Shockwaves

     Shock waves can be classified into two broad categories.Waves that send particles oscillating back and forth in the same direction as the waves are traveling are called primary.
    Primary waves, sometimes called compressional waves, travel through the earth beneath the crust. Secondary waves cause vibrations which move perpendicular to the wave. These waves travel on the surface of the earth and move much slower than primary waves. Thus, when an earthquake occurs, seismic centers throughout the world record primary waves before the secondary waves arrive.