A small airplane banks for Merrill Field as a plume of ash and steam from Mount Spurr dominates the Anchorage, Alaska, skyline on, Oct. 2, 1992. (Paul Souders/Anchorage Daily News via AP)

ANCHORAGE, Alaska (AP) — An increase in the number of earthquakes under a volcano near Alaska’s largest city this year has geologists paying attention.

Mount Spurr, about 80 miles (129 kilometers) northwest of Anchorage, last erupted in 1992, spewing an ash cloud nearly 12 miles (19 kilometers) into the air, prompting flights to be canceled and people to don masks. Another eruption at the 11,100-foot (3,383-meter) stratovolcano could be severely disruptive to the city, according to the Alaska Volcano Observatory.

The observatory raised its alert status for Mount Spurr in October — from green to yellow — when the increase in seismic activity became pronounced and a ground deformation was spotted in satellite data. Observatory scientist David Fee said Friday there have been about 1,500 small earthquake below the volcano this year, compared to about 100 in a normal year.

While that might seem like a lot, it’s "not an enormous amount,” Fee said. It could be a precursor to an eruption — or not. Similar seismic unrest occurred from 2004 to 2006 before subsiding without an eruption.

“We don’t see any significant change in our data that would tell us that an eruption is imminent,” Fee said. “Things have been kind of this low-level unrest for a while now and we’re, of course, watching it very closely to detect any changes and what that might mean.”

Scientists are monitoring seismic stations, global satellite data and a webcam for additional changes that would signal an impending eruption. If magma is moving closer to the surface, there would be an increase in earthquakes, ground deformations, the creation of a summit lake or fumaroles, which are vents that open in the surface to vent gas and vapors.

The volcano last erupted in 1992 from the Crater Peak flank vent, located about 2 miles (3 kilometers) south of the summit. The eruption dropped about a quarter-inch of ash in Anchorage that year, prompting residents to stay inside or to go out donning masks, and the cloud drifted as far as Greenland.

A similar eruption from the same vent happened in 1953. The last known eruption from the summit was more than 5,000 years ago.

Volcanic ash is angular and sharp and has been used as an industrial abrasive. The powdered rock can cause a jet engine to shut down, which prompted Anchorage and other nearby airports to close during the 1992 eruption.

Closing airports is always an inconvenience in a state with few roads, but can be more than just inconvenient. Business would also be impacted since the Anchorage airport today is among the world’s busiest cargo hubs with Memphis, Hong Kong and Shanghai, mainly because of Alaska's proximity to Asia.

Mount Spurr, located on the Volcanic Ring of Fire, is one of 53 volcanoes in Alaska that have been active within the last 250 years.

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