An earthquake is a sudden shaking of the ground caused by the rapid release of energy within the Earth, usually when rocks undergo sudden fault movement. The released energy travels away from the source as seismic waves, producing vibrations that may be weak enough to go unnoticed or strong enough to damage buildings and infrastructure. Earthquakes can occur at many depths and in many geological settings, although the majority are associated with tectonic processes.
The word earthquake describes the physical event rather than a particular measurement scale or level of damage. A very small earthquake and a great earthquake are both earthquakes even though their energy release and consequences differ enormously. Earthquakes can also be associated with volcanic activity, landslides, mining, fluid injection and other processes, but tectonic fault rupture is the dominant natural mechanism globally.
The physical process begins when stress within the crust causes a fault or fractured region to rupture. The rupture releases elastic strain energy that has accumulated in surrounding rocks. Seismic waves then propagate through the Earth and along its surface. The resulting ground motion depends on the earthquake source, the distance from the source, the geological materials through which waves travel and local surface conditions. A single earthquake can therefore produce very different levels of shaking in different locations.
Soft sediments can amplify particular frequencies of ground motion compared with hard bedrock, while basin structures can prolong shaking. The duration and character of shaking also depend on the size and geometry of the rupture. Earthquake effects are consequently determined by both the earthquake itself and the environment through which its energy propagates.
Earthquakes are commonly described according to their magnitude, depth, location and focal mechanism. Magnitude represents a property of the earthquake source and is related to the amount of energy released. It is not the same as intensity, which describes the observed effects or strength of shaking at a particular location. A single earthquake therefore has one principal magnitude value for a given measurement method, while it can have many intensity values because shaking varies geographically.
Depth is measured from the Earth's surface to the earthquake focus. The focal mechanism describes how the fault moved, such as normal, reverse or strike slip motion. These parameters provide complementary information and are essential for understanding the physical characteristics of an earthquake rather than relying solely on descriptions of damage.
Earthquakes occur in several major tectonic settings. At subduction zones, one plate descends beneath another and can become locked against the overriding plate. Large ruptures across these interfaces can produce some of the world's most powerful earthquakes. Continental collision zones can generate earthquakes through compression and faulting within thickened crust. Transform boundaries produce earthquakes associated with lateral motion along faults. Divergent boundaries commonly generate smaller earthquakes associated with crustal extension and magma related processes.
Earthquakes also occur within continental interiors where old faults are reactivated by regional stresses. The location of an earthquake therefore provides information about geological processes operating in the region. Seismologists combine earthquake locations with plate motion measurements, geological mapping and geophysical data to understand the underlying tectonic environment.
Earthquakes can be accompanied by several secondary phenomena. Strong ground shaking can cause landslides, rockfalls, liquefaction, structural collapse and failures of roads, bridges, dams and other infrastructure. Undersea earthquakes can displace the seabed and potentially generate tsunamis. Earthquakes can also produce surface faulting when rupture reaches the ground surface. Fires and hazardous material releases may occur when infrastructure is damaged. In mountainous areas, earthquake induced landslides can create temporary dams across rivers, which can later fail and produce flooding. These effects are not inherent to every earthquake. Their occurrence depends on earthquake magnitude and depth, local geology, terrain, infrastructure quality, population distribution and environmental conditions. The social consequences can therefore vary dramatically between earthquakes with similar physical characteristics.
The study of earthquakes is called seismology. Seismologists use instruments called seismometers to detect and record ground motion. Networks of instruments allow scientists to determine earthquake locations, depths, magnitudes and focal mechanisms. Historical records and geological evidence can extend knowledge beyond the period covered by modern instruments. Paleoseismology examines geological evidence of prehistoric earthquakes, particularly surface ruptures and displaced sediments, to estimate the recurrence behaviour of faults. Geodesy using techniques such as satellite positioning and radar interferometry can measure slow crustal deformation before and after earthquakes. Together these methods provide information about both individual events and long term seismic behaviour.
Earthquakes vary enormously in size. Small earthquakes may occur constantly in many active regions and may be detectable only by sensitive instruments. Moderate earthquakes can cause local damage, while large earthquakes can affect extensive areas. The largest known tectonic earthquakes occur mainly at subduction zone megathrusts, where enormous fault surfaces can rupture. Magnitude scales are logarithmic, so an apparently modest numerical difference represents a very large physical difference in energy release.
Earthquake frequency also follows statistical patterns, with many small events and relatively few very large events. After a major earthquake, numerous aftershocks commonly follow, although the exact sequence varies. Earthquake size, frequency and spatial distribution are therefore studied statistically as well as through individual case studies.
An earthquake is fundamentally a geological rupture event that converts stored mechanical energy into seismic energy and other forms of energy such as heat and permanent deformation. It is not simply the shaking itself, because the shaking is an observable consequence of the underlying source process. Understanding earthquakes requires distinguishing source properties from effects at the surface.
The earthquake source is described by parameters such as seismic moment, rupture area and average slip, while ground motion at a location depends on propagation and local site conditions. Modern earthquake science combines plate tectonics, fault mechanics, seismology, geodesy, engineering and statistics. This integrated approach allows scientists to investigate why earthquakes occur, how their waves travel and how their effects can be reduced, even though exact earthquake prediction remains beyond current scientific capability.
