An earthquake is the sudden release of accumulated elastic strain energy in the Earth's crust, typically along a fault, that generates seismic waves and causes ground shaking. Earthquakes range from imperceptible microtremors to catastrophic events that reshape cities and claim hundreds of thousands of lives.
An earthquake is a sudden release of elastic strain energy that has accumulated in the Earth's lithosphere, most commonly along pre-existing faults. The release occurs when the accumulated shear stress exceeds the frictional strength of the fault, causing rapid slip along the rupture surface. The energy released propagates outward as seismic waves, which cause ground shaking and can trigger secondary hazards including landslides, liquefaction, tsunamis, and fires.
The elastic rebound theory, formulated by Harry Fielding Reid after the 1906 San Francisco earthquake, remains the foundational framework for understanding earthquake mechanics. It states that rock masses on either side of a fault deform elastically under tectonic loading, and when the accumulated strain exceeds the fault's strength, the rock rebounds to an undeformed state β releasing stored energy as seismic waves. The cycle of strain accumulation and release repeats over decades to millennia, producing characteristic earthquake cycles on individual faults. Modern geodesy (GPS, InSAR) allows direct observation of strain accumulation, transforming seismic hazard assessment from a purely historical to a partly predictive science.
Earthquakes are classified by depth as shallow (0β70 km), intermediate (70β300 km), or deep (300β700 km), and by tectonic setting as interplate (occurring at plate boundaries) or intraplate (occurring within a plate). The vast majority of global seismicity concentrates along plate boundaries, particularly the Pacific Ring of Fire, the Alpide belt, and mid-ocean ridges. However, intraplate earthquakes β such as those in eastern North America, peninsular India, and parts of Iran β can be highly destructive despite their lower frequency, precisely because they occur in regions with little historical seismic awareness. Earthquake prediction remains an unsolved problem; modern practice focuses on probabilistic hazard assessment and structural resilience rather than short-term prediction.