Glossary of Earthquake Terms

Worldwide glossary of common earthquake-related terms (compiled by James Daniell)

(shortened list of the most important terms)

Acceleration
The rate of change of velocity of a reference point. Commonly expressed as a fraction or percentage of the acceleration due to gravity (g) where g = 980 cm/s2. - (USGS)
Active Fault
A fault that is considered likely to undergo renewed movement within a period of concern to humans. Faults are commonly considered to be active if they have moved one or more times in the last 10,000 years, but they may also be considered active when assessing the hazard for some applications even if movement has occurred in the last 500,000 years. (USGS)
Aesthenosphere
The highly viscous mechanically weak region of the upper mantle of the Earth. It lies below the lithosphere, at depths between 100 and 200 km below the surface, but perhaps extending as deep as 400 km. (Wikipedia)
Aftershock
Secondary tremors that may follow the largest shock of an earthquake sequence. Such tremors can extend over a period of weeks, months, or years. USGS)
Blind fault
A fault that does not extend upward to the Earth™s surface. It usually terminates upward in the axial region of an anticline. If is dip is less than 45 degrees, it is a blind thrust. (IASPEI)
Body Waves
A seismic wave that propagates through the interior of the Earth, as opposed to surface waves that propagate near the Earth's surface. P and S waves are examples. Each type of wave has distinctive strain characteristics. (USGS)
Building Code
A building code, or building control, is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings and non-building structures. The main purpose of building codes are to protect public health, safety and general welfare as they relate to the construction and occupancy of buildings and structures. A seismic code, refers to a building code which uses earthquake-resistant design principles. (USGS - CEDIM)
Continental Crust
Outermost solid layer of the earth that forms the continents and is composed of igneous, metamorphic, and sedimentary rocks. Overall, the continental crust is broadly granitic in composition. Contrast with oceanic crust. (PDC)

Critical Structures
1) Structures whose ongoing performance during an emergency is required or whose failure could threaten many lives. May include (1) structures such as nuclear power reactors or large dams whose failure might be catastrophic; (2) major communication, utility, and transportation systems; (3) involuntary- or high-occupancy buildings such as schools or prisons; and (4) emergency facilities such as hospitals, police and fire stations, and disaster-response centers. , 2) The primary physical structures, technical facilities and systems which are socially, economically or operationally essential to the functioning of a society or community, both in routine circumstances and in the extreme circumstances of an emergency.2) Critical Loss Facilities are hospital and health care facilities, public buildings, telecommunications, airports, energy systems (coal, nuclear etc.), bridges and other facilities that are critical to the recovery and rehabilitation of a region post-earthquake. (IASPEI-UN/ISDR)
Crust
The outermost major layer of the Earth, ranging from about 10 to 65 km in thickness worldwide. The continental crust is about 40 km thick in the Pacific Northwest. The thickness of the oceanic crust in this region varies between about 10 and 15 km.The crust is characterized by P-wave velocities less than about 8 km/s. The uppermost 15-35 km of crust is brittle enough to produce earthquakes. The seismic crust is separated from the lower crust by the brittle-ductile boundary. (USGS)
Deep Earthquake
An earthquake whose focus is located more than 300 kilometers from the earth's surface. Earthquake-report.com differs from the official notification calling earthquakes with a depth of more than 100 km as "Deep". This is mainly because of the non-damaging impact of these earthquakes. (USGS)

Depth
The distance (usually measured in km) below the surface of the earth delineated by 0km (the mean spheroid). Also known as earthquake depth “ Earthquakes can occur anywhere between the Earth's surface and about 700 kilometers below the surface. For scientific purposes, this earthquake depth range of 0 “ 700 km is divided into three zones: shallow, intermediate, and deep. (USGS-CEDIM)
Disaster
An event that causes major disruption on the economy, society and the environment. Its origin or causes may be directly derived from natural phenomena, i.e. geophysical (as volcanic or seismic events that cause collapse of infrastructure, landslides or liquefaction, etc.) or climatic (as hurricanes, typhoons, tornadoes, major variation in rainfall both in terms of excess or deficit causing drought). Although usually not covered by the methodology, disasters may also have a human or anthropic origin as chemical spills, industrial accidents, or voluntarily caused events such as war, terrorist actions, etc. Disaster consequences or damage will always be associated with human intervention before, during and after the event (the œdisaster cycle). (UN)
Duration
Time interval between the first and last peaks of strong ground motion above a specified amplitude. (EQCanada)
Earthquake
Ground shaking and radiated seismic energy caused most commonly by sudden slip on a fault, volcanic or magmatic activity, or other sudden stress changes in the Earth. An earthquake of magnitude 8 or larger is termed a great earthquake.
