Earthquake Glossary — Key Terms Explained

Earthquake reporting uses a lot of words that sound simple until you actually need to understand them: magnitude, intensity, depth, epicenter, aftershock, fault, rupture, ShakeMap, and more.

This glossary explains the most useful earthquake terms in plain English. It is designed for readers who want to understand earthquake maps, USGS data, news reports, and regional earthquake pages without needing a geology degree first.

A

Active Fault

An active fault is a fault that has moved in the recent geologic past and may move again in the future. “Recent” can mean thousands of years in geology, so an active fault does not need to produce earthquakes every year to be considered important.

Aftershock

An aftershock is an earthquake that happens after a larger earthquake, usually in the same general area. Aftershocks occur as the crust adjusts after the main rupture.

They are usually smaller than the mainshock, but they can still be damaging, especially if buildings, roads, slopes, or bridges were already weakened. Aftershocks may continue for days, weeks, months, or even years after a major earthquake.

Amplitude

Amplitude is the size or height of a seismic wave recorded by an instrument. Larger wave amplitudes usually mean stronger ground motion at that recording station.

Amplitude is not the same as magnitude. Magnitude describes the earthquake as a whole, while amplitude is measured at a particular place.

Attenuation

Attenuation is the weakening of seismic waves as they travel through the Earth. In general, shaking becomes weaker with distance from the earthquake source.

Ground type matters too. Some soft soils can amplify shaking, while harder rock may reduce it.

B

Blind Fault

A blind fault is a fault that does not break the surface. Because there may be no visible fault trace, blind faults can be harder to identify before they rupture.

The 1994 Northridge earthquake in Southern California was associated with a blind thrust fault.

Body Waves

Body waves are seismic waves that travel through the inside of the Earth. The two main body waves are P-waves and S-waves.

P-waves are the fastest and usually arrive first. S-waves arrive after P-waves and usually produce stronger shaking.

C

Convergent Boundary

A convergent boundary is where two tectonic plates move toward each other. In some places, one plate is forced beneath another in a process called subduction.

Convergent boundaries can produce very large earthquakes, especially along major subduction zones.

Creep

Fault creep is slow, steady movement along a fault without a large earthquake. It can gradually offset roads, curbs, fences, pipes, and other structures built across the fault.

Creep does release some stress, but it does not automatically remove the risk of larger earthquakes.

Crust

The crust is Earth’s outermost solid layer. Earthquakes occur mostly in the crust and uppermost mantle where rocks can store stress and suddenly break or slip.

D

Depth

Depth is how far below Earth’s surface an earthquake begins. It is usually reported in kilometers.

Depth matters because shallow earthquakes generally produce stronger local shaking than deeper earthquakes of the same magnitude.

Common depth groups are:

Shallow: 0 to 70 km
Intermediate: 70 to 300 km
Deep: 300 to 700 km

Did You Feel It?

Did You Feel It? is a USGS system that collects public shaking reports after earthquakes. These reports help show where shaking was felt and how strong it was in different places.

Dip-Slip Fault

A dip-slip fault is a fault where movement is mostly vertical along the slope, or dip, of the fault plane. Normal faults and reverse faults are both types of dip-slip faults.

Divergent Boundary

A divergent boundary is where two tectonic plates move apart. These boundaries are common along mid-ocean ridges and can produce earthquakes as new crust forms and plates separate.

E

Earthquake

An earthquake is the shaking of the ground caused by the sudden release of energy in the Earth. Most earthquakes happen when blocks of rock suddenly slip along a fault.

Earthquake Early Warning

Earthquake early warning systems detect the first fast-moving seismic waves from an earthquake and send alerts before stronger shaking arrives in some locations.

In the western United States, ShakeAlert supports earthquake early warning in California, Oregon, and Washington. These alerts may provide seconds of warning, depending on how far you are from the earthquake.

Early warning is not earthquake prediction. The earthquake has already started.

Epicenter

The epicenter is the point on Earth’s surface directly above the hypocenter, where an earthquake begins underground.

News reports often describe earthquakes by their epicenter, but the actual starting point is below the surface.

F

Fault

A fault is a break or zone of breaks in Earth’s crust where rock has moved. Many earthquakes happen when stress builds up along a fault and is suddenly released.

