Anatahan volcano starts to wake up

After a temporarily lull early this month, Anatahan’s volcano is again showing signs of activity, with long-period earthquakes occurring, albeit sparsely.

The volcanic earthquakes indicate magmatic movement that may lead to another eruption.

The Emergency Management Office and the U.S. Geological Survey said in a recent report, however, that tremor levels have been low, a few percent of peak levels recorded in June.

While the volcano emitted no significant ash this month, there were at least three periods of elevated tremors on Sept. 15, 16 and 19.

Both agencies also said that a 4.7-intensity earthquake occurred on Sept. 18, at about 1:22am. They located the quake’s epicenter at approximately 225 kilometers south-southwest of Guam.

EMO geophysical instrument specialist Ramon Chong, who went to Anatahan recently to revive the seismic station on the island, reported the presence of a small, grey plume moving northwesterly. But the EMO team reported no volcanic explosion. The seismic stations on Anatahan and Sarigan are currently functional.

“The surface is a uniform wet grey with no evidence of recent dry deposits that might have indicated surges. They [Chong’s team] collected some red, low-density rocks from the surface. Photos show that east the crater is quite deep, with discolored water in the bottom,” the agencies reported.

The agencies noted no volcanic activity on Anatahan beginning Sept. 3 after about eight months of continuous eruptive activity. The situation lasted only several days, but activity still remained at low levels. No ash emissions are currently visible through satellite monitoring by the Air Force Weather Agency and the Washington Volcanic Ash Advisory Center.

The USGS and the EMO reported no volcanic activity on Anatahan for the first time since eruptions began to escalate in January this year. Since January, tremor levels on Anatahan continued but with fluctuating magnitudes. Occasionally, strombolian explosions occurred.

The volcano had its strongest historical eruption on April 6, with the volcano spewing ash to 50,000 feet. The USGS and the EMO estimated the volume of ash emissions in that eruption at about 50 million cubic meters.

The volcano first erupted on May 10, 2003 after centuries of dormancy, with ash plume rising to an altitude of over 30,000 feet and covering over 1 million square kilometers of airspace above the Pacific Ocean. That eruption, which ceased by mid-June that year, deposited about 10 million cubic meters of material over Anatahan island and the sea.

The second batch of eruptions began about April 9, 2004, after more than a week of increasing seismicity. The second eruption consisted of passive extrusion during mid-April, which later became strombolian explosions every minute or two on April 24. The explosions continued every minute or two through mid-July, often sending a thin plume of gas and ash upward a few thousand feet and 100 km downwind.

According to the agencies, the second eruption essentially ended on July 26, although visitors to the island three months later could still see very small amounts of steam and ash rising 100-200 ft above the crater rim and could smell sulfur dioxide near the crater.

Anatahan remains off-limits to the public, except for government and approved scientific missions, pursuant to a continuing emergency declaration by Gov. Juan N. Babauta.