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The Impact: Study Suggests Back-to-Back, Multi-Fault Earthquakes Can Hit Seattle; WA EMD Talks Seismic Survival Strategies

Mike McClanahan profile by Mike McClanahan

Photo courtesy of Bryan Black

A new tree-ring study, involving ancient sunken trees that were killed by earthquakes in the Seattle area indicates that large quakes can happen on separate faults at the same time or in very rapid succession in the area.

The study, led by the University of Arizona, concluded that the Seattle fault and the Saddle Mountain Fault both ruptured around the same time 1100 years ago, causing dramatic changes to the landscape. The researchers say they can precisely date the event because of a powerful solar storm that left a specific signature on trees that were alive during the time..

Lead researcher Bryan Black with the University of Arizona’s Laboratory for Tree Ring Research says Washington’s seismic building codes don’t account for two rapid fire earthquakes or two that happen at the same time.

 “The geological evidence suggests this was the biggest, most damaging event of the last, probably 16,000 years in the region. So it was, it was a big event. Fortunately, they seem to occur infrequently, but there were widespread indications, widespread scars on the landscape of this earthquake event. And as far as landslides, faults thrusting up 20 to 25 feet throughout the region, that would significantly change the landscape, also generate local tsunamis and Puget Sound,” said Black.  “It’s hard for me to comment on what the damage would look like, but the single large earthquake would be on par with the massive earthquake that happened in San Francisco in 1906, or the rapid double quake would be something similar to the terrible earthquakes that happened on the Turkey-Syria border back in February of this year,” said Black.

Developing earthquake safety reflexes is the point of an annual training exercise called the Great Washington Shakeout which always falls on the third Thursday in October. It’s a simulated earthquake drill where participants practice the drop, cover, and hold on survival strategy.

The Washington Emergency Management Division is heavily involved in coordinating emergency preparedness for Washington’s seismic risks. There are a lot of them.

The agency’s Earthquake/Volcano Program Coordinator, Brian Terbush, said at least 1.4 million people were registered for the annual October earthquake safety drill this year.

“We can’t predict when an earthquake is going to happen. And unfortunately, but also fortunately here in Washington, we don’t get many opportunities to practice for earthquakes. I mean, California, they’re constantly getting these messages about the earthquake that’s happening there. And people are just used to earthquakes. Our last big one that a lot of people felt, well, if you don’t, count last weekend ,was our Nisqually earthquake back in 2001,” said Terbush.

Terbush cautions against trying to run outside during an earthquake.

“That is going to be a tough instinct to fight, which is why we suggest practicing. You’ve got to get that muscle memory so that when you’re in the stressful situation of an earthquake, you fight that urge because again, you’re running, you’re exposed if things are falling from above,” said Terbush. “Unfortunately, back in 1949 and 1965, when we had similar earthquakes here to Nisqually, that was one of the main ways that people were killed in that earthquake. I think we had about six and seven deaths respectively in those earthquakes. I mean, some of those were even schoolchildren running outside, especially in some of our buildings, which are unreinforced masonry. They’re brick buildings that are not secured properly to their foundations, the walls not secured to the floors. And they have these parapets up above that are unsecured. When they start shaking from side to side, bricks start falling off and glass starts falling off. And running outside the building puts you in danger of all those falling bricks. We saw this a lot in the Nisqually earthquake back in 2001.”