There are three main fault types:

Normal fault: rock above the fault moves downward.
Reverse or thrust fault: rock above the fault moves upward.
Strike-slip fault: rock moves horizontally past the other side.

Fault Plane

The fault plane is the surface along which rocks slip during an earthquake. It may be vertical, angled, or gently dipping depending on the fault type.

Fault Trace

A fault trace is where a fault intersects the Earth’s surface. On maps, it is often shown as a line marking the surface position of the fault.

Focus

Focus is another word for hypocenter: the underground point where an earthquake starts.

Foreshock

A foreshock is an earthquake that happens before a larger earthquake in the same general area.

The tricky part is that scientists usually cannot identify a foreshock until after the larger earthquake happens. At the time, it may just look like a normal earthquake.

Frequency

Frequency describes how many wave cycles pass a point each second. In earthquake science, frequency helps describe the character of shaking.

Lower-frequency waves can affect larger structures, while higher-frequency shaking may be more noticeable in smaller structures and objects.

G

GeoJSON

GeoJSON is a data format used for geographic information. USGS earthquake feeds use GeoJSON, which is why earthquake data can be shown on maps and in tables.

Ground Acceleration

Ground acceleration describes how quickly the ground changes speed during shaking. It is one way scientists and engineers measure earthquake motion. A common related measure is peak ground acceleration, or PGA

Ground Failure

Ground failure is a broad term for ground movement caused by earthquake shaking. It can include landslides, liquefaction, lateral spreading, settlement, and cracking.

Ground Shaking

Ground shaking is the movement people feel during an earthquake. It is usually the main cause of earthquake damage.

Shaking depends on magnitude, depth, distance, fault type, local soil, and building design.

H

Hanging Wall

The hanging wall is the block of rock above an angled fault plane. In a normal fault, the hanging wall moves down. In a reverse or thrust fault, it moves up.

Hazard

Earthquake hazard describes the chance that a location will experience a certain level of earthquake shaking or related effects over a period of time.

Hazard is not the same as risk. Hazard is about the natural event. Risk also includes people, buildings, and infrastructure that may be affected.

Hypocenter

The hypocenter is the point underground where an earthquake begins. The epicenter is the point directly above it on the surface.

I

Induced Seismicity

Induced seismicity refers to earthquakes caused or influenced by human activity. Examples can include wastewater injection, reservoir filling, mining, geothermal activity, or other underground fluid changes.

Not all human activity causes earthquakes, but in some places it can increase earthquake rates.

Intensity

Intensity describes how strongly an earthquake is felt at a specific location.

Unlike magnitude, intensity changes from place to place. It depends on distance from the epicenter, depth, local soil, building type, and other factors.

In the United States, intensity is often described using the Modified Mercalli Intensity scale.

Intraplate Earthquake

An intraplate earthquake happens within a tectonic plate, away from the plate boundaries where most large earthquakes occur.

Intraplate earthquakes are less common than plate-boundary earthquakes, but they can still be damaging.

L

Landslide

A landslide is the movement of rock, soil, or debris down a slope. Earthquake shaking can trigger landslides, especially on steep or unstable slopes.

Lateral Spreading

Lateral spreading is a type of ground failure where blocks of ground move sideways, often on gentle slopes or near riverbanks, coastlines, or artificial fill.

It is commonly associated with liquefaction.

Liquefaction

Liquefaction happens when water-saturated, loose sediment temporarily loses strength during earthquake shaking and starts behaving more like a liquid.

This can cause buildings, roads, bridges, and underground utilities to sink, tilt, crack, or shift. Areas built on loose fill, river deposits, or coastal sediments can be more vulnerable.

Love Wave

A Love wave is a type of surface wave. Love waves move the ground side to side, horizontally, and can produce strong shaking near the surface.

Love waves are one of the two main surface wave types, along with Rayleigh waves.

M

Magnitude

Magnitude is a number that describes the size of an earthquake at its source. It is based on seismic recordings and does not change depending on where you are.

A magnitude 6 earthquake is not just slightly larger than a magnitude 5. The scale is logarithmic, meaning each whole step represents about 32 times more energy released.

See Earthquake Magnitude Scale for a fuller explanation.

