Overview
An earthquake is the abrupt shaking of the ground produced when stress accumulated in Earth’s crust is released. That release radiates energy as seismic waves that travel through and across the surface. The initial point of rupture deep inside the ground is called the hypocenter or focus (hypocenter), and the location on the surface directly above it is called the epicenter (epicenter). Small earthquakes happen every day worldwide; only a few reach sizes that cause major damage or loss of life.
Causes and seismic waves
Most earthquakes are associated with the motion of tectonic plates. Plates slide, collide, or move past one another along faults; when friction temporarily locks a segment of a fault, strain builds until it is released in a sudden slip. That slip generates different types of seismic waves: compressional P-waves, shear S-waves, and slower surface waves that usually do the most damage. Seismology (seismology) is the scientific study of these waves, their sources, and what they reveal about Earth’s interior.
Detection and measurement
Ground motions are recorded by instruments called seismometers or seismographs (seismometer), which allow scientists to determine an event’s location, depth, and timing. Earthquake size is described in two different ways: magnitude and intensity. Magnitude expresses the energy released by an event; traditional local magnitude scales such as the Richter scale (Richter scale) are now largely superseded by the moment magnitude scale (Mw) for large earthquakes. Intensity measures the strength of shaking and its effects at particular places, commonly expressed with the Modified Mercalli Intensity scale. A large historic quake reached magnitude 9.5 in Chile in 1960, the largest instrumentally recorded event.
Hazards and effects
Direct effects of earthquakes include ground rupture, severe shaking, and secondary hazards. Shaking frequently triggers landslides (landslides) on slopes or causes liquefaction in water-saturated sediments. Underwater earthquakes can displace large volumes of water and generate tsunamis (tsunami) that travel great distances and amplify coastal damage. The distribution of earthquakes is not random: most events cluster along plate boundaries and known fault zones (fault lines), though damaging intraplate earthquakes also occur.
Human response, mitigation, and notable facts
Because the exact timing of earthquakes cannot be reliably predicted, preparedness focuses on reducing vulnerability: seismic building codes, retrofitting older structures, public education, land-use planning, and early-warning systems that detect initial P-waves and send rapid alerts can provide seconds to tens of seconds of advance notice. After a major rupture, foreshocks and aftershocks are common; the latter can continue for days to years and complicate recovery.
- Who studies them: seismologists and earthquake engineers analyze causes and design resilient structures.
- Measurement tools: seismometers, accelerometers, and networks of stations improve location and magnitude estimates.
- Important distinction: magnitude (energy released) vs intensity (local shaking and damage).
Understanding earthquakes combines field observations, laboratory experiments, and computer models to refine hazard maps and inform policy. While prediction of exact events remains beyond current capability, improved monitoring and engineering have reduced loss of life in many earthquake-prone regions. For further technical information see seismology resources (seismology) and instrument descriptions (seismometer).
Additional references and regional guides may be found via geological surveys and emergency management organizations; examples and educational material are available through many public portals (hypocenter, epicenter, Richter, fault, tsunami, landslide).
