Tokyo Institute of Technology, changed in unspecified ways on 3 June. Warnings were announced for the immediate area of Yugama crater lake; the area within 1 km of the lake was potentially at risk if a small eruption occurred, particularly a gas explosion accompanied by ballistics. JMA recommended that no one enter the area and for any visitors to be mindful of gas hazards that could persist in low-lying areas (dense gases could collect within small valleys).

On 22 June, JMA reported the appearance of damaged vegetation; trees were dying in a forested area 3 km SW of Motoshiranesan cinder cone (location is the S-most labeled feature on the maps of figures 10 and 12). Note that Motoshiranesan is the highest peak belonging to the Kusatsu-Shirane complex (Hayakawa and others, 1981). An aerial survey was conducted on 23 June and observers noted that the affected area was ~10 m². White vapor emissions with volcanic gas odors were noted and measurements from an infrared camera determined a maximum temperature of ~40°C. JMA reported that there was no apparent link between these changes and the ongoing seismicity.

In July, JMA maintained Alert Level 2 and reported that elevated seismicity continued, although tremor was not detected. More than 80 earthquakes were detected from the Yugama crater lake area on 24 and 31 July (figure 13). Fluctuations in deformation occurred and magnetic surveys indicated that regional temperatures were leveling off. Infrared camera data determined that no significant changes were occurring at the fumarolic sites. Gas emissions measured continued to suggest changes in the volcanic system.

Figure 13. For Kusatsu-Shirane, the daily number of earthquakes recorded during July 2013-July 2014. Earthquakes increased abruptly in March 2014. Two months later, JMA raised the Alert Level from 1 to 2 due to persistent unrest (see text). Note that earthquake counts exceeded the y-axis on three occasions: 5 June 2011 (86 earthquakes), 24 July 2014 (149 earthquakes), and 31 July 2014 (100 earthquakes). Courtesy of JMA.

Reference: Hayakawa, Y., Aramaki, S., Shimozuru, D., and Ossaka, J., 1981, Kusatsu-Shirane Volcano, in Aramaki, S., ed., Field Excursion Guide to Fuji, Asama, Kusatsu-Shirane, and Nantai Volcanoes: Volcanological Society of Japan, International Association of Volcanology and Chemistry of the Earth's Interior, p. 49-63.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1 3 4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/).

April 2018 (BGVN 43:04) Cite this Report

Phreatic explosion at Motoshiranesan cone on 23 January 2018 results in one fatality and several injuries

The large Kusatsu-Shiranesan volcanic complex comprises three overlapping pyroclastic cones and numerous summit craters; it is located about 150 km NW of Tokyo in the Gunma Prefecture of central Japan. Intermittent short-lived historic activity has been reported from the northernmost Shiranesan cone since the beginning of the 19th century. An explosion at the southernmost Motoshiranesan cone in January 2018 resulted in one fatality and several injuries. Information about the event was gathered from the Japan Meteorological Agency (JMA) and various news sources.

Summary of activity during 1976-2014. Small phreatic explosions in the Mizugama and Yugama craters at the northernmost part of the Kusatsu-Shiranesan volcanic complex occurred in 1976, 1982, and 1983 (figure 14). Larger ash-bearing explosions in November and December 1983 sent tephra 30-40 km to communities downwind to the SE from the Yugama and adjacent Karagama craters on the Shiranesan cone. Intermittent increases in seismic activity near the Yugama crater coincided with water discoloration in the crater lake, and possible ejections of debris from hydrothermal activity in 1989 and 1996. Increased hydrothermal activity was noted on the N flank of Yugama during 2013-2014. Seismic swarms, deformation, thermal, and fumarolic activity increased briefly during early June 2014 in the area around the Yugama crater lake, but no eruption was observed. In late June 2014, JMA reported dying vegetation in a forested area 3 km SW of the Motoshiranesan summit area.

Figure 14. Subfeatures of the Kusatsu-Shiranesan volcanic complex as seen in Google Earth imagery, looking N. The northernmost bleached area includes the historically active Yugama, Mizugama, and Karagama craters, part of the Shiranesan cone. In the center of the complex is the Ainomine cone which has a ski area on the S flank. The southernmost edifice is the Motoshiranesan cone which has multiple craters at its summit, including Kagamiike or "Mirror pond". The explosions of 23 January 2018 occurred at Kagamiike and the adjacent crater to the N, in area referred to by JMA as Honkonoyama. Courtesy of Google Earth.

Activity during 2014-2017. Seismicity remained elevated from March to mid-August 2014 around the Yugama crater area. Ground deformation data suggested inflation between March 2014 and April 2015 in that area. Field surveys conducted on 4-5 and 10-11 November 2014 indicated fumarolic areas on the N and NE flanks of the Mizugama crater, but no other significant activity. Short-lived increases in seismicity were observed during January-February 2015. A field survey in May 2015 confirmed ongoing thermal activity on the N and NE wall of the Yugama crater, and the N and NE flank of the Mizugama crater. A small-amplitude, 2-minute-long tremor during late June 2015 was the first since January 2013; it was not accompanied by eruptive activity. The fumarolic activity on the N wall of the Yugama Crater was higher during a field survey in October 2015 than in had been the previous May.

Thermal activity was ongoing at Yugama and Mizugama craters during 2015-2017 along with intermittent fumarolic activity in the same general area, but no significant seismicity was reported. By June 2017 the decrease in the concentration of components derived from high-temperature volcanic gas in the lake, and the stable low-level seismicity in the area, led JMA to lower the warning level from 2 to 1 (on a 5 level scale) on 7 June 2017; they noted that the thermal activity continued around the Yugama crater throughout the rest of the year (figures 15-17).

