Sometimes we need to look Down!

What makes earthquakes? Although there are many causes, including volcanoes, the most common thing that causes them are tectonic motions, which also cause tsunamis. But as valuable as it is to understand other planets in our solar system and in other star systems, sometimes it’s important to understand what’s going on inside our own planet.

The crust of the Earth actually is made up of a number of plates, which rub against one another and move over time. Who’s to blame? I fault the liquid hot magma.

So what happens is that these plates slip against each other in one of three ways, as shown below,

and these cause earthquakes. But what’s going on deep down in these faults? Well, that’s what a team of scientists in Japan intend to figure out, and here’s the BBC news article about it!

Turns out that the biggest quakes (the magnitude 8 or 9 ones) mostly happen at sea. So they’re actually going to the sea floor, digging out cores that go over a kilometer deep (1.4 km, seriously!), and then bring them back to the lab to analyze them and try to understand what’s going on beneath the Earth’s skin. Here’s a picture:

This is basically like taking the Earth to the doctor and having it X-rayed to see what’s wrong with its insides. Now nothing’s wrong, of course, except from our perspective! (We tend to not like these natural disasters.) But by doing this, they’re able to make 3D maps of the Earth’s density, and figure out what’s likely to happen where. I don’t know that this will lead to one of the holy grails of geology, earthquake forecasting, but that’s the hope!

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  1. #1 Dave M.
    April 21, 2008

    Quite an interesting coincidence you writing this article right after three quakes we felt there in St. Louis, epicenter Bellmont, IL and Mount Carmel, IL. The first was a 5.2 early Friday morning (4:37am the 18th, then a 4.5 later that morning at 10:14am and finally a 4.0 early this morning at 12:37am. Of course the later two quakes were the “feelable” aftershocks from the 5.2. There have been about 15-20 aftershocks that were less than 3.5 since the first quake.

    All this in an area that very rarely feels earthquakes. Of course we are on the New Madrid fault, but it’s just a little surprising.

    The quakes were either “S waves” or “Love waves” since the ground seemed to shift from side to side as opposed to up and down.

  2. #2 ethan
    April 21, 2008

    That’s a very good conclusion about the shifting of the ground. For all that we learn in Earth Science (for those of us who learn anything in Earth Science, Richard Feuerstein), P-wave-dominated earthquakes are actually relatively rare. I believe that a very frightened Pamela Gay wrote about these earthquakes you talk about.

    I remember getting a 3.8 earthquake when I was a kid in New York City; who gets earthquakes in NYC? But it sure would be nice to know how the Earth works underneath these fault lines, and they’re starting to figure some things out, like the stryations in the Earth. (They happen to be along the fault lines, surprise surprise.) We’re still a long way from long-term predictions of earthquakes, but maybe someday…

  3. #3 Brian
    April 22, 2008

    I’m a seismologist who grew up and went to grad school in the middle of the New Madrid Seismic Zone. All earthquakes emit seismic waves of two types: compressional (aka: longitudinal waves, meaning motion is in the same direction as propagation) and shear (aka: transverse waves, meaning motion is perpendicular to the direction of propagation). These two types of waves propagate differently depending on the elastic properties of the material. P, or primary, waves are compressional waves, and S, or secondary, waves are shear waves. P waves travel faster than S waves. All earthquakes have both types. Due to the fact that seismic waves disperse, P waves tend to be much higher in frequency compared to S waves and the later surface waves. This means that the amplitude (amount of sharking) is often greater with the later waves compared to P waves.

    By the way, I used to research the seismic hazard at the New Madrid Seismic Zone. Although it’s true that seismologists don’t fully understand the reasons for the seismicity there, the actual seismic hazard is probably very low. You’re at much greater risk from earthquakes in Alaska, Hawaii, California, or other western states compared to middle America.

    I won’t rule out the possibility of ever being able to predict earthquakes, but at this time it seems a long, long way off. All we can do now is predict *where* an earthquake is likely to happen, not *when*.

    Getting at Ethan’s original post, he mentioned that most great earthquakes occur underwater but didn’t say why. This is because the Earth’s biggest earthquakes happen at subduction zones. This is where one tectonic plate converges with another and slides underneath it. Most subduction zones happen either in the ocean or at the edges of continents just off shore. The so-called “Ring of Fire” in the Pacific is comprised of many subduction zones that surround the Pacific and produce a majority of earthquakes, volcanoes, and tsunamis.

  4. #4 ethan
    April 22, 2008

    My friends are smart.

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