Although the latest eruptive period ended in late July (BGVN 29:08), the volcanic system at Anatahan continued to exhibit unrest in the following months. On 27 September, several hours after the onset of a series of intense tropical depressions and storms, the first long-period seismic events since July 2004 were recorded. However, only a few, small events occurred. Beginning on 12 October, several episodes of small, regularly-spaced long-period events were recorded at intervals of 4-15 seconds. On 18 October, people in Saipan smelled H₂S during very hazy visibility, but no plume was detected on satellite imagery by the Washington Volcanic Ash Advisory Center (VAAC).

Based on a pilot report to the Guam Forecast Office, the Washington VAAC reported that ash from Anatahan was at a height of ~ 3 km altitude on 2 December. Ash was not visible on satellite imagery, but a hotspot was briefly evident on infrared imagery.

Eruptions in January 2005. Major eruptive activity at Anatahan resumed on 5 January 2005, preceded by two days of small long-period earthquakes and a day of harmonic tremor. The airport tower at Guam confirmed that a plume of diffuse ash and gas up to ~ 200 m above the vent was visible at first light on 6 January, and at the 1225 hours VAAC reported a plume 60 km long and 20 km wide, blowing W.

Frequent Strombolian explosion signals began on 6 January and continued during 7 and 8 January, accompanied by a change in the seismic signals, from harmonic tremor to a broader band tremor, with explosions recorded by microphones several times per minute. The eruption type and activity level were both very similar to the peak eruptive activity during the eruption of April-June 2004 (BGVN 29:04-29:06). During an overflight on 7 January, personnel from the Emergency Management Office (EMO) reported ash rising well above 1,500 m and a plume that likely extended up to 100 km downwind. A dome was visible in the crater and bombs were observed rising less than 600 m.

On 7 January ash rose to ~ 3 km and bombs a meter or more in diameter were expelled to ~ 100 m and formed a new cinder cone ~ 120 m in diameter. The amplitude of the explosion signals increased slowly after 6 January to about double these values by noon on 10 January, with explosions every 3-10 seconds. Explosion signals amplitudes then plunged suddenly to half the values at the start of 10 January. The amplitudes surged again, nearly doubling by approximately 0400 on 11 January, dropping to half the value again by about noon. The eruption apparently stabilized at that level through 14 January. During 15-19 January, the eruption appears to have stopped twice for a few hours but swiftly resumed at higher levels. National Oceanic and Atmospheric Agency (NOAA) satellite photos show a plume of vog (volcanic smog) trailing ~ 60 km downwind.

Near mid-day on 20 January seismicity dropped abruptly to near background levels, whereas microphone noise became fairly continuous, indicating that the explosions had ceased but that degassing may have been continuing. The apparent cessation of Strombolian activity lasted until late 22 January, when explosions resumed. The eruption peaked about 0700 on 23 January, at which time a SIGMET (significant meteorological forecast) was issued by the FSS (Flight Service Station) Honolulu, based on pilot reports of ash up to 3-4.6 m. The explosions then decreased somewhat but were still frequent and strong through 24 January, based on the seismicity.

The EMO placed Anatahan Island off limits until further notice and concluded that, although the volcano was not currently dangerous to most aircraft within the CNMI airspace, conditions may change rapidly. Aircraft should pass upwind of, or beyond 30 km downwind from, the island, and exercise due caution within 30 km of Anatahan.

Synopsis of recent eruptions. Anatahan had no historical eruptions prior to 2003. On 10 May of that year, after several hours of increasing seismicity, a phreatomagmatic eruption sent ash to over 10 km and deposited about 10 million cubic meters of material over the island and sea. A very small craggy dome extruded during late May and was destroyed during explosions on 14 June, after which the eruption essentially ceased. A second eruption began about 9 April 2004, after more than a week of increasing seismicity. The eruption consisted of passive extrusion during mid-April, then increased to Strombolian explosions every minute or two on 24 April. The Strombolian explosions continued through mid-July, often sending a thin plume of gas and ash upwards a few hundreds of meters and 100 km downwind. Activity decreased substantially on 26 July, though visitors to the island three months later could still see very small amounts of steam and ash rising 30-40 m above the crater rim and could smell SO₂ near the crater.

Geological Summary. The elongate, 9-km-long island of Anatahan in the central Mariana Islands consists of a large stratovolcano with a 2.3 x 5 km compound summit caldera. The larger western portion of the caldera is 2.3 x 3 km wide, and its western rim forms the island's high point. Ponded lava flows overlain by pyroclastic deposits fill the floor of the western caldera, whose SW side is cut by a fresh-looking smaller crater. The 2-km-wide eastern portion of the caldera contained a steep-walled inner crater whose floor prior to the 2003 eruption was only 68 m above sea level. A submarine cone, named NE Anatahan, rises to within 460 m of the sea surface on the NE flank, and numerous other submarine vents are found on the NE-to-SE flanks. Sparseness of vegetation on the most recent lava flows had indicated that they were of Holocene age, but the first historical eruption did not occur until May 2003, when a large explosive eruption took place forming a new crater inside the eastern caldera.

Information Contacts: Juan Takai Camacho and Ramon Chong, Emergency Management Office of the Commonwealth of the Northern Mariana Islands (CNMI/EMO), P.O. Box 100007, Saipan, MP 96950, USA (URL: http://www.cnmihsem.gov.mp/); Frank Trusdell, U.S. Geological Survey, Hawaiian Volcano Observatory (HVO), PO Box 51, Hawaii National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/nmi/activity/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch, NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/); NOAA/ National Weather Service, National Centers for Environmental Prediction, Aviation Weather Center, Volcanic Ash SIGMETS (URL: http://adds.aviationweather.gov/airmets/)