Earthquake Risk
The expected (or probable) life loss, injury, or building damage that will happen, given the probability that some earthquake hazard occurs. Earthquake risk and earthquake hazard are occasionally used interchangeably. (USGS)
Epicenter
The point on the Earth's surface vertically above the point (focus or hypocenter) in the crust where a seismic rupture nucleates. (EQCanada)
Fault Plane
The surface on which the earthquake movement takes place. (CEDIM)
Fault Rupture
See rupture front (CEDIM)
Fault Scarp
Steplike linear landform coincident with a fault trace and caused by geologically recent slip on the fault. (USGS)
Fault Trace
Intersection of a fault with the ground surface; also, the line commonly plotted on geologic maps to represent a fault. (USGS)
Faults
A fracture along which there has been significant displacement of the two sides relative to each other parallel to the fracture. (USGS)
Focal Depth
A term that refers to the depth of an earthquake focus. (USGS)

Focus
See hypocenter (USGS)
Ground Motion (Shaking)
General term referring to the qualitative or quantitative aspects of 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. (USGS)
Gutenberg-Richter
Earthquakes appear to follow a pattern through time in terms of no. of earthquakes vs. magnitude. This is called the Gutenberg-Richter criterion. (CEDIM)
Hazard
Any physical phenomenon associated with an earthquake that may produce adverse effects on human activities. This includes surface faulting, ground shaking, landslides, liquefaction, tectonic deformation, tsunami, and seiche and their effects on land use, manmade structures, and socioeconomic systems. A commonly used restricted definition of earthquake hazard is the probability of occurrence of a specified level of ground shaking in a specified period of time. (USGS)
Hypocenter
The point within the Earth where an earthquake rupture initiates. Also commonly termed the focus. (USGS)
Intensity
A subjective numerical index describing the severity of an earthquake in terms of its effects on the Earth's surface and on humans and their structures. Several scales exist, but the ones most commonly used in the United States are the Modified Mercalli scale and the Rossi-Forel scale. (HAZUS Guidelines)
Intermediate Earthquake
An earthquake whose focus is located between 70 to 300 kilometers from the earth's surface. Earthquake-report.com differs from the official notification calling earthquakes with a depth of more than 40 to 00 km as "Intermediate". This is mainly because of the limited damaging impact of these earthquakes. (USGS)
JMA
The Japan Meteorological Agency seismic intensity scale is a measure used in Japan and Taiwan to indicate the strength of earthquakes. This scale is a numerical system, assigning earthquakes levels 0-7. (Wikipedia)
Landslide
An abrupt movement of geological materials downhill in response to gravity. Landslides can be triggered by an earthquake or other natural causes. (EQCanada)
Latitude
Angular distance north or south from the earth's equator measured through 90 degrees. (NASA)
Liquefaction
The transformation of a granular material from a solid state into a liquefied state as a consequence of increased pore water pressures and reduced effective stress. In engineering seismology, it refers to the loss of soil strength as a result of an increase in pore pressure due to ground motion. This effect can be caused by earthquake shaking. (IASPEI)
Lithosphere
The outer solid part of the Earth, including the crust and uppermost mantle. The lithosphere is about 100 km thick, although its thickness is age dependent.The lithosphere below the crust is brittle enough at some locations to produce earthquakes by faulting, such as within a subducted oceanic plate. (USGS)
Local site conditions
A qualitative or quantitative description of the topography, geology, and soil profile at a site that affect ground motions during an earthquake. ( IASPEI
Locked Fault
A fault that is not slipping because frictional resistance on the fault is greater than the shear stress across the fault. Such faults may store strain for extended periods that is eventually released in an earthquake when frictional resistance is overcome. A locked fault condition contrasts with fault-creep conditions and an unlocked fault. (USGS)
Logarithm
It is simply the exponent required to produce a given number. For a certain base 10, in this case 1 = 10, 2 = 10X10 = 100, 3=1000 and so on. (CEDIM)
