Two tectonic plates shift within Earth at the focus of the earthquake. The energy released by their movement ripples outward to the surrounding rock in waves, called earthquake waves, seismic waves, or shock waves. The waves cause the rock to expand and then compress as the energy moves through it.
Imagine holding a taut piece of rope tied to a doorknob in front of you. If you flick your wrist, a pulse, or wave, will travel through the rope similarly to the way in which a shock wave travels.
You can easily see the up-and-down wave action. These vertical waves are called transverse waves. Not easily seen are longitudinal waves, which expand and compress the rope horizontally. It is easier to visualize longitudinal waves by thinking of snapping a rubber band. The elastic expands and compresses in the same way that rocks affected by shock waves do.
These shock waves travel from the focus of an earthquake to the epicenter. When they reach the surface, and are no longer bound by neighboring rock, their energy can cause severe damage.
An earthquake’s focus marks where underground rock can no longer bear the stress of Earth’s internal shifts. Shock waves travel out from the focus, causing adjacent rock to expand and compress as the earthquake’s energy passes through.
The epicenter of the quake is where the shock waves break through Earth’s surface.