Report on Anatahan (United States) — 4 February-10 February 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report,
4 February-10 February 2004
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2004. Report on Anatahan (United States). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 4 February-10 February 2004. Smithsonian Institution and US Geological Survey.
16.35°N, 145.67°E; summit elev. 790 m
All times are local (unless otherwise noted)
After more than 5 months of very low-level seismic activity, long-period earthquakes began at Anatahan on 1 February with a maximum magnitude of 2. On 7 February just before 0600 seismicity peaked, with up to 15 events occurring per hour. It then decreased dramatically, but remained well above the levels during the previous few months. By 8 February the size of the volcanic earthquakes had diminished greatly, but their frequency had increased to as many as one every minute. The amplitude of the low-frequency tremor increased. During 9 and 10 February the volcanic earthquakes had become so small that they were essentially not recorded by the seismic network. The amplitude of the low-frequency tremor remained high, but decreased from its peak level during 5-7 February.
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.