“As seismologists gained more experience from earthquake records, it became obvious that the problem could not be reduced to a single peak acceleration. In fact, a full frequency of vibrations occurs.” -Charles Francis Richter
You’ve all been around long enough to be familiar with the severe damage that earthquakes can cause, rattling and cracking the ground, shaking down buildings, and creating catastrophic tsunamis
tidal waves. In short, the largest ones that occur in the wrong places will cause billions of dollars worth of damage and will kill thousands of people.
As you may well imagine, the Earth is hardly the only world that is geologically active like this, spontaneously and naturally quaking. Other planets like Venus have quakes, the Moon has them, even stars like the Sun have quakes!
Of course, all of these quakes release large amounts of energy; the quakes on the surface of the Sun release somewhere in excess of 40,000 times the energy of the 1906 San Francisco quake, one of the most destructive in recorded history! But what you might not realize is that all of that energy has to come from somewhere, and the naïve explanation — tectonic plates colliding — simply won’t do.
Plate tectonics tells you where earthquakes are most likely to occur, and the geophysics of the Earth’s crust tells you the different types of faults that cause these quakes, but neither of these tells you where the energy for these quakes comes from.
There’s a hint, though, if you’ve ever tried spinning with your arms out, and then brought them in.
There are only a few quantities in this physical Universe that are fundamentally conserved, and one of them happens to be angular momentum. Angular momentum, in plain English, is the product of your rate of rotation (known as your angular velocity) and how your mass is distributed (known as moment of inertia). When you bring your arms in, your moment of inertia goes down, and hence — in order to conserve angular momentum — your rate of rotation needs to go up!
Well, guess what we notice about the Earth after each-and-every measurable earthquake?
By just the smallest of amounts, the rotational period of a day shortens. For example, the 2011 Japan earthquake (including aftershocks) shortened the day by 1.8 microseconds, the 2010 Chile earthquake shortened the day by 1.26 microseconds, and the 2004 Sumatra quake shortened the day by an astounding 6.8 microseconds!
These are tiny numbers, of course, considering that we lose about 14 microseconds from the 24-hour-day each year just due to tidal friction from the Moon-Earth-Sun system, but it’s measurable nonetheless. And since angular momentum is conserved and the Earth’s angular velocity changes, that tells us that with each earthquake, the Earth’s moment of inertia must change, too.
But unlike you when you bring your arms in, the Earth has no external source of energy to cause this change-in-configuration. There’s only one place for this energy to come from: gravitational potential energy!
The Earth, as you well know, is (naturally) made up of all the stable and quasi-stable elements in the periodic table, which includes all the elements from 1 (hydrogen) to 94 (plutonium), exempting the three unstable elements along the way.
You can imagine an Earth quite unlike the one we enjoy today: one that was perfectly segregated by element in concentric layers, like some kind of atomic onion. On average, the most massive stars in the Universe do a remarkable approximation of this, and so you can imagine arranging the Earth in a perfect onion, with a tiny core of plutonium, enveloped by a thick spherical shell of uranium, in turn enveloped by protactinium, thorium, and so on.
Like I said, ultra-massive stars do something very close to this, and to some extent, so does Earth, already.
But we’re not perfectly layered; it’s not even close. Each big earthquake that causes an ~1 microsecond speed-up in the Earth’s rotational day is the equivalent of moving some ten billion tons of uranium from the Earth’s surface all the way to the center of the inner core! Of course, real earthquakes involve a much slighter rearrangement of much larger amounts of material: usually quintillions of tons of mass moving by just centimeters.
Still, each time we have an earthquake, that’s one tiny step closer to our ideal density configuration!
So where does an earthquake’s energy come from?
From the gravitational potential energy stored in the elemental configuration of the Earth itself! And when an earthquake occurs, that’s the Earth slightly rearranging its mass to lower its moment of inertia, increase its angular velocity, and to turn that gravitational potential energy into some very frightening and destructive kinetic energy!
As the Earth continues to cool, shrink, and radiate its heat away, more rearrangements like this are inevitable.
For the intricacies of earthquakes and the Earth’s interior, you need to understand an awful lot about geology and geophysics, but for the basic physics of earthquakes, and where they get their energy from? It’s as simple as rearranging atoms, and bringing the heavier elements closer to the core!