As discussed in our previous report (BGVN 29:12), Anatahan's third historical eruption began on 5 January 2005. Ongoing eruptions continued through at least 18 February 2005.

Anatahan lies in the Commonwealth of the Northern Mariana Islands (CNMI) ~ 120 km N of Saipan and ~ 320 km N of Guam. The first historical eruption of Anatahan began 10 May 2003 (BGVN 28:04 and 28:05); after several hours of increasing seismicity, a phreatomagmatic eruption sent ash to over ~ 9 km (~ 30,000 feet) and deposited about 10 million cubic meters of material over the island and sea. A small craggy dome extruded in late May and was destroyed during explosions on 13 and 14 June, after which the eruption ceased. The second historical eruption began about 9 April 2004 after a week or so of increasing seismicity (BGVN 29:04). That eruption primarily comprised Strombolian explosions every minute or so and occasionally sent ash up to a few thousand feet. The eruption ended 26 July 2004.

Charles Holliday (US Airforce Weather Agency, AFWA) contributed a series of remotely sensed images showing plumes in February (table 3, figure 13). The plume in the 4 February Terra image (figure 13, top) contains a brownish tinge suggesting considerable ash. The Anatahan region was on the western edge of the Terra pass. The image contains an artifact reminiscent of Venetian blinds (commonly called the bow-tie effect), which arose due to pixel replication in the mapping/processing algorithm filling in for missing data on the edge of the scan.

Table 3. A list of some of the satellite images recording Anatahan plumes during 3-15 February 2005. Those shown with an asterisk appear in the next figure. Date and time are both UTC; for example, 04 Feb 2005 at 0105 is the date and time in UTC, in this example equivalent to 04 Feb 2005 at 1105 local time. Names affiliated with satellites are as follows: DMSP (Defense Meteorological Satellite Program), NOAA (National Oceanic and Atmospheric Administration), NASA (National Aeronautics and Space Administration). Courtesy of Charles Holliday, U.S. Air Force Weather Agency.

Figure 13. Two remotely sensed images of Anatahan plumes during February 2005 (N is upwards). (top) S-blown steam and vog on 4 February at 0105 UTC clearly identifiable to Saipan and Tinian islands; reported as the source of health problems in Guam news reports (see text). (bottom) A modest ash/steam plume in unusually clear conditions, imaged at 2303 UTC on 13 February (DMSP F16 visible) reached 260 km (140 NM). Courtesy of Charles Holliday, U.S. Air Force Weather Agency.

Randy White of the U.S. Geological Survey noted that the energy release from seismic stations monitoring Anatahan dropped to near zero on 13 February 2005, yet a monitoring microphone continued to indicate considerable acoustic-energy release. Corresponding to this, a MODIS image clearly showed ash still being emitted early on 14 February (see table 3). In other words, the seismicity failed to accurately portray the eruption's vigor.

Reporters Katie Worth and Natalie Quinata wrote in the 5 February 2005 issue of the Pacific Daily News that many students in school on Guam had been sent home after experiencing dizziness or nausea because of the foul-smelling 'vog' or volcanic smog hovering over the island from the eruption. Guam lies ~ 320 km S of Anatahan.

John Ravelo wrote a news article for the Saipan Tribune published on 15 February with the title "Anatahan ash cloud continues to hinder flights." Ravelo said that, "An aircraft coming from Manila to Saipan experienced zero visibility before landing at the Francisco C. Ada-Saipan International Airport yesterday morning, prompting the carrier's pilots to fly around the island and search for a clearer approach to the runway. The passenger aircraft landed safely at the airport, but the hazy condition delayed its arrival."

At about 0909 on 14 February, the Washington Volcanic Ash Advisory Center (VAAC) reported that a plume of ash extended SW of the volcano at an altitude of 4.3 km. The plume was 18-28 km wide. Later in the afternoon, the Washington VAAC reported an ash plume below an altitude of 2.7 km that extended SW of the volcano for about 460 km. The VAAC also forecasted that the plume would shift to a more westerly direction within the next 12 hours.

According to Ravelo's 15 February Saipan Tribune article, the CNMI Emergency Management Office and the U.S. Geological Survey "said in a joint report that the magnitude of the volcanic eruption declined during the past few days." During the eruption's peak on 26 January and 1 February 2005, however, the article stated that both agencies noted that the volcano sent ash to about 4.6-6.1 km altitude.

A message from Holliday filed at 0100 UTC on 17 February 2005 included a series of remarks, mainly from unnamed scientists on the scene in the field. As background prior to presenting those remarks, we note that the term 'RSAM' (real-time seismic amplitude) signifies estimates the average amplitude of ground shaking. RSAM values increase with increases in tremor amplitude or the rate of occurrence and size of earthquakes. The RSAM estimates the seismicity during intervals when many earthquakes might occur, times when rapid earthquake-magnitude assessments might become impractical. The remarks follow.

"Over the past 24 hours, the eruptive activity at Anatahan apparently continued to decline, with RSAM levels at the seismic station ANAT now only marginally above the levels recorded just before the 5 January eruption began. Microphone amplitudes have also dropped to similar levels.

"The 2003 crater floor is now essentially entirely covered by fresh lava [with] a diameter of about one kilometer. The current eruption peaked during the period between 26 January and 2 February [2005], during which the volcano sent ash as high as 15,000 to 20,000 feet a.s.l. [~ 5,000 to ~ 6,000 m] . . .. In the days following, ash blew as far as 100 nautical miles [185 km] and vog blew nearly 600 [nautical] miles [~ 1,100 km] downwind.

"The third historical eruption of Anatahan began on 5 January, after three days of precursory seismicity. On 6 January frequent strombolian explosion signals began and by the next day ash was rising to 10,000 feet [~ 3 km] and blowing 40 nautical miles [72 km] downwind. Bombs a meter in diameter were being thrown hundreds of feet in the air [ 1 foot = 0.305 m]. By January 20 explosions were occurring every 3 to 10 seconds and fresh ejecta and small lava flows had filled the innermost crater to nearly the level of the pre-2003 East Crater floor.

"The Emergency Management Office, Office of the Governor, CNMI, has placed Anatahan Island off limits until further notice and concludes that, although the volcano is not currently dangerous to most aircraft within the CNMI airspace, conditions may change rapidly, and aircraft should pass upwind of Anatahan or beyond 30 km downwind from the island and exercise due caution within 30 km of Anatahan."

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: Charles Holliday, U.S. Air Force Weather Agency (AFWA), Offutt Air Force Base, Nebraska 68113, USA; Randy White, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025-3591 USA; Katie Worth and Natalie Quinata, Guam Pacific Daily News, P.O. Box DN, Hagatna, Guam 96932, USA (URL: http://www.guampdn.com/); John Ravelo, Saipan Tribune (15 February 2005), PMB 34, Box 10001, Saipan, MP 96950, USA (URL: http://www.saipantribune.com/); Operational Significant Event Imagery (OSEI) team, World Weather Bldg., 5200 Auth Rd Rm 510 (E/SP 22), NOAA/NESDIS, Camp Springs, MD 20748, USA (URL: https://www.nnvl.noaa.gov/); 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/).