What is a tectonic fault?

A fault is a crack in the Earth crust along which rock masses have shifted past one another, driven by the slow movement of tectonic plates. Stress builds up as the plates push, pull, or grind against each other, and when the rock can no longer withstand it, the two sides slip suddenly, releasing energy as an earthquake. Faults come in several types: normal faults, where rock pulls apart; reverse or thrust faults, where rock is pushed together; and strike-slip faults, where blocks slide horizontally, like the famous San Andreas Fault in California. Faults can range from tiny cracks to boundaries thousands of kilometres long. The point underground where a slip begins is the focus, and the spot directly above it on the surface is the epicentre.