Earthquake glossary

Definitions are short summaries based on the general usage of institutions such as AFAD, Kandilli Observatory, USGS and EMSC. See the source links for detail.

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Magnitude

A measure of the size of an earthquake at its source; a logarithmic scale.

Magnitude is a single number computed from seismometer recordings that expresses the size of the earthquake at its source. The scale is logarithmic: one unit more corresponds to about 10 times larger ground-motion amplitude and about 32 times more released energy. Agencies use different methods and stations, so small differences for the same earthquake are normal.

Source: USGS Earthquake Glossary; AFAD Earthquake Department

ML – local magnitude

Magnitude from the largest amplitude at nearby stations (the modern form of the Richter scale).

ML is computed from the largest wave amplitude recorded at stations close to the earthquake. It is widely used for small and moderate local earthquakes; because it saturates for very large events, Mw is preferred for those.

Source: USGS Earthquake Glossary; AFAD Earthquake Department; Boğaziçi University Kandilli Observatory and Earthquake Research Institute

Mw – moment magnitude

Magnitude based on the seismic moment (fault area, slip and rock rigidity).

Moment magnitude is derived from the seismic moment (fault area × average slip × rigidity). It does not saturate for large earthquakes and therefore best describes the size of large events. Variants such as Mww or Mwr indicate the computation method.

Source: USGS Earthquake Glossary; AFAD Earthquake Department

Md – duration magnitude

Magnitude from the duration of the recorded signal; used for small earthquakes.

Md is based on the duration of shaking in the recording. Some networks use it for small local earthquakes.

Source: USGS Earthquake Glossary; Boğaziçi University Kandilli Observatory and Earthquake Research Institute

mb – body-wave magnitude

Magnitude from the P-wave amplitude at distant stations.

mb is computed from the P (body) wave amplitude measured at distant stations. It is used for quick first assessments of remote earthquakes and saturates for large events.

Source: USGS Earthquake Glossary; EMSC – Euro-Mediterranean Seismological Centre

Focal depth

Depth of the point where the earthquake started (the focus), in km.

Focal depth is how deep below the surface the earthquake started. Earthquakes are generally classed as shallow (0–70 km), intermediate (70–300 km) and deep (over 300 km). Shallow earthquakes of the same magnitude are usually felt more strongly. The first computed depth may be revised later.

Source: USGS Earthquake Glossary; AFAD Earthquake Department

Hypocentre (focus)

The point underground where the earthquake rupture began.

The hypocentre is the point below the surface where the rupture on the fault started. Its position is given by latitude, longitude and depth.

Source: USGS Earthquake Glossary; AFAD Earthquake Department

Epicentre

The point on the surface directly above the hypocentre.

The epicentre is the point on the Earth's surface directly above the hypocentre. The earthquake location shown on maps is the epicentre; in large earthquakes the rupture may extend for tens or hundreds of kilometres.

Source: USGS Earthquake Glossary; AFAD Earthquake Department

Intensity

An observational measure of the effects of shaking at a place (e.g. MMI, EMS-98).

Intensity describes the effects of an earthquake on people, buildings and the environment at a particular place, expressed in Roman numerals (I–XII). An earthquake has one magnitude but many intensity values that vary from place to place. Historical earthquakes are mostly known from intensity observations.

Source: USGS Earthquake Glossary; EMSC – Euro-Mediterranean Seismological Centre; AFAD Earthquake Department

Aftershock

Smaller earthquakes recorded in the same area after a larger one.

Aftershocks are earthquakes recorded on and around the ruptured fault after a mainshock. Their number tends to decrease over time. This platform groups aftershocks only by the time and location proximity of recorded earthquakes; it provides no information about future earthquakes.

Source: USGS Earthquake Glossary; AFAD Earthquake Department

Foreshock

An earthquake that preceded a larger one in the same area; identifiable only in hindsight.

An earthquake is called a foreshock only in hindsight, if a larger earthquake later occurs in the same area. Whether an earthquake is a foreshock cannot be known at the time.

Source: USGS Earthquake Glossary

Mainshock

The largest earthquake in a sequence.

The mainshock is the largest earthquake in a sequence of earthquakes related in time and space; the following events are called aftershocks.

Source: USGS Earthquake Glossary

Earthquake swarm

Many earthquakes in the same area over a short time without a distinct mainshock.

A swarm is a burst of many earthquakes in an area over a short time without a clear mainshock. The "unusual activity" indicator on this platform is a statistical comparison of the number of records with the area's past average; it is not an earthquake prediction.

Source: USGS Earthquake Glossary; AFAD Earthquake Department

Fault

A fracture in the crust along which two blocks move relative to each other.

A fault is a fracture surface along which two blocks of rock move relative to each other. By motion, faults are strike-slip (horizontal), normal (extension) and reverse/thrust (compression). Active fault data for Türkiye is published by MTA. An earthquake being close to a fault does not mean it occurred on that fault.

Source: General Directorate of Mineral Research and Exploration (MTA); USGS Earthquake Glossary

b-value (Gutenberg–Richter)

A statistical parameter describing the ratio of small to large earthquakes.

By the Gutenberg–Richter relation, the logarithm of the number of earthquakes at or above a magnitude decreases linearly with magnitude; the slope is the b-value, around 1 in most regions. On the platform the b-value is a descriptive summary of the records in the selected period and is not computed when there are too few records.

Source: USGS Earthquake Glossary

Magnitude of completeness (Mc)

The threshold above which a catalogue is assumed to record all earthquakes.

Mc is the threshold above which the station network records nearly all earthquakes in an area. Events below Mc may be missing from the catalogue; statistics such as the b-value use records above Mc.

Source: USGS Earthquake Glossary; EMSC – Euro-Mediterranean Seismological Centre

PGA – peak ground acceleration

The largest ground acceleration measured at a point, in m/s² or g.

PGA is the largest acceleration the ground reaches during shaking, recorded by accelerometers. It is an important parameter in structural engineering. Community sensor measurements are informational and do not replace official station records.

Source: USGS Earthquake Glossary; AFAD Earthquake Department

Seismic energy

An approximate value of the energy radiated as seismic waves.

On the platform energy is approximated from magnitude with the Gutenberg–Richter relation (log E = 1.5M + 4.8, joules). The TNT equivalent only gives a sense of scale; values depend on the magnitude type.

Source: USGS Earthquake Glossary

Automatic and reviewed solutions

The automatic solution first published by agencies may be updated after expert review.

Agencies publish an automatic location and magnitude minutes after an earthquake; these values may be updated after review by seismologists. The platform keeps every update in the revision history and names the source agency.

Source: AFAD Earthquake Department; Boğaziçi University Kandilli Observatory and Earthquake Research Institute; EMSC – Euro-Mediterranean Seismological Centre

UTC and Turkish time

Earthquake times are given in UTC; Turkish time (TSİ) is UTC+3.

Seismological agencies usually publish event times in Coordinated Universal Time (UTC). Turkish time is 3 hours ahead of UTC. The platform shows times converted to the user's time zone.

Source: AFAD Earthquake Department; EMSC – Euro-Mediterranean Seismological Centre