The “Good Friday” earthquake in Anchorage, Alaska is the largest earthquake in North American history, and the third largest earthquake in terms of richter size, in the history of the world (since earthquake records have been kept). It measured a 9.5 and shook the ground for over 4 straight minutes (compared to roughly 10 seconds or so for normal earthquakes).
It also caused tsunamis all the way down the coast to Northern California, where 12 people were killed; in Oregon, 4 children were swept off a beach; and a Canadian inlet, 55 houses were totally washed away.
The Alaska quake was a subduction zone earthquake, which occurs when an ocean plate slides beneath a continental plate or another ocean plate. These produce the deadliest quakes (think the 2004 Indian Ocean quake). As it turns out, there is a subduction zone 50 miles off the coast of Oregon. Last time it unleashed a massive tremblor was in 1700, where it reportedly caused tsunamis in Japan. According to seismologists quoted in The Myth of Solid Ground: Earthquakes, Prediction, and the Fault Line Between Reason and Faith by David Ulin, which I’m obsessing over at the moment, this particular zone should blow off steam (i.e., produce a massive, devastating quake) every 200-500 years. If it were to happen now, it would completely wipe out the entire coast of Oregon.
As it turns out, there are three huge fault lines running underneath Portland, two of which are directly underneath downtown. One, the Portland Hills Fault, is about 6 blocks west of where Tom and I live. Scientists are unsure if any of these faults are even still active or have ever really produced substantial earthquakes, but they all agree that the Portland Hills Fault is the most likely to blow any time soon (in fact, they give it about a 10% chance of producing a sizable quake in the next 30 years). If it goes, the west hills would completely liquefy, and everything on them (all those million dollar homes precariously placed over the city on pencil-thin stilts) would come crashing into downtown, making a lot of people really unhappy. There was a fairly unpleasant earthquake in Portland in 1993, which shook residents out of their beds and damaged some buildings, but strangely, geologists aren’t sure where it came from. They all agree that there is a far greater chance of Portland being disturbed by earthquakes in Washington, as it was when Portland residents felt the richter 6.8 Seattle quake in 2001. And in 1949, there was a huge earthquake in Olympia that killed 11 residents there.