However, our planet's seemingly stable surface is made up of enormous pieces of rock that are slowly but constantly moving. Those pieces continually collide with and rub against one another, and sometimes their edges abruptly crack or slip and suddenly release huge amounts of pent-up energy. The stored energy is released in the form of an earthquake. An earthquake emits its power as three waves of energy . Primary, or waves are felt as a sudden jolt . Secondary , or S-waves arrive a few seconds later and are felt as a more sustained side- to- side shaking. S-waves and P-waves are two types of wave with different movements.