The seismic tremors or earth movements that occur in the earth's crust when fracturing rocks or underground movement along a fault plane send out a series of three distinct sets of shock waves (earthquake waves).
The energy released by (or magnitude of) an earthquake is recorded on a seismograph, using the Richter scale. Magnitudes of earthquakes range from less than 3.4 where seismic activity does not impacted greatly, to a magnitude 8.6 where seismic activity releases energy equivalent to roughly 10,000 atomic bombs and can cause enormous damage.
Another scale used to describe earthquakes is the Modified Mercalli (MM). It rates the amount of shaking felt and damage caused (or intensity).