Could a Japan-like mega-tsunami hit America? Yes, and there's proof.

It is likely that the U.S. will be hit with a devastating tsunami within the next 50 years.
Written by David Gewirtz, Senior Contributing Editor

Every day, the news reports out of Japan detail more and more horrific conditions. The power of the offshore earthquake, followed by a mega-tsunami, is almost beyond comprehension. The level of devastation and human loss is extreme and the worries over reactor disasters only compound the dispair.

Could it happen here?

And yet, one question -- above all -- keeps running through my head: could it happen here? The short answer is "Yes". In fact, it already has.

Brian Atwater is a geologist for the U.S. Geologic Survey. Atwater has been investigating ruins in the Pacific Northwest that show strong evidence of a previous geological event that took place about 300 years ago.

He found a native American fire pit covered by a layer of tsunami sand from about the year 1700. He also was able to correlate that archeological information with Japanese records of a tsunami hit in or around Honshu, also in 1700. The Japanese records narrow it down. They say it hit in January.

The Earth's lithosphere -- the planet's hard shell -- is made up of a series of tectonic plates. These are enormous slabs of the Earth's crust, running about 60 miles deep and as big as continents. There are seven major plates, and a number of smaller plates across the planet.

These plates move and are subject to subsurface pressure. Over time, they rub against each other and push against each other. Imagine two small boats floating side-by-side in a pond. They might drift slightly into each other, but if the speed of the drift was slow enough, there'd be no harm.

Now imagine two aircraft carriers in the ocean cruising side by side, say just a few yards apart. If a wave came up, it could throw the first carrier into the other, and the harder it was thrown, the more damage would be done.

Tectonic plates

Tectonic plates are like those carriers, except much, much bigger. If one plate is thrown into another because of subterranean forces, the resulting geological event can be felt across much of the world.

This is what happened in Japan last week. Two plates came together and...popped. Basically, a big chunk of underwater planet shot up suddenly, displacing the water that was once in its place. That water displacement became the tsunami.

Just west of Seattle is the Juan de Fuca plate, named after a 16th century explorer who mapped much of America's west coast. The Juan de Fuca plate rubs up against the North American plate, just off the coast of Vancouver, Washington, Oregon, and northern California.

This tectonic bump-and-grind zone is known as the Cascadia subduction zone. In 1700, the two plates didn't just nudge each other, they bashed into each other with tremendous force, causing an earthquake of a similar magnitude to the one in Japan.

And -- just like in Japan -- the megathrust under the ocean's floor displaced a tremendous amount of water. That water exploded over the shores of the Pacific Northwest and -- just as if the Pacific were really a giant bathtub -- also flooded some of Japan.

So, we now know that not only could a devastating megathrust quake/tsunami hit America, we also know it did.

The next obvious question is this: can it happen again?

Next: Why it could happen »

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There've actually been a number of earthquakes in the Pacific Northwest region, but most have been the more traditional (if no less devastating) deep earth quakes, like those that took place in 1949, 1965, and 2001.

The Cascadia zone has had a regular cycle of major events, happening every 300 to 900 years. Geological information shows that previous megathrust events have taken place (working backwards in time) in the years 1700, 1310, 810, 400, 170 BC and 600 BC.

It's relatively easy, then, to answer our can-it-happen-again question. Based on previous activity, the answer is "almost definitely".

When could it happen here?

So, let's work through what we know. We know the tsunami that's currently devastating Japan was caused by a megathrust quake that took place off the coast and was the result of underwater tectonic action that displaced large amounts of ocean water.

We know that a similar tsunami hit American shores in the year 1700, the result of a similar sort of geological activity as took place in Japan.

We know that the Cascadia subduction zone generates massive geological events every 590 years, on average.

That, of course, brings us to the next question: when could it happen here?

At least two sets of Cascadia events took place only a few hundred years apart. The last known Cascadia event was 300 years ago. The math is not promising.

Let's let U.S. Geologic Survey geologist Atwater bring the story home with the most disturbing of conclusions. This is an interesting video put out some years ago by the BBC. Not only does Atwater talk about the geology, you can also see how the U.S. would respond if the event took place.

The problem is that Atwater isn't some lone wacko predicting the end of the world. He's a well-respected scientist and his work is backed up by other research. For example, research by Oregon State University marine geologist Chris Goldfinger makes a case that the Pacific Northwest is due for another quake, sometime in the next 50 years.

That means that if you live in the northwestern part of the United States or western Canada -- near Seattle, Portland, Vancouver, Victoria, or Tacoma -- either you or your kids will experience a tsunami of the magnitude that just hit Japan.

Reactors and reactions

There is one piece of good news in all of this. Unlike Japan, the U.S. has no operating nuclear reactors within the Cascadia subduction zone area of effect. That's something, at least.

If Katrina taught us anything, it's why disaster planning is so important. It's also why developing and building to disaster-aware building codes can be a life-saving practice.

It is likely that the U.S. will be hit with a devastating tsunami within the next 50 years. How much it hurts depends on how well we prepare.

Wayne's world

Update: My colleague, Wayne Rash, former Executive Editor at eWEEK, had some very interesting additional notes. These are his words:

If you have room for an update, that tsunami in 1700 was probably much worse than the one that struck Japan. The configuration of the sea floor in that area resulted in waves that were much higher, and in some cases significantly higher than what hit Japan.

But that doesn’t hold a candle to a tsunami that hit Alaska that in one area had waves around a thousand feet high. Yes, it was in a bay with an unusual sea floor configuration, but still…

The other tsunami zone that mostly goes ignored by emergency planners is the area of the mid-Atlantic ridge. The ridge in the North Atlantic is subject to large underwater landslides that can generate tsunamis. One of the areas of concern – Manhattan. But that would just scare everyone so we don’t want to plan for that possibility.

Thanks, Wayne!

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