The Icelandic Meteorological Office has warned airlines of heightening seismic activity near the Bardarbunga volcano, which lies under the Vatnajokull glacier in southeast Iceland.
On Monday, the office, which is responsible for monitoring natural hazards, raised the volcano's status level from green, which indicates no eruptive activity, to orange, which indicates a heightened and escalating unrest in the volcano with a possible chance of eruption.
(Orange is the fourth of a five-level system used by the International Civil Aviation Organization to warn airlines of any seismic activity.)
Beginning Aug. 16, small continuous earthquakes were measured in two "swarms," one east of the volcano's caldera and another at the glacier's northeast border, a statement from the office said. Early Monday morning, an eruption in the northeast reached a 4.5 magnitude level, the strongest in the area since 1996, and prompted the office to raise the volcano's status and issue the warning.
"As evidence of magma movement shallower than 10 kilometers (6 miles) implies increased potential of a volcanic eruption, the Bardarbunga aviation color code has been changed to orange," the statement said. "Presently there are no signs of eruption, but it cannot be excluded that the current activity will result in an explosive subglacial eruption, leading to an outburst flood (jokulhlaup) and ash emission."
Earthquakes reaching a magnitude of 3 or less continued throughout the week. On Wednesday, an intrusion about 15 miles long had formed about three miles beneath the nearby Dyngjujokull Glacier, and Iceland's National Crisis Coordination Center had been activated to prepare for a possible eruption.
On Thursday a joint daily status report from the meteorological office and the University of Iceland's Institute of Earth Sciences said that measurements to date did not suggest that an eruption was imminent.
"Previous intrusion events in Iceland have lasted for several days or weeks, often not resulting in an eruption," the report said. "However an eruption of Bardarbunga cannot presently be excluded, hence the intense monitoring and preparation efforts."
Friday's status report noted a 4.7 magnitude earthquake that was measured in the Bardarbunga caldera Thursday night and said there was no sign of the seismic activity decreasing.
Icelandair does not anticipate any flight disruptions at this time, Michael Raucheisen, a marketing and communications coordinator for the airline, said in an email on Wednesday.
"We are closely monitoring the situation and will advise of any schedule changes should they occur," Mr. Raucheisen said. "Earthquakes and volcanic eruptions are commonplace in our network hub of Iceland, but in case an event does occur, contingency plans are in place to deal with any potential outcome."
In 2010, ash and debris from Eyjafjallajokull in Iceland disrupted flights in Europe for almost a week, stranding passengers and causing billions of dollars in loss for airlines.
But while an eruption at Bardarbunga would also have the potential to disrupt flights, the biggest issue would be flooding.
"The risk of any disruptive ash cloud similar to the one in 2010 would depend on how high any ash would be thrown, how much there would be and how fine-grained it would be," Martin Hensch, a seismologist at the Met Office, told The Guardian newspaper. But the biggest problem for Iceland, he said, would be flooding caused by an eruption under the glacier.
"We've known for some time that Bardarbunga was going to do something – we just didn't know what," Dave McGarvie, a volcanologist at the Open University in Edinburgh, Scotland, said in a statement. "The good news for air travel is that both these clusters are away from the heart of the main volcano, as it's in the heart that the kind of magma is produced which leads to highly explosive eruptions that produce the abundant fine ash capable of being transported long distances through the atmosphere."
Bardarbunga's last eruption was in 1797.