Seismic waves are vibrating movement of the ground. Seismic waves can be caused by underground explosions, volcanic eruptions and man-made explosions that can vibrate the ground. Seismic waves go through the Earth’s layers. The speed of seismic waves depends on the type of materials they meet. When they meet different kind of material, they bounce or go in different directions. Seismic waves can be detected by seismographs.