When is this thing going to blow?

That was the question on the minds of people calling the Alaska Volcano Observatory from Alaska and far beyond as Mount Redoubt continued simmering into its second week.

Well, according to the AVO, the answer is that Redoubt is on the brink of an explosive eruption that could shower Southcentral Alaska with several millimeters of ash. Or not.

Scientists with the U.S. Geological Survey said on Monday that activity has been "waxing and waning" at the volcano 106 miles southwest of Anchorage and that it remains more likely than not to erupt within days to weeks, although it could also cool down without erupting.

"There is very good evidence now that we have new magma involved in this process," said Tina Neal, a USGS geologist with the AVO. "Unfortunately, we just don't know enough yet to know exactly where this magma is and whether or not it will actually make its way out of the ground."

The uncertainty surrounding a volcanic eruption has prompted hundreds of people to call scientists to get the skinny and thousands more to flood the AVO's Web site -- intense traffic Friday caused the site to crash. But the No. 1 question is the one scientists can't answer for certain.

"Everybody wants to know when it's going to erupt," said Bill Lukas, spokesman for the USGS. "There's a pretty good likelihood that that's going to happen, but there's still a chance that it could not."

Steve McNutt, a volcanologist with the University of Alaska Fairbanks and coordinator at the AVO, said interest has been fairly high for an event that hasn't even happened yet; generally, things pick up after an eruption. McNutt's voice mail has been filling up with messages from people wanting information, he said.

"It comes in all flavors," McNutt said. "When something like this happens, you get this wide variety of stuff. You get a real clear sense of the incredible variety of views and educational backgrounds in the public."

One recent caller had a ticket to Hawaii and wanted to ensure the volcano wouldn't disrupt the flight, he said.

"They said please do everything I can to make sure the volcano doesn't erupt until after 8 o'clock," said McNutt, adding that the observatory tries to respond to all inquiries. "I gave it the attention it deserved."

The AVO has been fielding an average of about 10 calls an hour from people wanting the most current information about the volcano, the potential direction of ash flow and plumes, and associated hazards, said Chris Waythomas, a geologist with the USGS.

The intense concern over Redoubt likely comes from its last eruption, in 1989, in which an Anchorage-bound KLM Royal Dutch Airlines 747 with 231 passengers aboard flew through the Redoubt ash cloud and nearly crashed into the Talkeetna Mountains, he said.

"It was a near miss, so I think that may be one of the reasons that Redoubt is getting a lot of attention," Waythomas said. "Occasionally we get some goofy calls, but most of the time people are genuinely concerned and they want to verify with an expert what's going on.

"We try to explain that it's a difficult prediction to make."

Volcanoes in Alaska have a tendency, due to their viscous magma, to erupt explosively, and they do so fairly frequently. But to have significant ash cover Anchorage is more unusual; that last happened in 1992, when Mount Spurr, about 75 miles to the west, smothered the town.

Spurr simmered for some nine months before that main event, said John Power, a USGS geophysicist. Such a long gestation period is not unusual for volcanoes.

However, when Redoubt erupted in 1989 -- the only Redoubt eruption scientists have detailed data to base a prediction on -- the volcano trembled for less than 24 hours before blowing, he said. This time around, Redoubt is experiencing some of the same symptoms it did then, such as shallow earthquakes near its summit, he said.

"Certainly with volcanoes like Redoubt, we see increases in seismic activity that can go on for any number of (months). There's quite a bit of variability," Power said. But, "the character of these earthquakes is such that we might expect things to play out much faster."

Redoubt has recently been experiencing what scientists describe as "heightened unrest." Earthquakes have been occurring with unusual regularity, and two new holes about three-quarters up the 10,197-foot mountain have been blowing out large plumes of water vapor and volcanic gas.

Also, a "collapse pit" has formed in the ice of the summit crater because of heat from below, Neal said. A pair of muddy water flows caused by thermal activity have also begun draining down the mountainside, she said.

Scientists from the Lower 48 and Canada have beefed up the number of people monitoring the volcano's activity 24 hours a day and conducting flyovers to observe the action and measure gas levels. Officials also plan to install new monitoring equipment, including a pressure sensor to detect explosions, Neal said.

In response to the unrest, the Federal Aviation Administration has temporarily banned aircraft from flying under 60,000 feet for a 10-mile radius around Redoubt out of concern over a possible ash plume.

Neal said an eruption would likely bring at most several millimeters of ash fall to Southcentral.

"It really depends on how long the eruption goes on, how much ash is coming out of the ground, which is sometimes very hard to determine in real time, and then, of course, which way the winds are blowing and how fast," Neal said. "In 1989 and '90, there were more than 20 episodes of ash-producing events, so it could be that this is a prolonged event that goes on for many weeks or even months."

Find James Halpin online at adn.com/contact/jhalpin or call him at 257-4589.