Increased seismic activity at Mount Iliamna during early spring prompted state scientists to elevate the volcano’s alert level to yellow; two levels below the alert system’s highest stage, red.
The Alaska Volcano Observatory experienced network problems in May, which reduced its ability to gauge Iliamna’s activity. Stations near the volcano went offline, but large earthquakes occurring underneath Iliamna were still detected at far-off stations, and scientists kept the elevated alert level in place, said Jeff Freymueller, coordinating scientist at the Geophysical Institute of the University of Alaska Fairbanks.
“Every time we think about taking the alert level down, we’ll see one of these earthquakes that are big enough to see somewhere else,” Freymueller said. “The fact is, the volcano doesn’t normally do that.”
Since March 9, the Volcano Observatory has recorded increased seismic activity at Iliamna, albeit with a crippled network. It expects to lower the alert level; the volcano’s unrest persists, however. Mount Redoubt, another volcano across Cook Inlet and north of Iliamna, has shown little activity for about three years.
A seismic swarm prompted the yellow alert level in March. A seismic swarm consists of many earthquakes occurring in an area during a short period of time, with a large earthquake occurring anywhere within that time. It differs from the classic big quake followed by aftershocks and follows routine time decay — fewer earthquakes as time passes.
The last time a seismic swarm occurred at Iliamna was 1996 through 1997. The swarm was not followed by an eruption, according to the Volcano Observatory’s website.
Before the volcano’s seismic stations failed, 10 to 15 earthquakes were occurring weekly. It is not uncommon for Iliamna to experience no earthquakes for one or two weeks, Freymueller said. Last year, a total of 569 earthquakes shook the volcano.
The seismic swarms of the mid-1990s decreased with time, he said, and the unrest that started in May showed similar regression.
When the seismic stations went offline tracking the volcano’s progress became difficult. Crews waited until early September to visit the field — it was the earliest they could do so. The stations are snowed-in through August. Additional problems with helicopters pushed repairs back another month.
Crews visited the stations during the past two weeks, and the seismic network is partially repaired, Freymueller said.
Earthquakes of magnitude two or greater kept the yellow advisory in place while the network was down. Also, the observatory conducted a gas-measure flight — the monitoring flyovers are expensive and occur less frequently at coast volcanoes.
During the flight, scientists detected elevated levels of sulfur dioxide and other gasses. Similar gas fluxes were measured during a previous flight in June.
“When gas emission rates are high there is new magma that’s come up below the volcano,” Freymueller said. “The new magma carries dissolved water, CO2 and other gasses. Those gasses start to come out of the molten rock and are eventually released into the atmosphere.”
Both seismic swarms and gas fluxes are generally caused by magma that intrudes underneath volcanoes. The intrusion causes a lot of stress within and beneath the volcano, he said.
“This activity is way above background level, but well below what has been historically observed at Alaska volcanoes and really getting ready to erupt,” he said.
According to a seismic activity timeline on the Volcano Observatory’s website, “questionable eruptions” have occurred at Iliamna 13 times since 1741, the last of which occurred in 1953.
George L. Snyder wrote in a 1953 geological survey published by the United States Department of the Interior that two distinct sources of eruption are claimed to have been sighted, one on top and one off the side of the volcano.
The Volcano Observatory has recorded no unrest at nearby Redoubt for about three years. The volcano’s activity is cyclical, Freymueller said, as eruptions are followed by periods of calm.
Redoubt erupted in 1902, 1966, 1989 and 2009, according to the website’s timeline. The eruptions of ’89 and ’09 sent out plumes of ash that spread over Cook Inlet communities and closed local airports, and authorities encouraged people to stay inside.
“When (Redoubt) does something it’s pretty disruptive,” Freymueller said, “because of the proximity to Anchorage and Kenai. We’re happy that it’s been calm.”
It would not be surprising if Redoubt remained calm for another decade, he added.
Jerzy Shedlock can be reached at firstname.lastname@example.org.