Measured by determining the amount of slip and the area of the rupture surface. Another measure is intensity - more about that later. Occur all along the fault plane, often outside the are that actually ruptured. Rock breaks, starting at a point.
Energy is released as the fracture occurs.
Most of this energy is dissipated by breaking and crushing of rocks, and by release of heat. A small part of the energy travels outward as seismic waves . When these waves reach the surface, they create ground motion that we feel as an earthquake. Body waves - travel within the Earth. These are generated by rock fracture and radiate outward along a spherical wavefront.