Earthquake

Most tectonic earthquakes occur when rock masses suddenly move along a fracture or fault in Earth's crust. Tectonic plates move slowly, but friction can lock sections of their boundaries; stress builds until the rocks slip, releasing energy that travels outward as seismic waves. Most earthquakes occur on faults associated with plate boundaries, although earthquakes also occur within plates, away from their edges. Smaller shocks that follow a larger event in the same area are called aftershocks and may continue for weeks or longer.

The point beneath the surface where an earthquake begins is its hypocenter, or focus; the point directly above it at the surface is the epicenter. Seismometers record ground motion and seismic waves. Because P waves and S waves travel at different speeds, the time gap between their arrivals helps estimate a station's distance from the source; readings from several stations can then locate the event. Magnitude is a single measure of an earthquake's size, whereas shaking intensity describes the effects at a particular place and can vary with distance and local ground conditions. Moment magnitude is generally preferred for measuring large earthquakes; the older Richter scale is not the standard for most large events.

The effects of an earthquake depend not only on its magnitude but also on its depth, distance from settlements, local geology and the strength of buildings and infrastructure. Strong shaking can damage buildings and bridges, rupture utilities and trigger landslides. A large, shallow earthquake beneath or near the ocean can also move the seafloor vertically and displace water, generating a tsunami. Not every offshore earthquake produces a tsunami; the rupture's location, depth and movement matter.

Networks of seismic stations allow scientists to detect earthquakes, locate their sources and estimate their magnitudes. This monitoring supports the study of active faults and the assessment of seismic hazard. Scientists cannot reliably specify the date, location and magnitude of a future major earthquake; instead, hazard assessments estimate probabilities for an area over a stated period. Building design and other long-term risk-reduction measures can reduce the harm caused when shaking occurs.