The Physics of an Earthquake

Just a few days ago on October 30th, my home suffered from an earthquake that had a magnitude of 6.5 (a rather large quake) and resulted in a drastic amount of damage and deaths. Besides the Philippines, the other affected countries include Indonesia and Malaysia. This event made me curious about the physics behind an earthquake and why it is that the Philippines seem to have many more than quakes than other countries.
The Philippines lies along an area which has been named the Pacific Ring of Fire which causes the country to have frequent seismic and volcanic activity. This large horseshoe shape around the Pacific Ocean is where many earthquakes and volcanic eruptions take place. Roughly 90% of the world’s earthquakes and around 81% of the world’s largest earthquakes occur along this horseshoe. The Ring of Fire is a direct result of the movement and collisions of plates.

So why do we experience earthquakes?
The earth is built in a very large, spherical layer cake. The inner core is the densest of all the layers whereas the earth’s crust is the least dense. This is a good thing as it makes the earth stable. This is because the more mass you have closer to the center of gravity, the more stable you are.

Therefore, if the earth wants to be as stable as possible, it would have the heaviest elements concentrated at the center and then each layer outward is progressively less and less dense. The earth wants to be packed as tightly as possible. However, unfortunately, in its current state the earth is not in its most stable possible configuration. Over time, gravity is trying to make it so that the earth becomes more stable. It does this by bringing denser and more massive pieces closer to the center of the earth and places lighter chunks closer to the surface. Therefore, in terms of energy, that is what happens every time there is an earthquake. The earth is keeping the mass the same but rearranging it. This changes the earth’s moment of inertia.
There is a consequence of this as the earth is spinning and when spinning objects change their moment of inertia the speed of the spin changes. This is because the more mass that is brought closer to the center of the earth, the faster the earth will rotate. Using the image of the figure skater below, it means that when she brings her arms and legs closer to the center of mass (her body) she spins faster.

Therefore, for every earthquake, the world should start to spin a little faster than it was previously spinning. For example, an earthquake that occurred in Japan in 2011 caused the earth’s rotation to increase 1.8 times faster than it previously was spinning. This corresponds to a massive shift in mass.
In conclusion, until the earth reaches its theoretically most stable configuration, it will remain alive geologically and will continue to have earthquakes and cause shifting, which will in turn speed up its rotation.