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Before an earthquake occurs, cracks would start to appear. This is due to the the pressure that builds up in the rock layers. The rocks will eventually break apart, producing seismic waves. The first waves are called body waves that travel through the earth, while surface waves travel through the surface of the earth, as denoted by the name. Body waves will soon reach the surface and some would be changed to surface waves. Such waves are what causes the violent jolts at the epicenter. Subsequent tremors after the first occur because the disturbed rocks settle into a new position. Waves can be divided into primary and secondary waves. Primary waves travel at very high speeds (up to 5 miles per second) through almost any medium, solid, liquid or air. They are also amongst the first waves to be released. Secondary waves come after primary ones. They travel at a slower speed (3.75 miles per second). Back (Types of plate boundaries) | Next (Damages) |