Continuous tremor, intervals with several explosions per day, and plumes rising to 5.5 km altitude were observed at Suwanosejima between 1 April 2013 and 14 December 2014 (BGVN 39:11). The data for this report, covering 5 January-11 September 2015, was gathered primarily from two key sources: the Tokyo Volcanic Ash Advisory Center (VAAC) and the Japan Meteorological Agency (JMA). Throughout the entire reporting period, no MODVOLC thermal anomalies were recorded, although the hazard status remained at Alert Level 2 (Do not approach the crater), on an increasing scale of 1-5. The Otake (also O-take) crater (figure 1) was the site of much of the activity during 2015.

Figure 19. Simplified map of the geology of Suwanosejima. The active crater, O-take (Oc), appears in the center of the small, sparsely populated island. Courtesy of Taketo Shimano.

In its Monthly Volcanic Activity Report for January 2015, JMA noted four explosive eruptions at the Otake crater, in addition to other occasional non-explosive eruptions. Grayish plumes accompanying the eruption rose as high as 1 km above the crater rim. On 25 January a field survey revealed a pit in the southeastern portion of the Otake crater which had formed since the previous survey on 8 November 2012.

Plumes in 2015 were reported by the VAAC in the months of January, February, April, July, August, and September. JMA served as the primary source for all of these VAAC notices; any additional sources are noted. The Tokyo VAAC reported that on 5 January ash plumes rose to altitudes of 1.5-1.8 km and drifted NE and SE, and were also observed by pilots. The VAAC also reported an explosion on 25 January, the same day as the field survey.

The Tokyo VAAC reported that during 11-12 and 14-15 February ash plumes rose to altitudes of 1.8-2.1 km and drifted E. JMA's monthly report for February 2015 indicated that twelve explosions occurred at Otake crater, in addition to occasional, non-explosive events. Grayish plumes accompanying the explosions rose as high as 1,500 m above the crater rim. According to the Suwanosejima branch of the Toshima Village administration, ash fall was observed at Kiriishi port (located ~3.5 km S. of Otake) on 26 February.

A very small eruption at the Otake crater on 5 March 2015 was noted by JMA. An event on 13 April reported by the Tokyo VAAC generated a plume that rose to an altitude of 2.1 km and drifted N. Explosions during 24-25 April generated plumes that rose to altitudes of 1.8-2.1 km and drifted N and SE.

JMA reported a continued high activity level at the Otake crater with very small eruptions recorded on 5 and 17 May 2015. No explosions were observed at the Otake crater in June. The Tokyo VAAC reported that ash plumes from small eruptions at Otake on 30-31 July rose to altitudes of 2.1-3 km and drifted E, SW, and W, as reported by pilots and seen in satellite data. Grayish plumes accompanying the eruption rose as high as 1,300 m above the crater rim. According to the Suwanosejima branch of the Toshima Village administration, ashfall was observed in a village ~4 km SSW of Otake on 31 July.

JMA's August 2015 report described small, occasional, non-explosive events at the Otake crater, with accompanying grayish plumes rising as high as 1.2 km above the crater rim. Volcanic "glow" was observed at the Otake crater occasionally at night with a high-sensitivity camera. According to the Toshima Village administration, ashfall 4 km SSW of Otake was again present on 1, 2, and 9 August. The Tokyo VAAC reported that ash plumes identified in satellite images rose to an altitude of 4 km on 2 August, and to 1.8 km on 21 August that drifted SE.

In the September 2015 report, JMA noted that volcanic activity had remained at high levels, with 89 explosions recorded at the Otake crater; 69 of those were on 24 September, the first time more than 50 explosions a day had been observed since 30 December 2013. Plumes accompanying the events rose as high as 1,500 m above the crater rim. Crater incandescence was observed at night with a thermal camera. According to the Toshima Village administration, ashfall was once again observed in a village 4 km SSW on 7 September. The Tokyo VAAC reported that on 13 September ash plumes rose to an altitude of 1.8 km and drifted SE. JMA noted that parts of local structures shook in association with explosions that occurred on 24 September. Explosions and rumbling were heard on the island.

Geological Summary. The 8-km-long island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two active summit craters. The summit is truncated by a large breached crater extending to the sea on the E flank that was formed by edifice collapse. One of Japan's most frequently active volcanoes, it was in a state of intermittent Strombolian activity from Otake, the NE summit crater, between 1949 and 1996, after which periods of inactivity lengthened. The largest recorded eruption took place in 1813-14, when thick scoria deposits covered residential areas, and the SW crater produced two lava flows that reached the western coast. At the end of the eruption the summit of Otake collapsed, forming a large debris avalanche and creating an open collapse scarp extending to the eastern coast. The island remained uninhabited for about 70 years after the 1813-1814 eruption. Lava flows reached the eastern coast of the island in 1884. Only about 50 people live on the island.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/); Tokyo Volcanic Ash Advisory Center (VAAC), Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/).