An earthquake is caused by the sudden movement of rock deep underground. It occurs when rocks along a fault slip, with the focus being where it originates and the epicenter being where it is felt on the surface. Before an earthquake, heavy objects should be secured and emergency supplies gathered. During shaking, one should drop, cover, and hold on to protect from falling objects. After, check for injuries, exit buildings cautiously, and watch for additional tremors. Being prepared can help remain calm and safe.