Longitude
the arc or portion of the earth's equator intersected between the meridian of a given place and the prime meridian and expressed either in degrees or in time. (NASA)
Magnitude
A number that characterizes the relative size of an earthquake. Magnitude is based on measurement of the maximum motion recorded by a seismograph(sometimes for earthquake waves of a particular frequency), corrected for attenuation to a standardized distance. Several scales have been defined, but the most commonly used are (1) local magnitude (ML), commonly referred to as œRichter magnitude, (2) surface-wave magnitude (Ms), (3) body-wave magnitude (Mb), and (4) moment magnitude (Mw). ML, Ms and Mb have limited range and applicability and do not satisfactorily measure the size of the largest earthquakes. The moment magnitude (Mw) scale, based on the concept of seismic moment, is uniformly applicable to all sizes of earthquakes but is more difficult to compute than the other types. In principal, all magnitude scales could be cross calibrated to yield the same value for any given earthquake, but this expectation has proven to be only approximately true, thus the need to specify the magnitude type as well as its value.An increase of one unit of magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released. In other words, a magnitude 6.7 earthquake releases over 900 times (30 times 30) the energy of a 4.7 earthquake - or it takes about 900 magnitude 4.7 earthquakes to equal the energy released in a single 6.7 earthquake! There is no beginning nor end to this scale. However, rock mechanics seem to preclude earthquakes smaller than about -1 or larger than about 9.5. A magnitude -1.0 event releases about 900 times less energy than a magnitude 1.0 quake. Except in special circumstances, earthquakes below magnitude 2.5 are not generally not felt by humans. (USGS-IASPEI)
Mainshock
The biggest earthquake in a series is termed the mainshock.
Mantle
The layer of rock that lies between the outer crust and the core of the earth. It is approximately 1,802 miles (2,900 kilometers) thick and is the largest of the earth's major layers. (PDC)
MMI
The Mercalli scale rates the intensity of shaking from an earthquake. The ratings vary from I (felt only under especially favourable circumstances) to XII (total destruction). (Wikipedia)
Moment Magnitude
See magnitude (Mw) (USGS)
Ocean Spreading Ridge
A fracture zone along the ocean bottom that accommodates upwelling of mantle material to the surface, thus creating new crust. This fracture is topographically marked by a line of ridges that form as molten rock reaches the ocean bottom and solidifies. (USGS)
Oceanic Crust
The outermost solid layer of Earth that underlies the oceans. Composed of the igneous rocks basalt and gabbro, and therefore basaltic in composition. Contrast with continental crust. (PDC)
Oceanic Trench
A linear depression of the sea floor caused by and approximately coincident with a subduction thrust fault. (USGS)
Oscillator
A mass that moves with oscillating motion under the influence of external forces and one or more forces that restore the mass to its stable at-rest position. In earthquake engineering, an oscillator is an idealized damped mass-spring system used as a model of the response of a structure to earthquake ground motion. A seismograph is also an oscillator of this type (USGS)
P Wave
A seismic body wave that involves particle motion (alternating compression and extension) in the direction of propagation. (USGS)
Peak Acceleration
The highest acceleration in terms of value. (USGS)
PGA
The maximum acceleration amplitude measured or expected in a strong-motion accelerogram of an earthquake. (IASPEI)
Phase
(1) A stage in periodic motion, such as wave motion or the motion of an oscillator, measured with respect to a given initial point and expressed in angular measure. (2) A pulse of seismic energy arriving at a definite time. (USGS)
Plate
a large, relatively rigid segment of the Earth™s lithosphere that moves in relation to other plates over the asthenosphere. (USGS)
Plate Tectonics
A theory supported by a wide range of evidence that considers the Earth™s crust and uppermantle to be composed of several large, thin, relatively rigid plates that move relative to one another. Slip on faults that define the plate boundaries commonly results in earthquakes. Several styles of faults bound the plates, including thrust faults along which plate material is subducted or consumed in the mantle, oceanic spreading ridges along which new crustal material is produced, and transform faults that accommodate horizontal slip (strike slip) between adjoining plates. (USGS)
