A gradual increase in the number of long-period (LP) earthquakes and tremor began at Anatahan on 5 June. Both LP and tremor events peaked during 2230-0030 on 6 June. During the peak in activity, more than 350 LP events occurred. Also, tremor amplitudes briefly reached a new high for the current eruptive activity and an ash column reached ~7.9 km (25,900 feet) a.s.l. On 6 June, tremor amplitudes returned to low levels. During the rest of 1-7 June, ash plumes reached a maximum height of 4.3 km (14,100 ft.) a.s.l.

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.

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