The San Andreas fault in California is very distinct in the Carrizo Plain east of the city of San Luis Obispo, CA. Many faults can not be seen at the Earth's surface like this. Click on image for full size (80 K JPG) Courtesy of USGS

When the giant blocks of rock that make up the Earth's crust are moved by plate tectonics, they are bound to bump into each other sometimes. These blocks of rock come in contact at faults. Sometimes they slide smoothly past each other. Other times the rocks get stuck - the rough surfaces of rock snag and they can't move along the fault. That might lead to an earthquake.

There might be no movement along a fault for a long time if the blocks of rock are hitched together. However, plate tectonic force continues to push the rocks so the energy builds up over decades, centuries, and sometimes even over millennia.

Eventually the energy is released as an earthquake when the force is large enough. The rock breaks, often very deep underground, and moves into a new position. Vibrations called seismic waves travel outward in all directions from the point where the energy was released. Like a stone tossed into a pond that sends concentric circles of ripples outward, the seismic waves radiate from the center of the earthquake. These seismic waves are what people on the surface of the Earth feel when they are in an earthquake.

There are different types of seismic waves. Some rumble the ground surface. Other types of seismic waves travel through the planet.