As two plates (sections of Earth’s crust) grind against one another in opposite directions, a great deal of force is exerted. If at some point along the boundary, the plates become locked together, the force will continually build. Suddenly, the locked plates can slip, causing a large movement in an instant. The shock of this sudden movement is transmitted through Earth’s crust in waves.
Similar to the way in which waves ripple outward when a stone is tossed into a pool, waves radiate outward from the focus, or origin, of an earthquake. While an earthquake usually begins underground, the epicenter is the point on the surface directly above the focus.
A quake’s severity is called magnitude, and is measured on a device called a seismograph.