If you have lived in Southcentral Alaska for a year or more, you are almost certain to have felt an earthquake. But a damaging quake is something else again. Experts tell us a quake as powerful as the Great Alaska Earthquake of fifty years ago isn’t likely any time soon, but it doesn’t take a Magnitude Nine to do big damage.
The next big one already happened. It’s just that nobody lived there.
The Denali Fault quake of 2002 was magnitude 7.9, but unless you lived in Tanacross, Tok or Northway it was remote. Only afterwards did geologists see the massive landslides that it caused. In a sense, seismologist Mike West says, tossing these big numbers around puts urban Alaska at risk of getting complacent.
“I’ve heard people – researchers, probably myself included, say things like ‘Oh it was only magnitude 8,” which is completely warped,” West said. “A magnitude 7 or 8 earthquake is a massive, massive event.”
But if it happens out in the Aleutian Trench, nobody even sees the damage and the main concern is whether it would generate a tsunami. West suggests that maybe Alaskans get the idea that they’ve already seen it all, and he’s here to tell us we haven’t.
“1964 is a nice benchmark,” West said. “It showed us some things that can happen in an earthquake, but there are countless scenarios for equally damaging or more-so earthquakes that aren’t that earthquake.”
“So there’s a small danger in treating 1964 as an example of everything that can happen.”
West points to the nature of the ground we build on in Alaska, and raises the question of what would happen if an earthquake of a magnitude we think we are used to were to happen directly underneath a populated area. And there’s a sobering example from three years ago in New Zealand, where a powerful quake damaged the city in 2010, but another one months later collapsed buildings already weakened.
“We’re talking about a magnitude 6.1 earthquake that killed 150 some people in a first world country,” West said. “And it occurred, it happened because the earthquake was right under town – it was very shallow. And Christchurch was built on soft, wet sediments. And much of Alaska is built on soft, wet sediments.”
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Federal authorities are not unaware of Anchorage’s vulnerability. The seismic hazard maps are being updated, and there is a set of boreholes installed by the U.S. Geological Survey in the city’s downtown park strip with seismic sensors spaced at different depths to about 200 feet to pick up ground waves from earthquakes and watch them travel.
“This is the so called ‘strong motion’ seismic network, and it’s a tremendous asset for the state and the municipality,” West said. “It’s really about the densest instrumentation of that type in the country.”
That seismic network continues on up the structure of the nearby Atwood office building, and at dozens of other locations around the city. West says down the road it might be possible to install an earthquake early warning system like is being experimented with in California, though there is no active planning for it at this time.