According to the Air Force Weather Agency (AFWA), on 19 March a hot spot at Anatahan was visible on satellite imagery and vog (volcanic fog) extended 200 km from the island. On 24 March around 1330, seismicity at Anatahan abruptly increased to about twice the background level. The seismicity consisted of low-amplitude tremor and small long-period earthquakes, similar to the seismicity on 17 and 18 March. On the 24th, vog from Anatahan was visible on satellite imagery extending W, then curling N. The plume was estimated to be below 1.2 km (4,000 ft) a.s.l. and no ash or hot spots were visible. Anatahan remained at Alert-level Advisory; Aviation Color Code Yellow (Volcanic activity has increased somewhat, but remains fairly low and is being closely monitored).

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)