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Science Sidebar 3: Relearning A Little Geology

Once again, stuff that missed the cut for the paper.

I discuss some geology stuffs.  That links to the article that I originally read, but I’ll be discussing a little of the general background, too.  Those of you on the west coast may find this particularly interesting… or boring, since you’re also likely to have heard it a thousand times before.

This is Mount St. Helens in 1982, after the big eruption.  (Shamelessly borrowed from Wikipedia.) Also, in a bout of tremendous false advertising, it is only tangentially relevant to the post at hand.

This is Mount St. Helens in 1982, after the big eruption. (Shamelessly borrowed from Wikipedia.)
Also, in a bout of tremendous false advertising, it is only tangentially relevant to the post at hand.


Surprise!  The west coast of the US is prone to earthquakes.

We can blame plate tectonics for that.  The surface of the Earth is like a big jigsaw puzzle where the pieces don’t quite fit.  Floating on top of the more-liquid mantle, they slowly slip past, under or over or into each other.  Except, sometimes, they get stuck, and then move in sudden spurts when the pressure gets to be too high.

That’s the rough explanation for how earthquakes happen.

On the US West Coast, there are three tectonic plates that are relevant.  The first is the North American plate, which includes very nearly all of North America, including Mexico, Greenland, and parts of Russia and Japan.  The second is the aptly named Pacific plate, which covers — shockingly — much of the Pacific Ocean.  Those two border each other along a large fraction of the famed Ring of Fire, but there is one much smaller plate, the Juan de Fuca plate, which is nestled between them along the coasts of Washington, Oregon, and northern California a little bit further south than Eureka.  (And a bit of southern Canada, we can’t forget them.)

The North American and Pacific plates are sliding past each other (mostly) on the West Coast, but the Juan de Fuca plate is actually moving apart from the Pacific one, and sliding underneath the North American one.  The result is…


And wow, volcanism.

Off the coast, Juan de Fuca sinks beneath the North American plate.  Deep underground, beneath the Cascade Mountains, the place eventually sinks away, ground up and melted into the Earth’s mantle.  While that happens, some of the material melts, and, being a lower density, rises up.  This forms mountains and powers a certain amount of volcanism — hence, Mount St. Helens.

For bonus points, the fault is large enough that when it slips it can cause very large earthquakes, although there have not been any truly massive quakes recently.  There was a large (~9) earthquake in 1700.  There was a big tsunami in Japan as a result.  The most recent earthquake I remember that was significant was around a 6.0 near Seattle.  (Now, remember that this is in the newer moment magnitude scale, instead of the old Richter magnitude scale, but yes, it’s logarithmic.)

But Is The Big One Going To Knock My House Down?

Don’t ask me.

The main point of the article I linked above is that the expert geologists and seismologists and geophysicists and paleoseismologists (and all appropriate variants thereof) are still working out the details.

The ongoing discussion is about how often earthquakes occurred in the past, over the last 10,000 years or so ago.  Scientists can estimate this by looking for sediment or rock layers deep in the ocean which show signs of having been disturbed by earthquakes.  This lets them estimate the frequency of large earthquakes in the area, and use that information to forecast how likely earthquakes are to occur in a given period in the future.

However, there are complications that depend on when the samples of the sea floor — called cores — were taken from the ocean, where they were taken, how well those positions are known, and how well understood the particular kinds of turbidite are.  In short, the jury is still out, and the science is ongoing.

So, depending on who you ask, the odds of another 9.0 quake happening along the Juan de Fuca subduction zone in the next fifty years is somewhere between 10 and 40 percent.

For bonus points, there’s some evidence that a large earthquake, particularly one that affects the southern part of the Juan de Fuca plate, could trigger a major earthquake along the northern part of the San Andreas fault a couple of decades later.  (Maybe.)

In short: earthquake preparedness, people.

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