During January and February, thin gas plumes from Anatahan were occasionally visible on satellite imagery, but became continuous and slightly more dense during 26 February to 19 March. On 17 March around 2200, seismicity abruptly increased by a factor of nearly two and continued at that level for 2 hours. On the 18th around 1400, seismicity again abruptly increased by a factor of nearly two and continued at that level for about 8 hours before returning to the baseline level prior to 17 March. The increased seismicity consisted of small (M 0-1) long-period earthquakes occurring approximately every minute or so, sometimes reaching two per minute. A total of about 600 such events were detected during 17 and 18 March. Volcanic Ash Advisories were issued by the Washington VAAC, but were cancelled when the plume was determined to contain gas and only insignificant amounts of ash. The Alert Level was raised from Normal; Aviation Color Code Green, to Advisory; Aviation Color Code Yellow around 20 March.

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)