Earthquakes: Causes, Measurement, Hazards, and Preparedness

Earthquake Definition and Seismicity

An earthquake is the shaking of Earth’s surface caused by a sudden release of energy in the lithosphere, producing seismic waves. Most earthquakes result from the rupture and movement of geological faults, although volcanic activity, landslides, mining, hydraulic fracturing, and explosions can also generate seismic events. The initial rupture point is the hypocenter, or focus, while the point directly above it on the surface is the epicenter. Earthquakes vary from imperceptible tremors to destructive events capable of damaging infrastructure, triggering landslides, causing soil liquefaction, and generating tsunamis when large offshore earthquakes displace the seabed. Seismicity describes the frequency, types, and sizes of earthquakes in a region over a given period, while seismicity at a specific location can also mean the average rate of seismic energy released per unit volume. Earthquakes occur most frequently near tectonic plate boundaries, especially around the Pacific Ring of Fire, where stress accumulates along normal, reverse or thrust, and strike-slip faults. According to the elastic-rebound theory, tectonic motion gradually stores strain energy until frictional resistance is overcome, causing sudden fault slip and energy release. Earthquake sequences may include foreshocks, mainshocks, aftershocks, or swarms, and approximately 500,000 earthquakes are detected worldwide each year, although only about 100,000 are generally felt.

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Earthquake Terminology

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