October 6, 2005
By DOUG O'HARRA
Anchorage Daily News
Photo: Small earthquakes have begun rumbling under Tanaga, pictured from the south along with East Tanaga in August 2003. (Photo by M.L. COOMBS / U.S. Geological Survey)
A sleepy volcano in the western Aleutian Islands began stirring this month, trembling with tiny earthquakes six to 12 miles underground, according to the Alaska Volcano Observatory.
The swarm beneath 5,925-foot Tanaga marks the first sign of unrest since the observatory wired the rugged cone with its own network of sensors two years ago, said volcanologist Rick Wessels of the U.S. Geological Survey. The volcano was last known to erupt in 1914.
Like other Aleutian Arc volcanoes, Tanaga gapes beneath one of the world's busiest airline routes, with dozens of flights jetting between North America and Asia there every day. Volcanic ash blasted five to six miles into the sky can damage or shut down jet engines, so the observatory listens and watches for eruptions around the clock.
Most Aleutian volcanoes produce tiny quakes every day, but Tanaga had been remarkably quiet for reasons that remain unclear, Wessels said.
"It had one reasonably measurable event every month or so, and now it's gone to several per hour," he said.
Tanaga rises steeply on its own uninhabited island, 63 miles from the nearest community in Adak and more than 1,200 miles southwest of Anchorage. It's one of 28 volcanoes monitored by the observatory for seismic action, hot spots and smoky plumes -- including the 11,070-foot Mount Spurr that looms on the horizon 80 miles due west of Anchorage.
Spurr, which last dusted Anchorage with ash during its 1992 eruption, continued to gurgle with its own quake swarm this week and remained under a restless "yellow" alert.
"It had some nice little seismic events going on this morning, at least a half dozen measurable ones," Wessels said Thursday.
Meanwhile, Tanaga, far to the west, began rumbling late Oct. 1 and has since produced 15 to 68 tiny earthquakes every day. Centered about a mile and half northeast of the summit, the quakes ranged from magnitude .5 to 1.7, far too small to be felt on the surface.
This unrest doesn't necessarily mean Tanaga will erupt anytime soon, and the volcano's alert was not raised from the dormant "green" status, Wessels said.
"We put out a release so that everybody knows that this volcano is doing something neat and interesting," he said.
The last known eruption of Tanaga occurred in 1914, when lava was seen flowing down its steep slopes. Smoke was reported from the summit in 1829, 1791 and 1763-70. But the volcano is so little seen that no one really knows its habits.
The new quakes aren't the kind of tremors produced when molten rock begins oozing toward the surface, Wessels said. A more likely cause may be hot gas shattering rocks under immense pressure far underground.
They could also be regular old earthquakes, related to the Pacific tectonic plate moving along the Aleutian Arc.
"Sometimes these volcanoes have to adjust themselves to the stress," Wessels said.
Daily News reporter Doug O'Harra can be reached at do'harra@adn.com.
For more about the Tanaga volcano:
www.avo.alaska.edu/volcanoes/volcinfo.php
http://www.adn.com/news/alaska/story/7055841p-6959982c.html