Figure 15. A minor thermal anomaly persisted inside the NE crater wall at Yagama Crater at Kusatsu-Shiranesan throughout 2015-2017. Both visual (upper) and thermal (lower) images were taken during an overflight on 1 November 2017. View is to the north. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, November 2017).

Figure 16. Thermal anomalies persisted on the N and NE flank of the Mizugama crater at Kusatsu-Shiranesan during 2015-2017. These visual (upper) and thermal (lower) images were captured on 1 November 2017. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, November 2017).

Figure 17. Daily earthquake frequency at Kusatsu-Shiranesan during 1 January 2011-30 November 2017. Although earthquake counts temporarily increased during March-August 2014 and in January and February 2015, no eruptive activity was reported. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, November 2017).

Activity during January-March 2018. JMA reported that at 0959 on 23 January 2018 an eruption began at Kusatsu-Shiranesan coincident with the onset of volcanic tremor which prompted JMA to raise the Alert Level to 3 (on a scale of 1-5); there had been no prior indications of an impending eruption. Skiers at the popular Kusatsu Kokusai ski resort, located on the Ainomine cone, took video showing a plume of tephra and ejected bombs rising from vents around the Kagamiiki and adjacent crater at the summit of the Motoshiranesan cone (see Information Contacts for Mainichi for video link). Motoshiranesan is immediately adjacent S of the Ainomine cone and about 2 km SSE of the Yagama Crater on the Shiranesan cone where all previous historical activity had been reported (figures 14 and 18).

Figure 18. Locations and images of the active vents at Kusatsu-Shiranesan during the eruptive event of 23 January 2018. Upper left: View is looking W at the Motoshiranesan summit craters. The crater with the pond in Box 1 is Kagamiike (yellow Japanese characters). Boxes 1 and 2 in the upper left photo are enlarged in the lower photos. Upper right topographic map shows the locations in red of the three vents. The upper red line and dot correspond to the vents shown in the lower right box 2. The lower red bar on the topographic map (near the small pond) corresponds to the vent shown in the lower left image as box 1. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, January 2018).

Photos and video posted in news articles showed tephra shooting tens of meters into the air, drifting E, and blanketing the nearby hillside (figure 19); JMA noted ashfall in Nakanojo-machi, in the Gunma Prefecture, about 8 km E. Tephra hit a gondola, shattering glass and injuring four skiers (figure 20). Material fell through the roof of a lodge, where about 100 people had already been evacuated. Ground Self-Defense Force troops were engaging in ski training at the time of the event; one member died from the impact of large tephra blocks, and seven others were injured.

Figure 19. Tephra from Mount Kusatsu-Shiranesan covers the N flank of the Motoshiranesan cone and much of the Ainomine cone in this view to the W taken on 23 January 2018. Photo by Suo Takeuma, AP, courtesy of CNN (Japanese man killed by falling rocks after volcano erupts at ski resort, 23 January 2018).

Figure 20. Fist-sized tephra blocks and ash ejected from the eruption of Mount Kusatsu-Shiranesan cover the floor of a damaged gondola at the Kusatsu Kokusai Ski Resort on 24 January 2018, courtesy of The Mainichi Japan (Damaged ski resort gondolas show the power of Gunma Pref. volcanic eruption, 25 January 2018).

The following day, on 24 January 2018, JMA noted that volcanic earthquakes were numerous but decreasing in number, and two 2-3-minute-long periods of volcanic tremor were detected at 1015 and 1049. Minor but elevated seismicity continued through 30 January, punctuated by periods of tremor. The largest fissure where the eruption occurred was oriented E-W, located just inside the N rim of the northernmost crater at the Motoshiranesan summit (figure 21). Kenji Nogami, a professor at the Tokyo Institute of Technology, confirmed that the event appeared to have been "a typical phreatic eruption" (Japan Times).

Figure 21. The largest fissure vent active in the 23 January 2018 explosion at Kusatsu-Shiranesan was still surrounded by ash and tephra when photographed during an overflight on 28 January 2018. The summit ropeway station of the ski area is at the image top just NW of the explosion vent. Courtesy of The Mainichi (Visitor traffic plunges in Kusatsu hot spring resort after deadly eruptions, 30 January 2018).

The Tokyo Volcanic Ash Advisory Center issued a single volcanic ash advisory on 23 January indicating a possible eruption, but it was not identifiable from satellite data. Observations made on 14 February 2018 confirmed the presence of the vents in the Kagamiike and adjacent crater, but there was no evidence of thermal activity and little fumarolic activity in the area (figure 22). Seismicity decreased steadily after the explosion on 23 January 2018 through the end of March 2018 and no further activity was reported (figure 23).

Figure 22. Vents from the 23 January 2018 eruption at Kusatsu-Shiranesan were still visible at the craters on 14 February 2014 during a helicopter overflight by JMA. The upper image, looking W, shows the large vent at the N side of the crater immediately N of the Kagamiike crater, as well as a smaller vent located to the W on the E flank of the adjacent slope. The lower image shows two smaller vents on the inner wall of the adjacent Kagamiike crater. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, February 2018).

Figure 23. Seismicity decreased steadily at Kusatsu-Shiranesan after the explosion on 23 January 2018. Graph shows the number of daily seismic events during 1 January-31 March 2018. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, March 2018).

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); The Mainichi (URL: http://mainichi.jp/english/, eruption video URL-https://mainichi.jp/movie/video/?id=121708141#cxrecs_s); The Japan Times (URL: https://www.japantimes.co.jp/); Cable News Network (CNN), Turner Broadcasting System, Inc. (URL: http://www.cnn.com/).