Fault
A fracture in the Earth's crust where blocks of rock move past each other, releasing quakes.
A fault is a crack in the Earth's crust where two blocks of rock can move relative to each other. Earthquakes happen when stress overcomes the friction holding a fault locked, and it slips suddenly.
Faults come in types based on motion: strike-slip faults slide horizontally, like the San Andreas; normal faults pull apart; and reverse or thrust faults push together, common in subduction zones. A fault's length and type set the maximum quake it can produce, which is why mapping faults is central to forecasting regional hazard.