Fault: The Crack in the Ground Where Earthquakes Begin

Hold a sturdy wooden chopstick by both ends and slowly add pressure. At first it bends like a bow and holds on. Then you pass its limit, and it snaps in two with a sharp crack. You feel a jolt and a buzz right in your fingertips.

The rock under your feet goes through the same process. The huge plates that make up Earth's surface push and pull on each other, and over a very long time, enormous force builds up underground. Even solid rock bends a tiny bit and stores that force. But it can't hold on forever.

Finally, at its limit, the rock snaps and slips. The elastic energy it had been storing bursts out in every direction in an instant and shakes the whole ground. That shaking is the earthquake (the seismic waves) we feel. A brand-new fault forms when rock breaks like this, but most earthquakes happen on faults that already exist. The two sides stay stuck together by friction, then slip again.

The crack left behind by that broken, shifted ground is called a fault. In other words, a fault is a scar from past breaking, and also a weak surface. Once force builds up again, it can slip again and cause the next earthquake.

A fault looks different depending on which way the force acts. Picture a tilted playground slide, with the ground on either side of it facing each other.

The first type is the normal fault, which forms when the ground is pulled apart. Because of that pulling, the block sitting on top of the slope slides down the slide and drops.

Now reverse it. If the ground is squeezed hard from both sides, the block on the slope gets pushed up and rides over the other. This is called a reverse fault, and it's mostly found where plates collide head-on.

The last type doesn't move up or down at all. The ground just slides sideways, like two trains passing each other. This is called a strike-slip fault. The huge San Andreas Fault in the United States is one, and it creates strange scenes where roads and streams are cut and shifted sideways.

When you hike or look at a cliff, you can easily spot lots of cracks in the rock. But geologists don't call every crack a fault.

If a crack simply opens in the rock and the rock on both sides stays right where it was, it's called a joint. Only when the ground on both sides has actually slid along the crack do we call it a fault. So the key test is visible evidence of displacement, a sign that things have been shifted out of place.

Among faults, one that has moved in geologically recent times and could move again is called an active fault. What counts as "recent" differs from country to country. In Korea, it usually means a fault with signs of movement during the Quaternary period (the last 2.58 million years or so). Finding and studying exactly where active faults are hiding under the places we live is very important for preventing earthquake damage.