Many ideas have been proposed for the causation of earthquakes. Some are based upon natural factors, while others incorporate superstitions and religious beliefs. Harry F. Reid's elastic rebound theory is still regarded as a valid explanation for the occurrence of shallow tectonic earthquakes, regardless of whether the motion is horizontal or vertical. Although earth scientists agree that aseismic creep is related in some way to earthquakes, controversy exists over its specific role. Some believe that creep relieves an adequate amount of strain to prevent major earthquakes from occurring on the part of a fault experiencing aseismic slip. Crustal strain is usually indirectly determined by measuring the distance between geodetic markers on opposite sides of a fault with a telescope and transit rod or by triangulation where lengths are determined by angles measured with a theodolite. The chapter summarizes the major distinguishing parameters of earthquake waves from the work of Charles. F. Richter.