The magnitude-6.3 earthquake felt across Southcentral Alaska and as far north as Fairbanks late Tuesday was unlike any to hit the Cook Inlet region in decades, the Alaska Earthquake Center said Wednesday.
As of Wednesday morning, about a dozen aftershocks had been recorded from the earthquake, which originated at a depth of 73 miles, said seismologist Natalia Ruppert.
That's a normal number for a "deeper event," Ruppert said, as compared to the magnitude-6.9 earthquake in the Fox Islands on Sunday, which originated at a depth of 38 miles and generated "hundreds of aftershocks."
The aftershocks from Cook Inlet's Tuesday quake have been relatively small -- the largest was magnitude 3, Ruppert said. But the quake was unusual given its magnitude and depth, she said.
"We looked up some of the history in this region, and the last time we had a similar earthquake in Cook Inlet, it was … in 1988," Ruppert said. Tuesday's quake was "the largest since that 1988 (quake) in the deeper portion of the seismic region," Ruppert said.
While the earthquake originated roughly 9 miles southwest of the Mount Iliamna volcano and 43 miles southwest of Mount Redoubt, the tremor was not related to volcanic activity, according to the Alaska Volcano Observatory.
"It's pretty clear that this was a tectonic earthquake not related to the volcanoes," said David Fee, acting coordinating scientist with the agency.
Among distinguishing factors, the depth of the earthquake and relatively large magnitude helped determine that it was tectonic. Most volcanic earthquakes in Alaska register at magnitude 1 or 2, Fee said.
"Volcanically, this is below us," Fee said, so the agency wasn't too involved in monitoring the seismic event.