ANCHORAGE - The last time Mount Spurr erupted, it covered Anchorage in a cloud of ash that drove residents indoors, shut down the airports and left the city with volcanic soot a quarter of an inch thick. The ash spewed 50,000 feet in the air and drifted on air currents as far away as Greenland.
Twelve years later, the volcano is again restless. Fifteen to 25 small earthquakes a day have been rumbling beneath Mount Spurr for the past month. Last week, the Alaska Volcano Observatory raised its level of concern from green (dormant) to yellow (restless).
On Tuesday, scientists found a giant hole, 165 feet in diameter and 82 feet deep, in Mount Spurr's ice cap that may have been formed by the heat under the surface of the volcano. Such a hole has not been seen on the mountain in memory, said John Power, a seismologist with the Alaska Volcano Observatory.
But this volcanic activity is different from that which preceded the three eruptions in 1992 and scientists say there is no imminent threat of the volcano erupting.
"It is unusual, that is why we raised the color of the level of concern," Power said Wednesday. "The most likely scenario is that this is going to die off with no eruptive activity."
But, he said, "most eruptions are preceded by earthquake swarms, and there is a possibility that this could lead to some sort of eruptive activity."
In 1992, earthquake swarms under the 11,100-foot Mount Spurr and nearby Crater Peak led to eruptions in June, August and September of that year. Before then, the volcano had been dormant since 1953.
The eruption in August lasted four hours and was the worst to hit Anchorage, 80 miles to the east across Cook Inlet. The ash cloud drove people with respiratory problems indoors and closed the Anchorage airport for 20 hours. The region's other air fields - Merrill Field, Lake Hood, Elmendorf Air Force Base and Kenai Municipal Airport - also were closed.
Afterward, the rumblings tapered off until Mount Spurr was quiet again in 1997, Power said.
Nearby state and local agencies have begun preparing for Mount Spurr's reawakening. Dennis Brodigan, director of emergency services for the Matanuska-Susitna Borough north of Anchorage and northeast of Mount Spurr, said Wednesday the borough was shipping 100 air filter masks to the residents of Skwentna. The village of 94 is one of the nearest communities to the volcano, just 60 miles away.
No roads lead to Skwentna, and emergency crews could have problems reaching the village if the volcano erupted, he said.
"This has happened several times in the past in Skwentna," Brodigan said. "Their biggest concern is shutting down their generators" due to an ash cloud.
The borough also is stocking up on air filters for its vehicles and protective equipment for emergency personnel.
The state Division of Homeland Security and Emergency Management on Wednesday was updating its Web site with information for residents on how to prepare their homes and cars for an ash cloud, said homeland security spokesman Jamie Littrell. If an eruption occurred, Littrell said, the homeland security office would contact the Alaska Volcano Observatory, alert the governor in Juneau and coordinate a response with other state and local agencies.