Watching the waves spill onto the beach in Clatsop County, Oregon, it's easy to forget that the nearby houses are doomed. The Pacific waters are frigid blue. The beaches run inland from white sand to tough, salt-sprayed grass. It's too peaceful to envision disaster.



Yet, at a tectonic convergence far out in the water, the geologic process that will invariably devastate Clatsop's coastline is sedately underway.



The convergence is called the Cascadia subduction zone, a 700-mile-long grind of hot and locked crust, only recently understood to be the United States' most dangerous fault line. It's where North America's continental shelf meets the Juan de Fuca tectonic plate, which has, for centuries, compressed the shelf eastward and upward, a few millimeters every year.



Think of a spring slowly being wound. For years, the spring winds and winds, its potential energy mounting. Then, suddenly, it recoils.



A recent article in the New Yorker titled "The Really Big One" chronicles this recoil and its consequences in stomach-churning detail. First, a single, isolated jolt called a compressional wave will ripple inland. Dogs will start barking. People will sit up in bed. Then, thirty seconds later, there will be an earthquake. A really big one.



FEMA currently estimates that 13,000 people will die in a Cascadia subduction-zone event and millions of others, mostly in Portland and Seattle, will be displaced or lose access to power, running water, and functioning sewer systems. Bridges will sway and snap, houses will slip from their foundations, and when the shaking is over, Cascadia will have jolted 30 feet west and 6 feet down.



A tsunami will arrive on the Northwest's coastline 15 minutes later, the series of waves towering between 20 and 100 feet tall, depending on the magnitude of the earthquake.



That tsunami will wipe Clatsop's coast clean.



There won't be any prior notice. As Sandi Doughton, science writer for the Seattle Times, stated in a recent Q&A, "Every attempt to predict earthquakes has fizzled." Instead, the problem will trade much more heavily in disaster preparedness and good old-fashioned risk assessment. So, if you live on or plan to visit the Pacific Northwest's beaches (where most casualties are projected to occur), what are the chances you'll be there when the really big one comes? How should you think about (or not bother to think about) the risk?



Risking the really big one today, tomorrow, and 20,000 days later



Recent research from Oregon State University professor Chris Goldfinger puts the chances of a magnitude-eight earthquake in Cascadia at one in three over the next 50 years. An earthquake of that power would, at best, unfold like San Francisco's famous 1906 earthquake, which killed roughly 3,000 people. Goldfinger puts the chances of a much more severe earthquake--a magnitude-nine event--at one in ten for the same time period. That would look something like Japan's 2011 Tohoku earthquake, which killed over 15,000 people in the most earthquake-prepared country on the planet.



So, one in three and one in ten over the next 50 years.



To simplify (optimistically), let's say there's a one-in-three chance of any sort of catastrophic subduction event in the next half century. That's a one-in-three chance of the really big one over the next 18,262 days, or--neglecting that the probability of this earthquake increases over time (which it does, slightly, up to a point)--a 0.002% chance on any given day.



Put another way, if you flip 54,786 coins, you'll get heads 54,785 times. You'll only get tsunami once.