Two significant earthquakes that shook Alaska's remote Aleutian Islands were followed Friday morning by two smaller quakes, but there were no reports of damage or injuries.
The temblors probably were aftershocks of a magnitude-7.2 quake that hit the seismically active but sparsely populated island chain Tuesday along a dynamic fault line, said geophysicist Guy Urban of the West Coast and Alaska Tsunami Warning Center.
"Maybe things are still moving around out there," he said. "The area is still settling."
The magnitude-5.9 and 6.1 quakes struck about one minute apart Thursday. They were centered about 100 miles west of Adak in the island chain, according to a preliminary report by the U.S. Geological Survey.
"There were two tonight and that was very unusual," said geophysicist Paul Huang of the tsunami warning center. "We're going to study this a little more closely." No tsunami warning was expected, he and Urban said.
Magnitude 4.8 and 4.6 quakes struck 14 minutes apart Friday morning, Urban said. He said there have been no reports that any of the quakes this week were felt by people there.
The Aleutian Islands are a chain of more than 300 islands that extend southwestward from mainland Alaska into the northern Pacific Ocean.
The earth's most active seismic feature, the circum-Pacific seismic belt, brushes Alaska and the Aleutian Islands, where more earthquakes occur than in the other 49 U.S. states combined.