Dozens of noticeable aftershocks above magnitude 4.0 are expected in the remote Aleutian Island region off Alaska in the days and weeks following a major 7.0 earthquake, the state seismologist said.
A dozen measurable aftershocks have already hit the region since Friday's quake, including one reaching magnitude 6.1 in strength, said seismologist Michael West. There have been more than 30 aftershocks measuring at least magnitude 2.5.
None of the aftershocks are expected to cause a notable tsunami, since the initial quake did not cause one. And Mr West said experts were not too worried this quake would trigger another significant one nearby in the near future.
"This is a very common area for earthquakes," he said. Temblors above magnitude 5.0 are felt every month.
The site of Friday's quake is quite active. Significant quakes were felt just to the east and the west of the earthquake in 1986, 1996 and 2003.
"This was exactly the earthquake that's supposed to happen," Mr West said, noting that it was part of a pattern, when examined in a scientific way.
The Pacific tectonic plate is always pushing under America. It builds up stress and then earthquakes happen. Of course, Mr West notes, he has be cautious about saying something will never happen, but he's not particularly concerned.
There have been no reports of damage or injuries from the earthquake, which was strongly felt in Atka, an Aleut community of 64 people, and the larger Aleutian town of Adak, where 320 people live.
The earthquake and the aftershocks did not trigger any tsunami warnings, but Michael Burgy with the West Coast and Alaska Tsunami Warning Centre in Palmer, Alaska, said the centre is monitoring for potential tsunamis caused by landslides, either on land or under water.
The Alaska Earthquake Information Centre said the primary earthquake was centred 67 miles south west of Adak, about 1,200 miles south west of Anchorage. Shaking lasted up to one minute. The 6.1 aftershock struck in the same general area at 10.39pm on Friday. The 7.0 quake occurred offshore in the subduction zone where plates of the Earth's crust grind and dive. By contrast, California's most famous fault line, the San Andreas, is a strike-slip fault. Quakes along strike-slip faults tend to move horizontally.