Smithsonian Institution, Bulletin of the Global Volcanism Network Volume 21, Number 1, January 1996 Merapi (Indonesia) Increased seismicity related to lava avalanches and rockfalls Merapi Java, Indonesia 7.54 S, 110.44 E; summit elev. 2,911 m During November-December 1995 glowing avalanches down the Boyong, Krasak, and Bebeng rivers reached up to 2.5 km from the source. Seismic activity was dominated by multiphase earthquakes, low- frequency earthquakes, and lava avalanches (rockfalls). The number of multiphase earthquakes decreased from 924 in November to 152 in December; low-frequency earthquakes also decreased from 74 in November to 42 in December. Seismicity associated with lava avalanches and rock falls increased from 816 events in November to 1,078 in December (figure 6). A deep volcanic earthquake (A- type) and two tremor events were recorded in November, three shallow volcanic earthquakes (B-type) occurred in December. Inflation increased since 17 November from 2.5 to 10.8 microrad/day. Tilt data collected at two stations in the summit area during November and December indicated inflation of 60 and 320 microrad, respectively. The geomagnetic intensity in early December was -14.5 nTs; it then decreased to -1.5 nTs by the end of the month. The emission rate of SO2 during November fluctuated between 27 and 275 t/d, averaging 95 t/d, and the plume velocity was ~3.2-3.5 m/s. In December the emission rate decreased to 70 t/d, fluctuating between 18 and 156 t/d; plume speed was slightly higher at 3.3-3.6 m/s. Merapi is one of the most active volcanoes in Indonesia. The stratovolcano has an exposed, summit lava dome, the source of abundant glowing avalanches directed down its S to W slopes toward a region settled by at least 50,000 people. In historical time, instability of the growing dome has led to pyroclastic flows that have caused many fatalities. Collapse of the growing lava dome in November 1994 triggered pyroclastic flows that killed 64 people (Bulletin v. 19, nos. 10 and 12). Additional collapses were reported in January and August-October 1995 (Bulletin v. 20, nos. 2 and 10). Information Contact: VSI (see Semeru); Steve O=FEMeara (see Semeru). Figure 6. Monthly number of rockfall, multiphase, and low-frequency earthquakes at Merapi, June-December 1995. Courtesy of VSI. Smithsonian Institution, Bulletin of the Global Volcanism Network Volume 21, Number 1, January 1996 Krakatau (Indonesia) Steaming and fumarolic activity; cone description Krakatau Indonesia 6.10 S, 105.42 E; summit elev. 813 m During an approved visit on 6 November, the volcano was steaming but not erupting. A large sulfur-stained plug of lava, ~20% the diameter of the summit cone, was bulging out of a black cinder cone just below and W of the summit; it appeared to be inside the SW margin of a broad depression. A smaller sulfur-stained plug was farther S in another depression. The landing site on the SE shore was a black-sand beach with tiny dunes of white pumice. While climbing the SE slope of the older cone, the party crossed water-eroded fields of pyroclastic material dotted with volcanic bombs. The ascent to the summit went through deep cinder deposits covered with a blanket of loose breadloaf-sized stones. From the summit complicated internal crater structures could be seen. One symmetrical cone (~100 m across) had a sulfur-lined inner rim that was fuming. The largest and most active fumaroles were inside this cone=FEs S rim. A smaller cone within the larger one was almost horseshoe-shaped and steeper to the S. Bombs on the summit cone were as large as 1-2 m in diameter. The current activity began in March 1994 (Bulletin v. 19, no. 4) after almost continuous activity from November 1992-October 1993. Activity was intermittent during 1995 (Bulletin v. 20, nos. 3, 6, & 7). This renowned volcano lies in the Sunda Strait between Java and Sumatra. Caldera collapse, perhaps in 416 A.D., formed a 7- km-wide caldera. Three subsequent volcanoes coalesced into pre- 1883 Krakatau Island. Caldera collapse during the 1883 eruption destroyed all but a remnant of one volcano. The post-collapse cone of Anak Krakatau (Child of Krakatau), located within the 1883 caldera, has had frequent eruptions since 1927. Information Contact: Steve O=FEMeara (see Semeru). Smithsonian Institution, Bulletin of the Global Volcanism Network Volume 21, Number 1, January 1996 Soputan (Indonesia) Vapor emission and intense tremor; possible high ash cloud Soputan Sulawesi, Indonesia 1.11 N, 124.73 E; summit elev. 1,784 m Activity in late 1995 consisted of whitish vapor emission to 25- 100 m above the summit. During November occasional volcanic tremors were recorded with a maximum amplitude of 1.5 mm. Aviation reports on 7 November indicated increased eruptive activity with an ash cloud rising as high as 4.5 km altitude. Satellite imagery showed a possible ash cloud extending 90 km to the SW. On 5 December, an increase in tremor amplitude up to 5 mm followed a tectonic earthquake felt throughout