Dr. Know: Monster Earthquakes

  If a monster earthquake strikes off the Oregon Coast, could the resulting tsunami come all the way up the Columbia River and flatten downtown Portland? —Seismic Citizen

This is a popular misconception, possibly due to confusion in the public's mind between "a tsunami" and "Godzilla." Luckily, this is one of the relatively few things Portlanders won't have to worry about when we finally get our promised magnitude 9 superquake. 

While tsunami waters may raise the level of the lower Columbia—no picnic for folks in, say, Astoria—the standard earthquake scenarios don't include a Poseidon Adventure-type wall of seawater coming to wash Portland proper into the sea. 

That said, if you want to ruin your next trip to the coast, have a look at the tsunami hazard map for whatever seaside town you happen to be visiting—the one for Cannon Beach is particularly grim—and ponder the likelihood that you could get yourself and your beer cooler 100 feet above sea level (or 2 miles inland) in the 15 minutes coastal dwellers are projected to have between “first tremor” and “watery annihilation.” 

Thank God we live here in the Rose City, where the only liquid death we have to worry about is the melting of the very ground beneath our feet. 

Say again? Well, it turns out that earthquakes can raise our underground water pressure in such a way that formerly solid ground will flow like a liquid. (Quicksand works the same way.) It's called "liquefaction," and as you might imagine, it's not good news for your favorite bridge, skyscraper or freeway overpass. 

Couple this with the unique soup-sloshing-in-a-bowl effect of Portland geology (see Dr. Know No. 1, Sept. 23, 2009) and you can see that, even without a tsunami, we'll still have lots to brag about. Now, if only we had a nuclear power plant….

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