That 7.1 quake shook different parts of Anchorage in very different ways

January 25, 2016

A map provided by Michael West, director of the Alaska Earthquake Center at UAF, shows the acceleration of shaking from instruments placed at various sites in the Anchorage bowl.

Sunday morning’s magnitude-7.1 earthquake shook Anchorage neighborhoods with vastly different force even less than two miles apart, evidence that some pockets of the city are much safer than others.

The finding comes from one of the best urban seismograph networks in the country, conceived almost 20 years ago, installed a decade ago and receiving its definitive test Sunday. …

Instruments in the network show the strength of shaking compared to the strength of gravity. Sunday’s measurements of 15% g in some neighborhoods — or shaking that is 15 percent as strong as the normal acceleration of gravity — are as high as Anchorage is believed to have experienced in the massive 1964 earthquake that transformed the region.

But that quake was 1,400 times stronger the Iniskin Earthquake, as Sunday’s event is called. West said the ’64 quake did vastly more damage with massively greater motion and much longer duration. The rupture in 1964 lasted 4 minutes, long enough to liquefy ground in Anchorage. The rupture in the Iniskin quake lasted only 10 to 15 seconds.

We felt Sunday’s shaking much longer than that — longer than a minute — because of the soft geology of Cook Inlet and the weak soils under the city, factors that amplified and extended the impact.

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