There were no immediate reports of injuries or damage.
The U.S. Geological Survey says the quake occurred at 5:21 a.m. Thursday and was centered 10.5 miles (17 kilometers) northwest of Anchorage.
The Alaska Earthquake Center tweets that the aftershock was closer to Anchorage than most others have been, which may explain why some people thought it was stronger than it actually was.
On Nov. 30, a magnitude 7.0 earthquake struck 7 miles (11.2 kilometers) north of Anchorage, damaging roads and structures Thousands of aftershocks have followed.
The Anchorage Daily News reports preliminary assessments have found the big quake cause $76 million in damage.