The USGS reported that elevated seismicity at Anatahan continued during 12-13 March, then dropped to near background levels on 14 and 15 March. Based on observations of satellite imagery, the Washington VAAC reported that an ash plume was spotted around the summit area on 13 March and later that day drifted W and NNE. On 15 March, a possible diffuse ash plume was visible on satellite imagery drifting W. During 16-18 March seismicity remained low but was punctuated by occasional short bursts (typically about a minute in duration) of increased tremor. 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)