May 10, 2003: Eruption begins
Anatahan began erupting suddenly from its east crater at about 5:00 p.m. on May 10. Within hours of the eruption's onset, a towering column of volcanic ash and gas rose to more than 10 km (33,000 feet) above sea level, and the prevailing wind blew the ash westward. An immediate concern that was felt far beyond the Western Pacific was the potential for the tiny abrasive ash fragments to damage aircraft passing nearby and downwind from the volcano. The first Volcanic Ash Advisory issued by the Washington Volcanic Ash Advisory Center reported volcanic ash at 35,000 ft. moving south at 35 knots and at 15,000 ft. moving west at 10-15 knots.
The single EMO seismic station on the island was not working at the time, but a broadband seismic instrument installed 6.5 km west of Anatahan crater on May 6 by scientists from Washington University recorded significant earthquake activity in the hours before the eruption began; the instrument was one of many installed to conduct a seismic experiment along the Mariana Trench. A preliminary review of the data after it was retrieved in late May clearly shows there was a rapid increase in the number of small-magnitude earthquakes (probably less than M 2) to more than 100 per hour beneath the volcano within a few hours of the eruption onset.
May 11-June 3, 2003: Continuous explosive activity excavates east crater
A smaller but nearly continuous eruption column rose from the east crater of Anatahan for the next several weeks. The resulting eruption clouds were generally below about 6 km (20,000 ft). On May 23 and 24, typhoon Chan-hom shifted the prevailing east winds to the south, blowing the eruption column toward Saipan and Guam. Light ash fall resulted in flight cancellations at the Saipan and Guam international airports. Residents of Saipan reported a rotten egg smell associated with the ash fall.
The ongoing explosive activity excavated a deep crater within Anatahan's east crater. Scientists estimated the inner crater was nearly at sea level by about May 20; before the eruption, the floor of east crater was 68 m above sea level.
June 4, 2003: Lava flow visible in east crater
The spiny surface of a lava flow was first observed in the inner crater on June 4. The flow appeared to form a mound-shaped lava dome, but its volume is unknown. Scientists also noted the presence of new fault scarps and slump features within east crater, as well as additional faulting west of the east crater. Such features commonly develop around active vents due to the rise and subsequent eruption of magma.
On June 5 the EMO seismic station was repaired and ash samples were collected from the site. Through June 12, the seismic records showed only continuous ground shaking to varying degrees. The most intense periods of tremor (ground shaking) lasted 3 to 10 hours and occurred about every 24-36 hours. On June 12, three long-period earthquakes were recorded, the largest with a magnitude of about 2. Other earthquakes followed on the late afternoon and early evening of June 13.
June 14-16, 2003: Energetic explosions destroy part of lava flow
Two strong explosions on June 14 removed much of the small new dome in the inner crater. Just before noon on June 14, earthquakes began to occur at intervals of 1-2 minutes. For the next two days, several episodes of intense tremor and earthquakes lasting about 1.5 hours occurred about every 12 hours. These episodes of increased seismic activity accompanied strong ash emissions from the east crater, with eruption columns higher than 2 km (8,000 ft.). Quiet intervals in which the eruption column consisted of little ash were accompanied by continuous low tremor.
June 17, 2003 to present: Low-level activity continues
Since June 16, 2003 the seismic activity of Anatahan has consisted only of low-level tremor, and even that low level is gradually shrinking.
The future course of the eruption is uncertain. It could continue at its present, relatively low level for weeks or months, perhaps punctuated by explosive activity that could destroy more of the lava dome and generate eruption columns higher than 6 km (20,000 ft). Depending on prevailing winds, such explosions could result in ash fall on nearby islands. A higher level of activity could result in continued growth of the lava dome, more explosions that destroy part of it, and attendant greater volcanic hazards on Anatahan Island.
For the present activity, which consists of sporadic extrusion of lava in the east crater and a nearly continuous eruption column punctuated by stronger explosive activity, the potential hazards are limited to Anatahan Island and to aircraft passing downwind of Anatahan below about 8 km (25,000 ft).
Larger explosive eruption(s) (VEI 4 to 5) accompanied by a large eruptive column and pyroclastic flows are possible from Anatahan. Such activity would result in far-reaching eruption clouds and ash fall. There would be a small chance of a local tsunami on Anatahan from pyroclastic flows entering the water or from flank landslides into the sea.
An unlikely scenario is a massive explosive eruption (VEI 6 or higher) that forms an even larger caldera than currently exists (for example, the 1991 eruption of Mount Pinatubo in the Philippines) or a large landslide (for example, the 1980 landslide at Mount St. Helens, USA). This activity would represent a major threat to aviation in case of a massive explosive eruption, and tsunami generation in either case with effects likely to be felt throughout the Mariana Islands. We emphasize the unlikely nature of such events.
Anatahan Volcano is a small volcanic island located 120 km (80 miles) north of Saipan Island and 320 km (200 miles) north of Guam. The island is about 9 km (5.6 miles) long and 3 km (2 miles) wide. Anatahan is a stratovolcano that contains the largest known caldera in the Northern Mariana Islands. In 1990, when geologists from the USGS Hawaiian Volcano Observatory (HVO), EMO, and the University of Hawai`i examined the rock layers of Anatahan, they discovered abundant evidence of ancient explosive eruptions that sent fast-moving flows of hot ash and rocks across the island.
In the 1980s and 1990s, HVO scientists assisted the EMO in installing one or two seismometers on several of the volcanoes of the Northern Mariana Islands, including Anatahan, but the single station on the volcano was not operating before and during the initial phase of the eruption (the station became operational on June 6). HVO scientists also installed and surveyed benchmarks on several of the volcanic islands to track ground swelling related to the rise of magma beneath the volcanoes.
The present activity was preceded by two known swarms of earthquakes in 1990 and 1993. These swarms likely accompanied the emplacement of magma high into the volcano.
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U.S. Department of the Interior, U.S. Geological Survey, Reston, Virginia, USA