That cheerful orange stripe is known as the Cascadia Subduction zone. It's a fault line, where one part of the Earth enters the other. Most of the time, this happens smoothly, as if Barry White were playing in the background. But every once in a while, our douche of a planet likes to get rough and America rejects the incoming plate. This is called a full-margin rupture, and it's as devastating as it sounds. The last time that happened was in 1700, and the earthquake was so big that it caused a giant tsunami that destroyed a bunch of coastal Japan across the ocean.
National Oceanic and Atmospheric Administration
Meanwhile, Australia wussed out and cowered behind New Guinea.
The Pacific Northwest is all but due for one of these, and it couldn't be more terrifying. When the next one happens, at roughly around a 9.0 (re: very bad) on the Richter scale, cities like Vancouver, Seattle, and Portland are in for what's been described as the worst natural disaster in the history of North America. FEMA projects 13,000 dying, one million homeless, and 2.5 million needing food. Or roughly five to six Katrinas. And that's not even counting Canada.
The current chances of a big earthquake happening there are one in three, while a giant, apocalyptic earthquake is at ten to one. Some good odds, assuming you're a complete psycho who bets on human misery.