Forecasting the aftershocks following Friday morning earthquake

The aftershocks of Friday morning’s earthquake north of Anchorage, could be felt for months.

Jana Pursley, a geophysicist with the U.S. Geological Survey’s National Earthquake Information Center in Colorado, said the largest aftershock for the magnitude 7.0 event was a magnitude 5.7.

“There will be scores of aftershocks,” she said. “Most of them appear to be smaller size. Right now I’m seeing them about magnitude 4. Aftershocks, they vary in tectonic settings and size. Some earthquakes have them for days, weeks, months. Some have them for years. The most noticeable ones will probably slow down within a couple months, but I would not be surprised if we would have an aftershock half a year from now that people feel.”

There’s been no reported tsunami danger from the earthquake, which struck about 8:30 a.m.

Aftershocks’ size and how long they last depend on the magnitude of the earthquake and the tectonic setting, Pursley said.

“In many cases, when the USGS locates large, deep earthquakes – for example, in Fiji – you will not see that many aftershocks,” she said. “In Alaska, you have two plates basically pushing against each other. You have the Pacific plate moving toward the northwest with respect to the North American plate. ... There is constant movement. Very small, of course – it’s about 5.7 centimeters per year – but it’s constantly happening.”

Pursley said she wasn’t expecting severe injuries, but that roads and bridges would be damaged.

“Any building that does withstand the earthquake, it doesn’t mean that people can necessarily go back inside afterward,” she said.

And, she said, the damage would likely be widespread.

“It basically encompasses near Anchorage, near Knik-Fairview and then it stretches a little into the bay,” she said. “It’s kind of an oval-shaped region around where the earthquake happened.”

Anchorage has a population of about 294,356 according to 2017 estimates from the U.S. Census Bureau.

The area is no stranger to major earthquakes, including a magnitude 7.1 in the Cook Inlet area in 2016 and a magnitude 9.2 in the Prince William Sound area in 1964.

After a magnitude 7.0 earthquake shook Alaska about 8:30 a.m. Friday, rumors began flying that a second, larger event was on its way.

It’s possible, but there’s no way of predicting that, said geophysicist Jana Pursley with the U.S. Geological Survey’s National Earthquake Information Center in Colorado.

“The risk of larger earthquakes is increased for a few days after a big quake happens,” she said. “There’s always a risk that there could be a second, larger one. But we have no way of saying when, where or even if it’s going to happen.”

She noted that there was an increased chance of a bigger event following the 2014 Napa earthquake in California – a magnitude 6.0 – but it didn’t happen.

Pursley said people living in earthquake-prone areas should have important documents and medications readily available in case they need to evacuate. They should also know how to get in touch with their families or, if necessary, the authorities.

“In events like these, you might expect power outages and cellphone outages, so have a backup plan,” she said.

The USGS’s website at usgs.gov has more information on preparing for an earthquake.

Friday’s earthquake struck north of Anchorage. There’s been no reported tsunami danger.