Earth‘s rocky crust is broken into about ten giant masses of rock, called plates, which move slowly on the mantle below. When the plates bump against each other or split apart, they break along cracks called faults.

Movement along faults is sometimes smooth. But other times, the rocks on each side of the fault get stuck. Over time, pressure builds and builds until—BAM!—the rocks are forced apart! This releases powerful shock waves underground. When these shock waves reach the surface, we feel an earthquake.

Scientists know where many of Earth‘s faults are, so they can often predict earthquakes will happen. Unfortunately, they can‘t predict exactly when they will occur.