The classic profile of Mt. Edgecumbe, a 3,200-foot stratovolcano, looms just 12 miles from downtown Sitka. Other craters of the associated volcanic field are visible to the right. Although the last major eruption was 4,500 years ago, Lingít oral tradition suggests “minor activity” as recently as 800 years ago. Alaska has 90 volcanoes in total, four of which are currently erupting. (USFS photo)

Seismologists have detected some unusual activity below the long-dormant Mt. Edgecumbe volcano near Sitka, whose last major eruption was 4,500 years ago.

The Alaska Volcano Observatory reports that a “swarm” of small earthquakes occurred somewhere deep below the iconic crater beginning on April 11 – but it’s too early to tell if it signals an eruption is on the way.

Mt. Edgecumbe is just 12 kilometers from the Queen Charlotte Fault, where the North American Plate and the Pacific Plate are slipping in opposite directions at the rate of about 2 inches per year. So there’s always a lot of ordinary background “tectonic activity” in the area. 

But the “swarm” on April 11 was out of the ordinary.

“What sort of makes this current bit of activity different is that there have been some larger earthquakes in sort of the magnitude 2 range that are locatable, but also many, many that are too small to be located,” said Dave Schneider, a geophysicist with the US Geological Survey’s Alaska Volcano Observatory in Anchorage. The “swarm” consisted of these larger magnitude 2 quakes accompanied by hundreds of smaller ones – all relatively shallow, at around 5 to 10 kilometers below sea level.

No April Fools this time: Sitkan Porky Bickar and some friends helicoptered a slingload of old tires into the Mt. Edgecumbe crater in the wee hours of April 1, 1974, and set them alight. The event is often hailed as one of the best April Fool’s pranks of all time.

The swarm didn’t knock anyone out of bed in Sitka, which is about 12 miles from the crater. But it was big enough for seismic stations in Sitka and elsewhere in Alaska to locate it accurately.

“A 2 is a good-sized rock breaking earthquake at a volcano, but also nothing that’s going to make your jaw drop and be really alarmed either,” Schneider said.

Schneider says the April 11 swarm has tapered off a bit, which is a good thing. But that doesn’t mean the event is over. He says that in the medical world, they call it “watchful waiting.”

“Seismic swarms of volcanoes can wax and wane,” he said. “I mean, they can start off with a bang and sort of fizzle out, they can sort of start with a with a whimper and increase, or they can sort of oscillate back and forth. And so, we’re just going to be in a period of just watching and sort of seeing what’s going on.”

“We’re a long way off from an imminent eruption, or an eruption at all,” said Jacyn Schmidt, the geoscience coordinator at the Sitka Sound Science Center. When she heard from a community member that there was a quake below Mt. Edgecumbe, she called the US Geological Survey and learned about the swarm, even before the Alaska Volcano Observatory issued its preliminary report.

“I had been talking to seismologists there who assured me that the earthquakes were very small,” she said, “which is true, but it’s unusual for them to be happening beneath Mt. Edgecumbe in the pattern that they’re seeing now.”

Schmidt considers Mt. Edgecumbe an exciting research opportunity. There are no concrete plans yet, but she hopes the Science Center can assist the USGS with local monitoring of the volcano. 

“There’s some talk of having equipment installed on Kruzof because right now, the equipment that they’re using to locate the earthquakes beneath Mt. Edgecumbe,” Schmidt said, “because those seismic instruments are in Sitka and in other places, but not right on the island itself.”

Dave Schneider shares Schmidt’s enthusiasm. There are 90 volcanoes in Alaska, four of which are currently erupting along the Aleutian chain. There’s even another seismic swarm occurring at the Davidoff volcano far out in the chain. That  Mt. Edgecumbe is showing signs of activity is not all that unusual, geologically speaking. The last major eruption was 4,500 years ago. He thinks there are a couple of ways this could go:

“The best case scenario for everyone – if you don’t like eruptions –  is that is that a dies out,” he said.

The other possibility is more appealing to fans of eruptions, and doesn’t necessarily mean disaster.

“The oral tradition is that 800 years ago, there was some activity, but it was minor,” Schneider said. “So if you look at the big scale of volcanic activity, minor activity is much more common than big activity.”

Schneider says that the Alaska Volcano Observatory will keep more than an eye on Mt. Edgecumbe. Satellite radar data collection is already underway to monitor the crater for deformation, in the event that magma or hydrothermal fluids cause the mountain to “inflate.” Other warning signs might be more earthquakes, larger earthquakes, or the emission of gasses. “That would help us home in perhaps on an eruption scenario,” said Schneider.