Most earthquakes are caused by fault movements associated with tectonic plates. The stresses are located within the crust and upper mantle of the Earth. Pressure builds up until the stress is so great that rocks eventually break, and the stored stress is released as energy in the form of earthquakes. This can be simply demonstrated in the snap of a pencil as it bends and breaks. In the case of the Earth, the transfer of energy can be great, and can extend over large distances. For instance, if you drop a pebble in a tub of water, you will notice that as the pebble hits the water, waves of energy migrate away from the initial drop. The ripples transfer the energy of the impact of the pebble a great distance from where it hit the water.
The energy released during an earthquake migrates from the focus of the earthquake (the location where the first rocks break) as waves. These waves are called seismic waves. The two general types of vibrations produced by earthquakes are surface waves, which travel along the Earth's surface, and body waves, which travel through the Earth. Surface waves usually have the strongest vibrations and probably cause most of the damage.
The vibrations produced by earthquakes are detected, recorded, and measured by instruments call seismographs. The zigzag line made by a seismograph, called a "seismogram," reflects the changing intensity of the vibrations by responding to the motion of the ground surface beneath the instrument. From the data expressed in seismograms, scientists can determine the time, the hypocenter, the focal depth, and the type of faulting of an earthquake and can estimate how much energy was released.
The severity of an earthquake can be expressed in several ways. The magnitude of an earthquake, usually expressed by the Richter Scale, is a measure of the amplitude of the seismic waves. The moment magnitude of an earthquake is a measure of the amount of energy released, an amount that can be estimated from seismograph readings. The intensity, as expressed by the Modified Mercalli Scale, is a subjective measure that describes how strong a shock was felt at a particular location.
