A moderate earthquake shook northwest Alaska on Monday, the fifth temblor of the same magnitude since April in an area with otherwise little activity, seismologists said.
The magnitude-5.7 quake struck at 4:01 a.m. Monday northeast of the village of Noatak, the Alaska Earthquake Center reported. As with other temblors in the earthquake swarm, the quake was felt in Noatak, an Inupiat Eskimo community of 560 people.
"It woke me up," said resident Alvin Ashby. "Some people slept through it."
People there aren't used to earthquakes, and these have some residents worried, said Ashby, who has lived in the community most of his life.
Before the swarm that began April 18, the last known quake of similar size in the area was a magnitude-5.5 quake that occurred in 1981, earthquake center seismologist Natasha Ruppert said.
The swarm of magnitude-5.7 quakes is connected to more than 300 smaller aftershocks, some with magnitudes in the high 3s, Ruppert said. The first quake and the others of that magnitude have been located in roughly the same area about 20 miles from Noatak. The community is 70 miles north of the Arctic Circle.
Monday's quake was preceded by one minute by a magnitude-4.2 foreshock and followed by at least 10 aftershocks.
Minor damage from the earthquakes includes cracks in walls in Noatak, but no injuries have been reported.
Ruppert said old seismic faults have been mapped in the area, but there are no known active faults, although that doesn't mean there aren't any. She said sometimes surface fault scars could be hidden by vegetation or glacial deposits.
"Obviously, since we are having all this earthquake activity, there must be some active faults in the area," Ruppert said.
Since the first of the swarms occurred, seismic sensors have been installed in Noatak and the regional hub town of Kotzebue, 55 miles south. Ruppert said the sensors help gather more accurate information about theearthquakes.
The series of earthquakes has scientists puzzled about activity that is considered very unusual in the area.
"At this point, we don't really understand the nature of these earthquakes," Ruppert said.
Ruppert said she doesn't believe there is any connection between the quakes and the Red Dog Mine, located about 35 miles north of Noatak.
"All mining activities are very near the surface," Ruppert said. "And all the earthquakes are miles below the surface."