Mainshock

The mainshock is the largest earthquake in a sequence. Smaller earthquakes before it may later be called foreshocks, and smaller earthquakes after it are called aftershocks.

Mercalli Intensity Scale

The Mercalli Intensity Scale describes the effects of earthquake shaking at a specific location, based on what people felt and what damage occurred.

The Modified Mercalli Intensity scale runs from I, meaning not felt, to XII, meaning extreme damage.

Moment Magnitude

Moment magnitude, often written as Mw, is a modern earthquake magnitude scale used for many larger earthquakes. It is based on seismic moment and gives a better estimate of very large earthquake size than older local magnitude scales.

Moment Tensor

A moment tensor is a scientific description of the forces involved in an earthquake source. It helps seismologists understand fault orientation, fault type, and how the earthquake rupture moved.

This is more technical than most public earthquake reports, but it is often used in scientific earthquake analysis.

N

Normal Fault

A normal fault is a dip-slip fault where the block above the fault plane moves downward relative to the block below. Normal faults are common where the crust is being stretched.

O

Oblique-Slip Fault

An oblique-slip fault has both horizontal and vertical movement. In other words, the fault movement is a mix of strike-slip and dip-slip motion.

Omori’s Law

Omori’s Law describes how aftershocks usually become less frequent over time after a mainshock.

This does not mean aftershocks stop immediately. After a large earthquake, aftershocks can continue for weeks, months, or even longer.

P

P-Wave

A P-wave, or primary wave, is the fastest type of seismic wave. It is a body wave that can travel through solids, liquids, and gases.

P-waves usually arrive first at seismic stations and are important for earthquake early warning systems.

Peak Ground Acceleration

Peak ground acceleration, or PGA, is the highest acceleration recorded or estimated during earthquake shaking at a location.

Engineers use PGA and related ground-motion measures to understand how strongly the ground moved during an earthquake.

Plate Boundary

A plate boundary is where two tectonic plates meet. Many earthquakes happen near plate boundaries because that is where stress builds up as plates collide, pull apart, or slide past each other.

Plate Tectonics

Plate tectonics is the scientific theory that Earth’s outer shell is broken into moving plates. The movement and interaction of these plates causes earthquakes, volcanoes, mountain building, and ocean trenches.

R

Rayleigh Wave

A Rayleigh wave is a type of surface wave. Rayleigh waves move the ground in a rolling motion, somewhat like ocean waves.

They can contribute to the longer, rolling shaking people may feel during larger earthquakes.

Reverse Fault

A reverse fault is a dip-slip fault where the block above the fault plane moves upward relative to the block below. Reverse faults are common where the crust is being compressed.

A thrust fault is a low-angle reverse fault.

Richter Scale

The Richter scale is an older local magnitude scale developed for Southern California earthquakes. It is still commonly mentioned in everyday language, but modern earthquake agencies use different magnitude scales depending on the earthquake.

For many larger global earthquakes, moment magnitude is the preferred scale.

Ring of Fire

The Ring of Fire is a major belt of earthquakes and volcanoes around the Pacific Ocean. It includes places such as Japan, Indonesia, the Philippines, New Zealand, Chile, Mexico, Alaska, the Pacific Northwest, and parts of California.

Rupture

Rupture is the actual breaking and slipping of rock along a fault during an earthquake. Larger earthquakes usually involve larger rupture areas and more slip.

Rupture Length

Rupture length is the length of the fault section that slipped during an earthquake. Very large earthquakes can rupture hundreds of kilometers of fault.

S

S-Wave

An S-wave, or secondary wave, is a seismic body wave that arrives after the faster P-wave. S-waves move material side to side or up and down, perpendicular to the direction the wave travels.

S-waves usually produce stronger shaking than P-waves and do not travel through liquids.

Seismic Hazard

Seismic hazard describes the chance that a place will experience earthquake shaking or related effects, such as liquefaction or landslides, over a certain time period.

It is used in building codes, planning, insurance, and emergency preparation.

Seismic Moment

Seismic moment is a physical measure of earthquake size. It is based on the area of the fault that slipped, how much it slipped, and the strength of the rock.

Moment magnitude is calculated from seismic moment.