Population Density
Population density (in agriculture standing stock and standing crop) is a measurement of population per unit area or unit volume. It refers to people in our case. (Wikipedia)
Preparedness
Definition: The knowledge and capacities developed by governments, professional response and recovery organizations, communities and individuals to effectively anticipate, respond to, and recover from, the impacts of likely, imminent or current hazard events or conditions. Comment: Preparedness action is carried out within the context of disaster risk management and aims to build the capacities needed to efficiently manage all types of emergencies and achieve orderly transitions from response through to sustained recovery. Preparedness is based on a sound analysis of disaster risks and good linkages with early warning systems, and includes such activities as contingency planning, stockpiling of equipment and supplies, the development of arrangements for coordination, evacuation and public information, and associated training and field exercises. These must be supported by formal institutional, legal and budgetary capacities. The related term œreadiness describes the ability to quickly and appropriately respond when required. (UN/ISDR)
Prevention
Actions or investments needed in the face of imminent hazards. Distinct from mitigation, which is a permanent strategy, prevention is seen as a pre-disaster set of activities. (UN/ISDR)
Primary Wave
See P Wave (CEDIM)
Residential
A building should be regarded as residential building when more than half of the floor area is used for dwelling purposes. Other buildings should be regarded as non-residential. Two types of residential buildings can be distinguished: * houses (ground-oriented residential buildings): comprising all types of houses (detached, semi-detached, terraced houses, houses built in a row, etc.) each dwelling of which has its own entrance directly from the ground surface; * other residential buildings: comprising all residential buildings other than ground-oriented residential buildings as defined above. (OECD)
Resilience
Definition: The ability of a system, community or society exposed to hazards to resist, absorb, accommodate to and recover from the effects of a hazard in a timely and efficient manner, including through the preservation and restoration of its essential basic structures and functions.Comment: Resilience means the ability to œresile from or œspring back from a shock. The resilience of a community in respect to potential hazard events is determined by the degree to which the community has the necessary resources and is capable of organizing itself both prior to and during times of need. (UN/ISDR)
Reverse Fault
Dip-slip faults are inclined fractures along which rock masses have mostly shifted vertically. If the rock above the fault is elevated by slip, the fault is termed reverse (or thrust fault). (USGS)
Risk
The probabilistic determination of the damages a certain hazard can cause given the existing vulnerability, location and time. (UN )
Risk Assessment
Definition: A methodology to determine the nature and extent of risk by analyzing potential hazards and evaluating existing conditions of vulnerability that together could potentially harm exposed people, property, services, livelihoods and the environment on which they depend.Comment: Risk assessments (and associated risk mapping) include: a review of the technical characteristics of hazards such as their location, intensity, frequency and probability; the analysis of exposure and vulnerability including the physical social, health, economic and environmental dimensions; and the evaluation of the effectiveness of prevailing and alternative coping capacities in respect to likely risk scenarios. This series of activities is sometimes known as a risk analysis process. (UN/ISDR)
Rupture
The instantaneous boundary between the slipping and locked parts of a fault during an earthquake. Rupture in one direction on the fault is referred to as unilateral. Rupture may radiate outward in a circular manner or it may radiate toward the two ends of the fault from an interior point, referred to as bilateral. (USGS)