the Mimahassa Peninsula on Sulawesi. The same day maximum tremor amplitude reached 200 mm and glow was observed from three points on the lava dome. About an hour later tremor reached a maximum amplitude of 40 mm. On 6 December, tremor was still being recorded, but maximum amplitude had decreased to 2 mm. Soputan, a small conical volcano on the rim of Tondaro Caldera, has had frequent historical eruptions. The eruption that began in October 1991 continued through at least mid-1993 with lava domes rising above the crater rim. Continuous endogenous growth and intense fumarolic activity were noted in July 1994 (Bulletin v. 19, no. 8). Information Contacts: VSI (see Semeru); Bureau of Meteorology, Northern Territory Regional Office, P.O. Box 735, Darwin, NT 0801 Australia. Smithsonian Institution, Bulletin of the Global Volcanism Network Volume 21, Number 1, January 1996 Lokon-Empung (Indonesia) Variable seismicity and minor vapor emissions Lokon-Empung N Sulawesi, Indonesia 1.36 N, 124.79 E; summit elev. 1,580 m Vapor emission was observed during November-December 1995 and whitish-gray plumes rose to 100 m above the active crater. Seismicity during September-October 1995 consisted of 1-8 A-type events/day. On 1 November there were 46 A-type events recorded, followed by very low seismicity over the next ten days. Activity then increased from 12 November through 31 December, but was highly variable with 4-21 events/day. B-type events remained at 0-8 events/day. The present activity is located at Tompaluan crater, in the saddle between the peaks of Lokon (1,579 m) and Empung (1,340 m). About 10,000 people evacuated following an explosion in October 1991 accompanied by a pyroclastic flow; the eruption ended in January 1992. Information Contact: VSI (see Semeru). Smithsonian Institution, Bulletin of the Global Volcanism Network Volume 21, Number 1, January 1996 Karangetang (Indonesia) Small ash plumes, incandescent ejecta, and increased seismicity Karangetang Siau Island, Indonesia 2.47 N, 125.29 E; summit elev. 1,784 m On 9 and 16 November "thunderclaps" were heard from the summit. A gray plume 500 m high was observed, and incandescent ejecta rose 10-50 m above the summit at night. On 17 December thunderclaps were heard again and ejecta rose 100 m above the summit. Seismicity increased from 26 October until the end of 1995. Daily counts of deep volcanic (A-type) earthquakes fluctuated up to 116 (figure 7). Karangetang volcano, the most active of Sulawesi province, is located at the N end of Siau Island and contains a line of summit craters. Activity during the summer of 1994 was characterized by the irregular occurrence of lava flows, explosions, Merapi-style pyroclastic flows and lahars (Bulletin v. 19, no. 8). Information Contact: VSI (see Semeru). Figure 7. A-type seismicity at Karangetang, September-December 1995. Courtesy of VSI. Smithsonian Institution, Bulletin of the Global Volcanism Network Volume 21, Number 1, January 1996 Suwanose-jima (Japan) Small eruptions in January; nine explosions throughout 1995 Suwanose-jima Ryukyu Islands, Japan 29.53 N, 129.72 E; summit elev. 799 m Monitoring from the Sakurajima Volcanological Observatory revealed nine explosions from Suwanose-jima in 1995. According to the Japan Meteorological Agency and the Kagoshima Prefectural Government, small eruptions during 10-13 January 1996 sent plumes 300-600 m above the volcano and caused ashfall to the S. Activity has been high since 1950, with 1-2 ash emissions every month, and some Strombolian explosions.=20 This 8 x 5 km volcano island consists of basaltic andesite and andesite. The largest historical eruption occurred in 1813-14; as a result, thick scoria beds covered the residential area and lava flows descended WSW from a new 300-m-diameter crater. Large eruptions also occurred in 1884-85 and a lava flow reached the E coast. The island was evacuated for about 70 years, but there are now around 50 residents. Information Contact: Volcano Research Center, Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://hakone.eri.u-tokyo.ac.jp/vrc/vrc.html). Smithsonian Institution, Bulletin of the Global Volcanism Network Volume 21, Number 1, January 1996 Sakura-jima (Japan) Explosive eruptions and ashfalls continue Sakura-jima Kyushu, Japan 31.58 N, 130.67 E; summit elev. 1,117 m Minami-dake crater was slightly active throughout January. The monthly total number of eruptions was 60, including 42 explosive ones. At the seismic station 2.3 km NW of Minami-dake crater (Station B), 601 earthquakes and 684 tremors were recorded. The highest ash plume of the month rose 2,300 m above the summit crater on the 21st. Ashfall measured at the Kagoshima Local Meteorological Observatory, 10 km W form the crater, was 41 g/m^2. The VRC noted that there were more than 200 eruptions in 1995; total amount of erupted material was estimated at 3-4 million tons by the Sakurajima Volcanological Observatory, Kyoto University. The latter has been observing continuous uplift on the N side of the volcano, implying accumulation of magma beneath the volcano. Sakura-jima volcano, located within the Aira Caldera, lies in the N half of Kagoshima Bay. The volcano formed an island that was joined to the Osumi Peninsula during an eruption in 1914. Frequent historical eruptions have deposited ash on the city of Kagoshima, which is located across Kagoshima Bay 7 km W of the summit. Sakura-jima=FEs frequent eruptions present a significant hazard to air transportation using Kagoshima=FEs airport. Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1- 3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan; VRC (see Suwanose- jima). Smithsonian Institution, Bulletin of the Global Volcanism Network Volume 21, Number 1, January 1996 Kuju (Japan) Earthquake swarm and eruptions on 13-14 January; continuous plume Kuju Kyushu, Japan 33.08 N, 131.25 E; summit elev. 1,788 m Frequent earthquakes during the night of 13 January and through the next morning were centered 3-4 km NW of the Hosho dome near Sujiyu spa; eruptions caused minor ashfall around the volcano. Instruments recorded 526 earthquakes during the 13-14 January episode, some of which were felt by local residents. Some earthquakes on 27 January were centered SW of the active dome. Overall, there were 861 earthquakes detected in January, but no tremor. The plume height remained at 100-300 m throughout most of the month. Scientists at the University of Tokyo noted that vesiculated glass was again observed in the 13 January material, and deflation near the crater area was continuing. The Kuju volcano group consists of more than 10 stratovolcanoes and lava domes NE of Aso caldera. Hosho lava dome is situated near the center of the 20 x 15 km complex. The Geological Survey of Japan reports historical phreatic or hydrothermal eruptions at Kuju in 1662, 1675, and 1738. Molten sulfur effusion may have occurred in 1675 and 1738. The youngest known magmatic activity, ~1.7 ka, produced the Kuro-dake lava dome at the E end of the complex, accompanied by pyroclastic flows. Information Contacts: JMA (see Sakura-jima); VRC (see Suwanose- jima). Smithsonian Institution, Bulletin of the Global Volcanism Network Volume 21, Number 1, January 1996 Fukutoku-okanoba (Japan) Discolored seawater observed for the second time in three months Fukutoku-okanoba Volcano Islands, Japan 24.28 N, 141.52 E; summit elev. -14 m (submarine) On 12, 22, and 23 January, an aviator from the Japan Marine Safety Agency (JMSA) reported distinct discoloration of seawater to yellowish green. Similar discoloration was seen during 25-28 November 1995 (Bulletin v. 20, no. 11/12). Prior to that, discolored seawater was last seen at this location in September 1993. Water discoloration has often been observed at Fukutoku-okanaba (Shin-Iwo-jima), 5 km NE of Minami-Iwo-jima island. Eruptions produced ephemeral islands in 1904, 1914, 1974, and 1986. A dark plume rose above the water surface in November 1992, and floating pumice was observed. Information Contact: JMA (see Sakura-jima). Smithsonian Institution, Bulletin of the Global Volcanism Network Volume 21, Number 1, January 1996 Minami-Hiyoshi (Japan) Discolored seawater plume 6 km long Minami-Hiyoshi Volcano Islands, Japan 23.507 N, 141.905 E; summit elev. -30 m (submarine) On 12 January, an aviator from the JMSA observed seawater discolored to yellowish green in an area 500 m wide and 6 km long, flowing from Minami-Hiyoshi seamount to the S. Discolored water was last observed in February 1992 (Bulletin v. 17, no. 2). Periodic water discoloration and water-spouting has been reported over this submarine volcano since 1975, when detonations and an explosion were also reported. Located 90 km SE of Minami-Iwo- Jima, Minami-Hiyoshi lies near the SE end of a coalescing chain of youthful seamounts, and is the only historically active vent. The morphologically youthful seamounts Kita-Hiyoshi and Naka- Hiyoshi lie to the NW, and Ko-Hiyoshi to the SE. Information Contact: JMA (see Sakura-jima). Smithsonian Institution, Bulletin of the Global Volcanism Network Volume 21, Number 1, January 1996 Unknown Submarine Activity (W Pacific Ocean) Acoustic signals in late January; source not locatable Unknown Submarine Activity western Pacific Ocean The JMA Ocean Bottom Seismograph off the Boso Peninsula (E of Tokyo) detected T-phase-like signals after 22 January, and clear T-phase signals on 27 January (figure 8). According to tentative analyses of arrival times at the detectors, the signals were interpreted to have propagated from the S. As of mid-February, JMA had not determined a specific source for these signals. However, discolored seawater was observed above two submarine volcanoes in the Volcano Islands during January: Minami-Hiyoshi on 12 January, and Fukutoku-okanoba on 12, 22, and 23 January. Information Contact: JMA (see Sakura-jima). Figure 8. Example of T-phase signals (spikes) detected by the Ocean Bottom Seismograph off the Boso Peninsula, Japan, 27 January 1996. Courtesy of JMA.