Earthquakes cause the fracturing of rock masses along faults. This causes sudden movement along faults. The point at which the Earth breaks at which the movement occurs is called the focus of the earthquake. The point on the surface of the Earth above the focus is the epicenter. An earthquake occurs when rocks have been stressed. This stress is stored until the actual break (the earthquake) releases the energy. This energy travels in the form of waves. Geologist record and study these waves.

There are many different types of waves generated from earthquakes. The major types are P (push/pull, compression, primary) and S (shear, secondary). P waves are faster than S waves. S waves cannot travel through liquid. P waves can travel through liquids and solids. These basic facts (plus many more) help geologists to interpret the inner structure of the Earth, and create a picture of the inside of the Earth. Without data from waves, we could not determine the boundaries. We cannot even drill through the upper part of the crust.