The USGS reported that emissions of sulfur dioxide from Anatahan were detected by the satellite-based Ozone Monitoring Instrument (OMI) during 20-25 February. Seismicity was elevated during 20-26 February, though levels varied greatly during 23-25 February. The Washington VAAC reported that plumes possibly containing some ash were visible on satellite imagery drifting WSW, SW, E, W, and SE during 20 and 23-24 February. A Volcanic Haze Advisory was issued by the Emergency Management Office (EMO) on 25 February for Tinian, Saipan, and Rota because of elevated sulfur dioxide levels. This advisory was cancelled on 26 February. The Volcanic Alert Level remained at Watch and the Aviation Color Code remained at Orange.

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.

Sources: Emergency Management Office of the Commonwealth of the Mariana Islands and United States Geological Survey Volcano Hazards Program, Washington Volcanic Ash Advisory Center (VAAC), Saipan Tribune