– Submitted by Cortlynd B., 13, San Diego, Calif.
Many earthquakes indeed happen as a result of plates rubbing against each other. You can explore “in the laboratory” how earthquakes occur by rubbing two wooden blocks against each other. If the blocks are really smooth, then you do not have to push hard. The blocks slide easily past each other. In the real world, this would cause only small earthquakes. Now use sanding blocks and attach sandpaper. If you use very fine-grained sandpaper, then you can still rub the blocks past each other easily, by exerting little force. But when you use very coarse sandpaper, then you need more force to move the blocks because the coarse grain on the sandpaper makes the blocks stick. The friction is increasing. By exerting more force, you put more pressure on the blocks and when they finally slide, more energy is released. In the real world, this means that a larger earthquake occurs.
When plates rub each other, and they get stuck along a contact zone, or fault, stress (another word for force) can accumulate before the large earthquake occurs. Some faults creep, which means they move all the time without accumulating much stress. They correspond to a sanding block with extremely fine or no sandpaper. It is thought that such faults likely do not produce large earthquakes. The size of the fault also matters. If you have very small sanding blocks, then you don’t have to push very hard. But with very large sanding blocks, you have to push harder. In the real world, this means that a long fault between large moving plates is more likely to have large earthquakes than a short fault.
So what makes the plates move? Earth’s plates move across the planet at about the same rate as a human fingernail grows, or even a bit faster, a few centimeters per year. Some plates move away from each other, and the consequence of this is that they are pushed against another plate at the other end. This push increases the stress along that fault, readying it for an earthquake. Another driving force is the aging of plates in the oceans. They get heavier and heavier. Eventually they sink, thereby pulling the rest of the plate with them.
Some earthquakes actually do not occur along the boundary between two plates but in the middle of a plate. In this case, the plate may have been cracked in the middle a long time ago, possibly because of the pulls in different directions at the edge of the plate, or because a hot blob of magma pushed from underneath. This crack can then act like an earthquake fault when the plate is later stressed. This explains why earthquakes occur in the middle of the North American continent, such as in the New Madrid seismic zone in Missouri.
— Gabi Laske, geophysicist, Cecil H. and Ida M. Green Institute for Geophysics and Planetary Physics