Earthquakes are motions of the earth's crust. Essentially, they are big sound waves that travel through the earth. There are two types of waves. The first type is called a longitudinal or P wave, where the earth compresses in the same direction as the wave travels. The second type is called a transverse, or S wave, where the compression in the earth is perpendicular to the direction of the wave. P waves travel roughly twice as fast as S waves.
