ANCHORAGE, Alaska — An earthquake and an aftershock that followed Sunday were the two largest recorded temblors to rock Alaska’s North Slope, according to quake experts.
A magnitude 6.4 earthquake shook the ground at 6:58 a.m. Sunday on the northern side the Brook Range, the mountains that run west to east from northern Alaska to Yukon Territory. The epicenter was 343 miles (551 kilometers) northeast of Fairbanks.
The previous most powerful recorded quake, going back “many decades,” was a magnitude 5.2 quake in 1995, state seismologist Michael West said. Sixty times more energy was released Sunday, West said.
Six hours later after the main quake, seismometers recorded a magnitude 6.0 quake, West said. It was one of a few dozen aftershocks measured at magnitude 4.0 or greater in the 24 hours after the initial shallow, strike-slip quake, which occur when blocks in a vertical fault line move horizontally.
“We often see vigorous aftershocks in that kind of quake. But it is prodigious, without question,” West said.
The big aftershock was about 6 miles (10 kilometers) away and possibly on the same fault line.
“We expect to have that in the next couple of days,” West said. “That’s where the aftershocks are tremendously valuable. The aftershocks paint in the fault line.”
The timing of the event was fortuitous. Alaska is hosting USArray, part of the National Science Foundation’s EarthScope experiment, a 15-year program to place a dense network of permanent and portable seismographs across the nation. About 260 seismographs were planned for Alaska.
The goal is an explanation of how continents formed and where dangerous earthquakes may occur in the future.
“They were a complete and utter game-changer in this event,” West said. “Of the dozen closest seismic stations, 10 of them were part of this temporary USArray project.”
A remaining question is whether the quake ripped open the surface.
“We are hopeful within the next few days that we will have satellite images that might tell us something about whether it ruptured the surface, and exactly where,” West said.
Alaska’s North Slope is the source for most oil extracted and exported from the state. The quake was 85 miles (137 kilometers) southeast of Deadhorse, where workers fly in for work at oil fields.
The University of Alaska Fairbanks’ Alaska Earthquake Center on Sunday fielded questions about whether the quake was a result of “induced seismicity.” West said that was unlikely.
Alaska averages 40,000 earthquakes per year, with more large quakes than the other 49 states put together. The state is at the convergence of two great tectonic plates, with the Pacific Plate slowly being pushed under the North America Plate.
As the Pacific Plate moves north, Alaska essentially is getting squished, West said. The compression is at the heart of its seismic activity — even a large quake in the northeast part of the state.
“Everything about it is consistent with what we might expect from Basic Plate Tectonics 101,” West said.
No damage was detected in oil fields or on the trans-Alaska pipeline. The pipeline was engineered for earthquakes and survived unscathed in a far larger temblor, the magnitude 7.9 quake of November 2002 on the Denali Fault, which passes under the pipeline.
The quake Sunday was south of the Arctic National Wildlife Refuge coastal plain, which Congress opened to drilling last year as part of a tax bill advocated by President Donald Trump.
The seismic activity Sunday would not be a roadblock to petroleum development, West said.
“We live all around the world with seismic hazards, and frankly, northern Alaska — we know very well it’s not a high hazard zone,” he said. “It’s no Anchorage. It’s no Japan. And no one should be using this earthquake to liken it to a place like that.”