The Hakoneyama volcano includes large calderas, post-caldera domes, and active thermal areas about 80 km SW of Tokyo. Phreatic explosions have been radiocarbon dated to the 12th or 13th centuries CE, and periodic seismic swarms have been recorded since the late 20th century when regular monitoring began. Renewed activity at the Owakudani hot spring area (1 km N of the Kamiyama dome summit), including two small explosions and the formation of several new fumaroles, was reported by the Japan Meteorological Agency (JMA) in 2015. The area is a popular resort destination with golf courses, spas, skiing, and aerial tramways.

Seismic swarms were previously reported in 1991 (BGVN 16:04), 2001, 2011, and 2013. A slight inflation of the volcanic edifice was recorded between December 2012 and March 2013. Increased seismicity and intense steam emissions at Owakudani were observed beginning in April 2015, and on 29 and 30 June 2015 two small explosions from a new fumarole created a mound of ash around the vent and scattered ash a few hundred meters N. Additional ash was observed in the early morning of 1 July. Strong degassing from the fumaroles continued through the end of the year, although seismicity dropped below detection by mid-August, and inflation stopped at the end of August.

Activity during 1991-2014. A swarm of about 300 earthquakes, all less than M 2.5 and located at 5 km depth below the central part of the caldera, was reported by JMA in April 1991 and felt by area residents (BGVN 16:04). Between June and August 2001 elevated seismicity from depths less than 5 km below the surface was associated with a small amount of inflation centered at the volcano; a small swarm was also recorded under the N end of Ashino-ko (Lake Ashi).

A brief period of increased seismicity was again recorded in the few days following the 11 March 2011 Tohoku Earthquake which was centered about 400 km NE. The activity was confined to the area around the Owakudani hot springs. On 21 March 2011, a M 4.2 earthquake occurred around the Komagatake dome (1 km SSE of Kamiyama). Fumes from the Sounjigoku geothermal area on the E side of the volcano were observed rising 50-200 m during March 2011. The tiltmeter network operated by the Hot Springs Research Institute of Kanagawa Prefecture showed no relevant changes resulting from the earthquake.

Between January and March 2013, JMA reported a slight increase in the number of shallow earthquakes in the area between Komagatake and Sengokuhara Hot Spring (4 km NE of Kamiyama dome). Strainmeter and tiltmeter observations recorded a slight inflation of the volcano that began at the end of 2012 and stopped by the end of February 2013.

Activity during 2015. Scientists conducted field surveys on 4-5 April 2015 and observed forceful steam emissions at the Owakudani hot spring complex that continued at least through 8 April. Six small seismic events were recorded on 10 April in the Yumoto area of the city of Hakone (8 km ESE). JMA reported an increased seismicity to tens of daily events at Hakoneyama beginning 26 April 2015. On 5 May, three more small seismic events were recorded in the Yumoto area. This led JMA to raise the Volcano Alert Level from 1 (Normal) to 2 (Do not approach the crater) on a 5-level scale on 6 May. There were 21 additional small seismic events in the Yumoto area during May 2015. The daily total of all seismic events rose significantly to 442 on 15 May, the highest since 2001.

Ground deformation data showed changes attributable to the seismic events. The Mainichi (a daily newspaper) reported on 17 May 2015 that an analysis of satellite data by the Geospatial Information Authority of Japan found that the ground level in a 200-m-diameter area in the Owakudani district rose 12 cm between 17 April and 15 May. Intense steam emissions continued at the Owakudani hot spring supply complex in May (figure 1).

Figure 1. An aerial photo taken by the Kyodo News helicopter shows steam rising from the Owakudani hot spring supply facility at Hakoneyama on 7 May 2015. Courtesy of The Japan Times.

Volcanic tremor lasting for around five minutes started at 0732 on 29 June 2015. At around 1245, fallout was observed in an area up to 1.2 km N and NE of the Owakudani hot spring complex. This fallout was interpreted by JMA to be sediment and debris ejected from a "blowhole" that formed a new fumarole at Owakudani. A field survey conducted on 30 June observed a mound of volcanic ash around the new fumarole. Additionally, volcanic ash was observed near the Owakudani Station of the Hakone Ropeway (a resort tramway) located about 300 m NNW of the hot springs area. As a result of the discovery of the ashfall, JMA raised the Alert Level at 1230 on 30 June from 2 to 3 (Do not approach the volcano).

Additional volcanic ash was found on a camera around 0500 on 1 July. Air shocks that were intermittently observed from 1600 on 29 June to 1 July may have occurred in association with eruptions, the last occurring between 0400 and 0500 on 1 July. A field survey conducted by the Hot Springs Research Institute of Kangawa Prefecture and JMA on 2 July confirmed continued forceful emissions of steam at the new fumarole, which had grown to 20 m in diameter, and was designated Crater 15-1. Three other new fumaroles were also observed around this crater.

No low-frequency earthquakes or volcanic tremors were recorded in July, but active degassing at Crater 15-1 and adjacent fumaroles continued. In a field survey on 6 August 2015, material described as "dark gray soil" was observed around the fumarole, inside the crater rim. Thermal infrared observation showed a high-temperature area on the E side of the crater from August through October. A single, minor seismic event was recorded on 17 August, the first one since 3 July. JMA downgraded the Alert Level at the volcano from 3 (Do not approach the volcano) to 2 (Do not approach the crater) on 11 September, and no further changes in the tiltmeter or strainmeter data were observed after August.

Field surveys on 3 and 29 September indicated that "dark gray soil" continued to be ejected from Crater 15-1, but the scale of the explosions were small and ejection heights were lower than the crater rim. Strong degassing was still occurring at the crater and all other fumaroles. By 20 November the slight weakening of the emissions from Crater 15-1 and overall stability and low seismic activity led JMA to reduce the Volcanic Alert Level further to 1 (Potential for increased activity). Field observations on 7 and 9 December 2015 confirmed continuing strong degassing with plumes to 600 m in the area around Owakudani, but no new ejecta.

Geological Summary. Hakoneyama volcano is truncated by two overlapping calderas, the largest of which is 10 x 11 km wide. The calderas were formed as a result of two major explosive eruptions about 180,000 and 49,000-60,000 years ago. Scenic Lake Ashi lies between the SW caldera wall and a half dozen post-caldera lava domes that were constructed along a NW-SE trend cutting through the center of the calderas. Dome growth occurred progressively to the NW, and the largest and youngest of these, Kamiyama, forms the high point. The calderas are breached to the east by the Hayakawa canyon. A phreatic explosion about 3000 years ago was followed by collapse of the NW side of Kamiyama, damming the Hayakawa valley and creating Lake Ashi. The latest magmatic eruptive activity about 2900 years ago produced a pyroclastic flow and a lava dome in the explosion crater, although phreatic eruptions took place as recently as the 12-13th centuries CE. Seismic swarms have occurred during the 20th century. Lake Ashi, along with the thermal areas in the caldera, is a popular resort destination SW of Tokyo.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/); The Mainichi, Japan's National Daily since 1922 (URL: http://mainichi.jp/english/); The Japan Times (URL: http://www.japantimes.co.jp/news/2015/05/07/national/volcano-alert-raised-mount-hakone/#.WHkqQHrcDy1).