Seismologists including Chris Goldfinger at Oregon State University, Brian Atwater with the USGS, and David Yamaguchi have studied something called the “Cascadia subduction zone”. Think of it like a major earthquake fault line in the ocean where two major tectonic plates slide, or attempt to slide, across each other. Based on the research of these scientists,

…we now know that the Pacific Northwest has experienced forty-one subduction-zone earthquakes in the past ten thousand years. If you divide ten thousand by forty-one, you get two hundred and forty-three, which is Cascadia’s recurrence interval: the average amount of time that elapses between earthquakes. That timespan is dangerous both because it is too long–long enough for us to unwittingly build an entire civilization on top of our continent’s worst fault line–and because it is not long enough. Counting from the earthquake of 1700, we are now three hundred and fifteen years into a two-hundred-and-forty-three-year cycle.

Kenneth Murphy, Direcor of FEMA’s Region X — the division responsible for Oregon , Washington, Idaho, and Alaska — says, “Our operating assumption is that everything west of Interstate 5 will be toast.”