What an earthquake is and how the ground actually moves

An earthquake — also called a quake, tremor, or temblor — is the shaking of the Earth's surface that results from a sudden release of energy in the lithosphere, energy that travels outward as seismic waves. Earthquakes range in intensity from those too weak to feel up through events strong enough to propel objects and people into the air, damage critical infrastructure, and destroy entire neighborhoods. The seismicity of an area is the frequency, type, and size of earthquakes experienced over time; the seismicity at a specific point in the Earth is the average rate of seismic energy released per unit volume.

The initial point of rupture is called the hypocenter or focus. The point on the ground surface directly above the hypocenter is called the epicenter. Most earthquakes are generated by movement along geological faults, but they can also be triggered by volcanism, landslides, and human activity such as deep-well wastewater injection, mining, and reservoir filling. The 1976 Tangshan earthquake in China killed more than 300,000 people; the 1960 Valdivia earthquake in Chile, at moment magnitude 9.5, remains the largest earthquake ever recorded.

Offshore earthquakes can displace the seabed and generate tsunamis, which often cause more damage than the original shaking. Earthquakes can also trigger landslides and soil liquefaction, where saturated loose soils briefly behave like a thick liquid. Faults are described by their motion: normal faults pull apart, reverse (or thrust) faults push together, and strike-slip faults grind sideways. Energy release follows the elastic-rebound theory — strain accumulates in the rock until friction is overcome, the fault slips, and the strain is released as seismic waves.

This pack is general reference, not professional engineering or emergency-management advice. For local guidance, references describe consulting national geological surveys (such as the United States Geological Survey), regional emergency-management agencies, and current building-code authorities.