Seismic Risk

Seismic risk describes the potential consequences of earthquake shaking. It combines hazard with exposure and vulnerability.

For example, a remote area may have high seismic hazard but lower risk if few people or buildings are exposed. A city near active faults may have much higher risk.

Seismic Swarm

A seismic swarm is a cluster of earthquakes in one area without one obvious mainshock. Swarms are common in volcanic and geothermal areas.

Seismic Wave

A seismic wave is energy that travels through the Earth after an earthquake or other sudden disturbance.

The main types are:

P-waves — body waves that arrive first
S-waves — body waves that arrive after P-waves
Love waves — surface waves with side-to-side motion
Rayleigh waves — surface waves with rolling motion

Seismicity

Seismicity means earthquake activity in a region. A highly seismic area has frequent earthquakes or a history of significant earthquakes.

Seismogram

A seismogram is the record produced by a seismometer or seismograph. It shows ground motion over time and helps scientists identify wave arrivals, estimate magnitude, and locate earthquakes.

Seismograph

A seismograph is an instrument system that records ground motion. People often use “seismograph” and “seismometer” casually, though technically the seismometer is the sensing part and the seismograph records the motion.

Seismometer

A seismometer is an instrument that detects ground motion. Seismometers are used by earthquake monitoring networks to locate earthquakes and estimate magnitude.

Seismology

Seismology is the scientific study of earthquakes and seismic waves.

ShakeAlert

ShakeAlert is the earthquake early warning system for the western United States. It supports alerts in California, Oregon, and Washington.

ShakeAlert does not predict earthquakes. It detects an earthquake after it begins and may warn some areas before stronger shaking arrives.

ShakeMap

ShakeMap is a USGS product that shows the estimated shaking intensity from an earthquake across a region.

ShakeMaps help show how shaking varied from place to place after a significant earthquake.

Slip

Slip is the amount of movement along a fault during an earthquake. Larger earthquakes usually involve more slip and/or a larger fault area.

Stress

Stress is force acting on rock. Earthquakes happen when built-up stress overcomes friction or strength along a fault and the rocks suddenly slip.

Strike-Slip Fault

A strike-slip fault is a fault where the two sides move horizontally past one another. The San Andreas Fault is a well-known example.

Subduction Zone

A subduction zone is a plate boundary where one tectonic plate dives beneath another. Subduction zones can produce some of the world’s largest earthquakes and tsunamis.

Surface Rupture

Surface rupture happens when an earthquake rupture reaches and breaks the ground surface. It can offset roads, fences, pipelines, streams, and other features crossing the fault.

Not every earthquake produces surface rupture.

Surface Waves

Surface waves travel along Earth’s surface. They usually arrive after P-waves and S-waves and can produce strong, prolonged shaking.

The two main surface wave types are Love waves and Rayleigh waves. Love waves move the ground side to side, while Rayleigh waves create a rolling motion.

T

Tectonic Plates

Tectonic plates are large pieces of Earth’s outer shell that move slowly over time. Their movement creates earthquakes, volcanoes, mountain ranges, and ocean trenches.

Thrust Fault

A thrust fault is a low-angle reverse fault. Thrust faults are common in compressional settings, including many subduction zones and mountain belts.

Transform Boundary

A transform boundary is where two tectonic plates slide horizontally past each other. The San Andreas Fault system in California is a famous transform boundary.

Tsunami

A tsunami is a series of ocean waves caused by sudden displacement of water. Large, shallow undersea earthquakes are one possible cause, especially when they move the seafloor vertically.

Not every large offshore earthquake creates a tsunami. Fault type, depth, and seafloor movement all matter.

U

USGS

USGS stands for the U.S. Geological Survey. Its Earthquake Hazards Program monitors earthquakes, publishes earthquake data, and produces tools such as ShakeMap and Did You Feel It?.

Earthquakes Worldwide uses public USGS earthquake data for maps, lists, and statistics.

W

Wave Amplitude

Wave amplitude is another way of referring to the size of a seismic wave. Higher amplitude generally means stronger recorded motion at that station.

Wavelength

Wavelength is the distance between repeating parts of a wave, such as one wave crest to the next. In seismology, wavelength helps describe how seismic waves interact with different materials and structures.

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