An earthquake is the motion or trembling of the ground produced by sudden displacement of rock in the Earth's crust.
Earthquakes result from crustal strain, volcanism, landslides, and collapse of caverns.
Stress accumulates in response to tectonic forces until it exceeds the strength of the rock. The rock then breaks along a preexisting or new fracture called a fault. The rupture travels in an irregular manner until the stress is relatively equalized. If the rupture disturbs the surface, it produces a visible fault. Although earthquakes in the United States occur most frequently in states west of the Rocky Mountains, devastating earthquakes have also occurred in the Midwest and East.