Earthquake are usually caused when rock underground suddenly breaks along the fault. This sudden release of energy causes the seismic waves that make the ground shake. When two plates are rubbing against each other, they stick a little. They don`t just slide smoothly; the rocks catch on each other. The rocks are still pushing against each other, but not moving. After a while, the rocks break because of all the pressure that`s built up. When the rocks break, the earthquake occurs. During the earthquake and afterward, the plates of rock start moving, and they continue to move until they get stuck again. The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus is called the epicenter of the earthquake.