If you believe what you see in the movies, you probably think that "the big one" — a mega-earthquake in a major North American city — will inevitably hit Los Angeles or San Francisco along the San Andreas Fault. But while these cities are certainly prone to devastating earthquakes, many experts now suspect that the site of the next really, really big one will be somewhere else entirely.

It may happen in the Pacific Northwest: home to coffee lovers, hipsters, grunge rock and rain. And also, possibly, the most ruinous, destructive earthquake to hit North America in modern times, reports The New Yorker. The characterization might seem unlikely to current residents — the Pacific Northwest rarely experiences large tremors — but the region sits at the crux of a major fault line called the Cascadia subduction zone, a tectonic plate boundary that stretches from northern Vancouver Island to northern California.

It just so happens that earthquakes along subduction zones, or plate boundaries where one plate is sinking dramatically underneath the other, are the most powerful earthquakes known to occur, capable of exceeding 9.0 on the Richter Scale. For example, it was movement along this same kind of tectonic boundary that caused the 2011 earthquake and tsunami in Japan.

The scale of earthquakes along these types of zones can often be proportional to the length of the fault, and as faults go, the Cascadia subduction zone is a particularly lengthy one. In other words, the entire region essentially sits upon (and, in fact, has been fundamentally shaped by) a perfect storm for mega-quakes.

So then, why are large earthquakes so uncommon in the Pacific Northwest? The truth is, they're actually not, at least not if you think in terms of geological timescales. Scientists now know that mega-quakes have occurred along this plate boundary once every 243 years on average.

The last big one occurred 315 years ago, in the year 1700. That is more than 100 years before explorers Lewis and Clark had even first traversed the region. Thinking in these timescales, it's easy to understand why modern residents and developers in cities like Seattle and Portland tend to have such lackadaisical concerns about earthquakes. Seattle and Portland were not even glints in the eyes of their developers (nor were the developers glints in the eyes of their ancestors, for that matter) when the last mega-quake occurred there.

Alarmingly, seismologists now predict that the odds of a mega-quake hitting the Cascadia subduction zone in the next 50 years are roughly one in three. The big one is coming, inevitably.

When a quake in excess of 9.0 in magnitude does occur along the entire length of the Cascadia zone, the resultant tremor will likely level much of the architecture in cities like Seattle, Tacoma, Portland, Salem and Eugene, which is by-and-large not designed to withstand such a violent shake. Structures that do survive the trembling could be inundated by a 20- to 100-foot tsunami wave that could be triggered up and down the coast. The entire map of the Pacific Northwest west of the I-5 corridor may need to be redrawn by the end of the disaster.

It's a terrifying scenario, especially considering how woefully unprepared the region is for handling such a disaster. The best that can be hoped for is that such a mega-quake occurs late along its typical interval, and that developers in the region wise up to the possibility of such a catastrophe and have plenty of time to up their preparedness.

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Bryan Nelson ( @@brynelson ) writes about everything from environmental problems here on Earth to big questions in space.

Mega-quake: The 'big one' could happen soon, but probably not where you think
The Pacific Northwest might be where the next huge tremors happen, say seismologists.