Data shows less shaking but more damage outside municipal building safety service area

Peak Ground Acceleration (PGA) measurements from two instruments at near equal distances from...
Peak Ground Acceleration (PGA) measurements from two instruments at near equal distances from the epicenter of November's earthquake. (KTUU)
Published: Apr. 16, 2019 at 7:10 PM AKDT
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Building officials, scientists and engineers in Anchorage are trying to determine exactly why Anchorage saw less damage than Eagle River and Chugiak as a result of the Nov. 30 magnitude 7.1 earthquake.

According to current FEMA totals, less than 4% of the structures built after 1990 required a disaster assistance request within Anchorage's Building Safety Service Area. Those numbers rose significantly when looking at post-1990 construction outside of the Anchorage bowl — 19.5% in Eagle River and 9% in Chugiak.

John Thornley is the chair of the Municipality of Anchorage Geotechnical Advisory Commission. He's also involved in a project to determine how earthquakes affect the movement of land in Anchorage and surrounding areas. But Thornley says that there's not much data available on Eagle River or Chugiak.

"We have roughly 45 instruments that have measured earthquakes in Anchorage and areas just outside of the Anchorage Bowl," he said.

One of those instruments is in Chugiak, and there are none in Eagle River.

Data from the sensor, located near AFD Station #31 in Chugiak, revealed that Peak Ground Acceleration — a measure of ground motion intensity — was lower in Chugiak (0.3g) than other areas of Southcentral. At AFD Station #12 near Dimond Blvd. and the Old Seward Highway — roughly the same distance as the Chugiak Fire Station from the epicenter — PGA was measured at 4.7g.

"The ground shaking was likely a little stronger mid-Seward Highway by Dimond," Thornley said.

Which begs the question, if ground motion was greater in Anchorage, why was there more damage in Chugiak?

While the data has not led scientists or engineers to any absolute conclusions, it does raise questions about whether or not changes need to be made to building inspection policies in communities located on the outskirts of Anchorage.

Dennis Berry is one of two structural engineers who hold seats on the MOA Geotechnical Advisory Commission.

Berry says he's heard from several community councils that sit outside the Anchorage Bowl considering making changes to the current building code and inspection scheme, but says that whether or not the current system will be altered has yet to be determined.

"I don't think we're seeing different ground shaking outside of the building service area," Berry said. "Based on what I've seen so far in terms of building construction, our problems really relate to the fact that buildings aren't being properly designed, or if they are, they aren't being properly constructed."