Earthquake Glossary

Aftershocks: Aftershocks are earthquakes that follow the largest shock of an earthquake sequence. They are smaller than the mainshock and within 1-2 rupture lengths distance from the mainshock. Aftershocks can continue over a period of weeks, months, or years. In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue.

Benioff zone: A dipping planar (flat) zone of earthquakes that is produced by the interaction of a downgoing oceanic crustal plate with a continental plate. These earthquakes can be produced by slip along the subduction thrust fault or by slip on faults within the downgoing plate as a result of bending and extension as the plate is pulled into the mantle. Also known as the Wadati-Benioff zone.

Body wave: A body wave is a seismic wave that moves through the interior of the earth, as opposed to surface waves that travel near the earth's surface. P and S waves are body waves. Each type of wave shakes the ground in different ways.

Compressional wave: A compressional wave or P wave, or, is a seismic body wave that shakes the ground back and forth in the same direction and the opposite direction as the direction the wave is moving

Displacement: Displacement is the difference between the initial position of a reference point and any later position. The amount any point affected by an earthquake has moved from where it was before the earthquake.


Earthquake hazard: Earthquake hazard or seismic hazard is anything associated with an earthquake that may affect the normal activities of people. This includes surface faulting, ground shaking, landslide, liquefaction, tectonic deformation, tsunamis, and seiches.

Earthquake risk: Earthquake risk or seismic risk is the probable building damage, and number of people that are expected to be hurt or killed if a likely earthquake on a particular fault occurs. Earthquake risk and earthquake hazard are occasionally incorrectly used interchangeably.

Epicenter: The epicenter is the point on the earth's surface vertically above the hypocenter (or focus), point in the crust where a seismic rupture begins.


Fault scarp: The fault scarp is the feature on the surface of the earth that looks like a step caused by slip on the fault.

Fault-plane solution: A fault plane solution is a way of showing the fault and the direction of slip on it from an earthquake, using circles with two intersecting curves that look like beach balls. Also called a focal-mechanism solution.

Fault plane: The fault plane is the planar (flat) surface along which there is slip during an earthquake.

Fault trace: The fault trace is the intersection of a fault with the ground surface; also, the line commonly plotted on geologic maps to represent a fault.

First motion: On a seismogram, the first motion is the direction of ground motion as the P wave arrives at the seismometer. Upward ground motion indicates an expansion in the source region; downward motion indicates a contraction.

Focal depth: The focal depth refers to the depth of an earthquake hypocenter. On earthquake information bulletin it is refer to as just depth.

Focal mechanism: The focal mechanism of an earthquake describes the deformation in the source region that generates the seismic waves. In the case of a fault-related event it refers to the orientation of the fault plane that slipped and the slip vector and is also known as a fault-plane solution.

Focus: Also known as hypocenter is the point within the earth where an earthquake rupture starts. The epicenter is the point directly above it at the surface of the Earth.


Frequency: The frequency is the number of times something happens in a certain period of time, such as the ground shaking up and down or back and forth during an earthquake.

Graben: A graben is a down-dropped block of the earth's crust resulting from extension, or pulling, of the crust

Ground failure: The term ground failure is a general reference to landslides, liquefaction, lateral spreads, and any other consequence of shaking that affects the stability of the ground.

Ground motion: Ground motion is the movement of the earth's surface from earthquakes or explosions. Ground motion is produced by waves that are generated by sudden slip on a fault or sudden pressure at the explosive source and travel through the earth and along its surface.

Harmonic tremor: Harmonic tremor describes continuous rhythmic earthquakes that can be detected by seismographs. Harmonic tremors often precede or accompany volcanic eruptions.

Horst: A horst is found together with a graben in an extensional environment. The graben is the downdropped block and the horst is the upthrown block that lies next to the graben.

Intensity: A measure of an earthquake's extreme strength, force, energy or effects at specific locations. The most commonly-used scale of intensity is the Modified Mercalli Intensity (MMI) index, which defines a range from one to twelve (Roman numeral: I to XII), with one being the lowest and XII the highest intensity. There are many intensities for an earthquake, depending on where you are, unlike the magnitude, which is one number for each earthquake.

Interplate: Interplate pertains to processes between the earth's crustal plates.

