Were you in an area that shook the hardest? Researchers map shaking felt across Anchorage in big quake

A shake map shows where the ground shook the most in Anchorage during the 7.0 magnitude earthquake on Nov. 30. Image courtesy of John Thornley.

ANCHORAGE (KTUU) - Data gathered in the aftermath of last month's earthquake is helping to minimize structural damage in future events, according to a geotechnical engineer.

That engineer is John Thornley, who wears many hats, including serving as geotechnical engineer for Golder Associates and chair of the Geo-Technical Advisory Commission for the Municipality of Anchorage.

He's also working on a Doctorate in earthquake engineering, focused specifically on Anchorage's geological response to seismic events. He and a team of researchers have developed "shake maps" in response to last month's earthquakes, looking at the parts of Anchorage that shook the hardest, as well as the least.

"It's a great opportunity," Thornley said. "I hate to smile at the large earthquake that we did have, but because there were no fatalities, I think it's great that we have this data now."

Data of seismic activity, Thornley said, was collected by multiple instruments called 3-component accelerators situated around Anchorage. They are part of what he refers to as "Anchorage's large seismic strong motion network."

He said the instruments help study shake strength in buildings as well as in the ground.

"They're oriented to face True North," Thornley explained. "They measure north-south vibrations, east-west vibrations, and also the vertical vibrations."

The shake maps developed from this data measure acceleration, represented in units of G, which is essentially the maximum amount an object shakes relative to the strength of a seismic wave.

He explained each of the three shake maps to Channel 2. The first analyzes peak ground acceleration, or PGA, looking at how the actual ground shook across Anchorage.

"What you can see here is we have our highest ground motion in South Anchorage," Thornley said, pointing at the PGA map. "Which is unique because that's the farthest location from the epicenter."

The second map shows a 0.2-second spectral acceleration. Thornley said it measures the shaking that smaller buildings experienced. The most intense shaking happened through the Midtown and Lake Hood areas, which Thornley said helps explain the liquefaction that occurred at Sand Lake.

The third map shows a 1-second spectral acceleration. Thornley said that measures what taller buildings felt during the earthquake, which was high levels of shaking from Mid- to downtown Anchorage, with the most shaking felt at the coastline. He said this map's indications are consistent with what scientists know from the '64 earthquake.

Thornley added that this research will help to inform building codes as to what worked well and what needs to improve in the future.

“There will be earthquakes for a long time to come in Anchorage and in Southcentral Alaska,” he said. "There will be another one, and hopefully we are even more ready for that one than we were for this one. And I think we did pretty well this time."


A shake map shows where tall buildings shook the hardest during the 7.0 magnitude earthquake Nov. 30. Image courtesy of John Thornley.
A shake map shows where short buildings shook the hardest during the 7.0 magnitude earthquake Nov. 30. Image courtesy of John Thornley.
A shake map shows where the ground shook the hardest during the 7.0 magnitude earthquake Nov. 30. Image courtesy of John Thornley.