Early on 28 April, the level of seismicity at Anatahan increased to its highest level since renewed seismic activity began on 24 April. At this time the lava dome probably increased in size. Seismicity decreased somewhat during the following ~5 days, but M 2-2.5 earthquakes and small explosions still occurred about every minute, and steam-and-ash emissions still rose several hundred meters above the volcano. Local authorities placed Anatahan Island off-limits until further notice. They also concluded that although the volcano was not currently dangerous to most aircraft within the Mariana Islands airspace, conditions could change rapidly and aircraft should pass upwind of Anatahan or more than 30 km downwind from the island and exercise due caution within 30-50 km of the volcano.

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: Associated Press, Emergency Management Office of the Commonwealth of the Mariana Islands and United States Geological Survey Volcano Hazards Program