Interplate coupling: Interplate coupling is the ability of a fault between two plates to lock and accumulate stress. Strong interplate coupling means that the fault is locked and capable of accumulating stress, whereas weak coupling means that the fault is unlocked or only capable of accumulating low stress.

Intraplate: Intraplate pertains to processes within the plates.

Isoseismal (line): An isoseismal (line) is a contour or line on a map bounding points of equal intensity for a particular earthquake.

Kinematics: Kinematic refers to the general movement patterns and directions of the earth's rocks that produce rock deformation.

Landslide: A landslide is a movement of surface material down a slope.


Locked fault: A locked fault is a fault that is not slipping because frictional resistance on the fault is greater than the shear stress across the fault (it is stuck). Such faults may store strain for extended periods that is eventually released in an earthquake when frictional resistance is overcome.

Love wave: A Love wave is a surface wave having a horizontal motion that is transverse (or perpendicular) to the direction the wave is traveling.


Mainshock: The mainshock is the largest earthquake in a sequence, sometimes preceded by one or more foreshocks, and almost always followed by many afterschocks.

Microzonation: Microzonation is the identification of separate individual areas having different potentials for hazardous earthquake effects.

Moho: The Moho is the boundary between the crust and the mantle in the earth. This is a depth where seismic waves change velocity and there is also a change in chemical composition. Also termed the Mohorovicic' discontinuity after the Croatian seismologist Andrija Mohorovicic' (1857-1936) who discovered it. The boundary is between 25 and 60 km deep beneath the continents and between 5 and 8 km deep beneath the ocean floor

Moment Magnitude: see magnitude

Moment Tensor: A mathematical representation of the movement on a fault during an earthquake, comprising of nine generalized couples, or nine sets of two vectors. The tensor depends of the source strength and fault orientation. See also seismic moment.

Normal fault: See dip slip.

Normal stress: The normal stress is that stress component perpendicular to a given plane. If you lean against a door after you close it, you are applying normal stress to the door. Normal stress can either be compressional or tensional.

Oceanic Trench: An oceanic trench is a linear depression of the sea floor caused by the subduction of one plate under another. The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. They are also the deepest parts of the ocean floor. Oceanic trenches are a distinctive morphological feature of convergent plate boundaries.

Paleoseismicity: Paleoseismicity refers to earthquakes recorded geologically, most of them unknown from human descriptions or seismograms. Geologic records of past earthquakes can include faulted layers of sediment and rock, injections of liquefied sand, landslides, abruptly raised or lowered shorelines, and tsunami deposits.

Peak Acceleration: See acceleration

Period: The period is the time interval required for one full cycle of a wave.


Rayleigh wave: A Rayleigh wave is a seismic surface wave causing the ground to shake in an elliptical motion, with no transverse, or perpendicular, motion.

Reflection: The reflection is the energy or wave from an earthquake that has been returned (reflected) from an boundary between two different materials within the earth, just as a mirror reflects light.

Refraction: Refraction is (1) the deflection, or bending, of the ray path of a seismic wave caused by its passage from one material to another having different elastic properties. (2) bending of a tsunami wave front owing to variations in the water depth along a coastline.


Ring of fire: The "Ring of Fire", also called the Circum-Pacific belt, is the zone of earthquakes surrounding the Pacific Ocean- about 90% of the world's earthquakes occur there.


Rupture velocity: The speed at which a rupture front moves across the surface of the fault during an earthquake.

Sand boil: A sand boil is sand and water that come out onto the ground surface during an earthquake as a result of liquefaction at shallow depth.

Segmentation: Segmentation is the breaking up of a fault along its length into several smaller faults. This can happen as a result of other faults crossing it, topography changes, or bends in the strike of the faults. Segmentation can limit the length of faulting in a single earthquake to some fraction of the total fault length, thus also limiting the size of the earthquake.

Seismic waves: Vibrations that travel outward from the earthquake fault at speeds of several kilometres per second. While fault slippage directly under a structure can cause considerable damage, the vibrations produced by seismic waves cause most of the destruction during earthquakes.



S wave: An S wave (secondary wave), or shear wave, is a seismic body wave that shakes the ground back and forth perpendicular to the direction the wave is moving.

Subduction zone: This is where two lithospheric plates come together, one overriding the other while another subducting beneath the other. Benioff zone, also known as the Wadati-Benioff zone is part of subduction zone where earthquakes occurred.