Earthquake Glossary

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Aftershock: A smaller earthquake that follows the largest shock (mainshock) of an earthquake sequence, within about one to two rupture lengths of it. Aftershocks can continue for weeks to years.

Depth: How far below the reference level an earthquake's hypocentre is, in kilometres. Earthquakes occur from near the surface to about 700 km down. Depth is usually the least certain part of a location.

Earthquake early warning: A system that detects an earthquake that has already started, rapidly estimates its location, magnitude and shaking, and sends alerts before strong shaking arrives at some places. It is not prediction.
Earthquake sequence: A group of related earthquakes close together in space and time, such as a mainshock with its foreshocks and aftershocks. Aftershock forecasts describe how a sequence is expected to continue.
Epicentre: The point on the Earth's surface directly above the hypocentre, where an earthquake rupture begins.

Fault: A fracture along which the blocks of crust on either side have moved relative to one another, parallel to the fracture.
Focal depth: The depth of an earthquake's hypocentre.
Forecast: A statement of the probability that earthquakes above a certain size will occur in a defined area within a defined time window. A forecast is not a prediction of a specific earthquake.
Foreshock: A smaller earthquake that precedes the largest earthquake (mainshock) of a sequence. An earthquake can only be identified as a foreshock afterwards, once a larger one has occurred, and not all mainshocks have foreshocks.

Geoid: A model of global mean sea level, extended under the continents, used as a reference surface for heights. USGS uses the WGS84 geoid as the reference datum for earthquake depths.

Hypocentre (focus): The point within the Earth where an earthquake rupture starts. The epicentre is the point directly above it on the surface.

Intensity: A measure of the strength of shaking and its effects at a particular place. One earthquake produces many intensities, which vary with distance, depth, rupture direction and local ground conditions.

Landslide: A movement of surface material down a slope. Strong earthquake shaking can trigger landslides in steep terrain.
Liquefaction: A process in which water-saturated sediment temporarily loses strength and behaves like a fluid during strong shaking, which can damage buildings and infrastructure built on it.

Magnitude: A number that characterises the relative size of an earthquake at its source, calculated from seismograph recordings. Scales are logarithmic: each whole step is about 32 times more energy.
Mainshock: The largest earthquake in a sequence, sometimes preceded by foreshocks and almost always followed by aftershocks.
Modified Mercalli Intensity (MMI): A scale of shaking intensity written in Roman numerals from I (not felt) to X and above (extreme), based on observed effects on people, objects and buildings.

P wave: A compressional seismic body wave that shakes the ground back and forth in the direction the wave travels. P waves are the fastest seismic waves and arrive first.
Plate tectonics (tectonic plate): The theory that Earth's crust and upper mantle form several large, relatively rigid plates that move relative to one another. Most earthquakes occur near plate boundaries.

Review status: A catalog field showing whether an event's solution was computed automatically or reviewed by a person (for example automatic or reviewed), or was deleted. Reviewed solutions can still be updated.

S wave: A shear seismic body wave that shakes the ground perpendicular to the direction the wave travels. S waves are slower than P waves and often cause stronger shaking.
Seismic wave: An elastic wave generated by an impulse such as an earthquake or explosion. Seismic waves travel through the Earth (body waves) or along its surface (surface waves).
ShakeMap: A USGS map product that estimates ground shaking across a region after an earthquake, combining instrument recordings, felt reports and models. It is an estimate, not a damage assessment.
Subduction: The process in which an oceanic plate descends beneath another plate into the mantle. Subduction zones produce the largest and the deepest earthquakes.

Tsunami: A series of sea waves caused by a large, sudden displacement of water, most often from a large undersea earthquake, but also from submarine landslides or volcanic activity.