“You can no more win a war than you can win an earthquake.” -Jeannette Rankin
As you all surely know, an 8.9 magnitude earthquake just struck Japan, devastating the island of Honshu.
Ask a geologist, and they’ll rightly tell you about plate tectonics, subduction zones, fault lines, and much, much more. But there’s a simple physics reason that they happen underlying all of it.
The Earth is built like a giant, spherical layer cake. The inner core — made up of mostly iron and nickel — is the densest of all the layers, while the lithosphere — the Earth’s crust — is the least dense.
This, by the way, is good.
Compare the left and right images, above. If I asked you which one was more stable, you’d immediately tell me the one on the left, obviously. Physically, this is because the more mass you have closer to the center of gravity, the more stable you are!
So this means, if we want the most stable Earth possible, it would be a perfect “onion”, with the densest, heaviest elements being concentrated at the center, and then — with each outer layer — a progressively lower and lower density. And it wants to be like that, packed as tightly as possible, the same way that a stone sinks in water, an iceberg floats on an ocean, or a helium balloon rises in air.
And, unfortunately, the Earth is not in its most stable possible configuration right now. But, over time, gravity is doing everything it can to help it get there. How? By bringing denser, more massive chunks closer to the center, and uplifting lighter, less dense pieces.
And that’s what happens — energy-wise — every time we have an earthquake! In physics terms, we’re keeping the mass the same, but rearranging it, which changes the Earth’s moment of inertia.
Now, the more astute among you will immediately think of a consequence of this, if this is what’s really happening. You see, the Earth is spinning, and you know what happens when spinning things change their moment of inertia.
The more mass you bring closer to the center, the faster it will rotate! For a figure skater, above, this means that bringing your arms and legs closer to the center causes you to spin faster.
For the Earth, this means that after every earthquake, tremor, or aftershock, the whole planet should start spinning slightly faster.
And it did. The Earth’s rotation is now 1.8 microseconds faster than it was before the earthquake on March 10th/11th (depending on where in the world you were), and most of that has happened because the Earth’s radius is now just a tiny bit smaller after the quake.
And this isn’t the first time that’s happened, either. According to figures from CBS news,
The 8.8-magnitude earthquake in Chile last year also sped up the planet’s rotation and shortened the day by 1.26 microseconds. The 9.1 Sumatra earthquake in 2004 shortened the day by 6.8 microseconds.
These might seem like tiny, tiny amounts of change, they actually correspond to a huge shifting in mass: something like moving more than 10 billion tonnes of rock from the crust of the Earth to its center!
And until we reach this theoretical “most stable configuration possible,” the Earth will remain geologically alive, quaking and shifting, and speeding up its rotation each time it does!
And for those of you who are celebrating Pi Day, check out this amazing visualization! (You may need to click on the Visualizing Pi link after a redirect, depending on your browser.)