S Wave Velocity
The velocity of a secondary or S wave. Generally measured in m/s. (CEDIM)
Secondary Wave
A seismic body wave that involves a shearing motion in a direction perpendicular to the direction of propagation. When it is resolved into two orthogonal components in the plane perpendicular to the direction of propagation, SH denotes the horizontal component and SV denotes the orthogonal component. Also known as S waves and shear waves. (PDC)
Seismic hazard
Risk of a certain ground motion occurring at a location (this can be defined by scenario modeling via stochastic catalogues, DSHA, PSHA or other such methods, and can include different types of earthquake effects) (CEDIM)
Seismic Risk
See earthquake risk, also the probabilistic risk is the odds of an earthquake occurring and causing damage within a given time interval and region. (EQCanada)
Seismic Station
A ground position at which a geophysical instrument is located for an observation. (U-Milwaukee)
Seismic Waves
An elastic wave generated by an impulse such as an earthquake or an explosion. Seismic waves may propagate either along or near the Earth's surface (for example, Rayleigh and Love waves) or through the Earth's interior (P and S waves). (USGS)
Seismicity
1) The geographic and historical distribution of earthquakes. 2) A term introduced by Gutenberg and Richter to describe quantitatively the space, time, and magnitude distribution of earthquake occurrences. Seismicity within a specific source zone or region is usually quantified in terms of a Gutenberg-Richter relationship. (ICWGroup/IASPEI)
Seismogram
A record written by a seismograph in response to ground motions produced by an earthquake, explosion, or other ground-motion sources. (ICW Group)

Seismometer
A seismometer is a damped oscillating mass, such as a damped mass-spring system, used to detect seismic-wave energy. The motion of the mass is commonly transformed into an electrical voltage. The electrical voltage is recorded on paper, magnetic tape, or another recording medium. This record is proportional to the motion of the seismometer mass relative to the Earth, but it can be mathematically converted to a record of the absolute motion of the ground. Seismograph is a term that refers to the seismometer and its recording device as a single unit. (NASA)
Sequence
Pertains to the possible foreshock, mainshock and aftershock occurrence and time period. (CEDIM)
Severity
Both intensity and magnitude “ i.e. the size/strength of an earthquake. (CEDIM)
Shakemaps
ShakeMaps, a product of the United States Geological Survey (USGS) Earthquake Hazards Program, are near-real-time maps of ground motion and shaking intensity that are produced following significant earthquakes. They appear as a set of links keyed to areas which have recently experienced an earthquake. The maps display instrumental intensity (modified Mercalli scale), peak ground acceleration, and peak ground velocity. They are downloadable as image (JPEG) or zipped postscript (PS) files, and datasets are downloadable as text, zipped shapefiles, KML, XML, or HTML files. Older shakemaps are stored in an archive. The most recent maps are also available as RSS feeds. (DLESE)
Shallow Earthquake
An earthquake whose focus is located within 70 kilometers of the earth's surface. Earthquake-report.com differs from the official notification calling earthquakes with a depth up to 40 km as "Shallow". This is mainly because of the possible damaging impact of these earthquakes. (Sci-Tech)
Strike-Slip
Strike-slip faults are vertical (or nearly vertical) fractures along which rock masses have mostly shifted horizontally. If the block opposite an observer looking across the fault moves to the right, the slip style is termed right lateral; if the block moves to the left, the motion is termed left lateral. (USGS)
Subduction
A plate tectonics term for the process whereby the oceanic lithosphere collides with and descends beneath the continental lithosphere. (USGS)
Surface Faulting
Displacement that reaches the Earth's surface during slip along a fault. Commonly accompanies moderate and large earthquakes having focal depths less than 20 km. Surface faulting also may accompany aseismic tectonic creep or natural or man-induced subsidence. (USGS)
Surface Wave
Seismic waves that propagate along the Earth's surface. Love and Rayleigh waves are the most common. (CEDIM)
Thrust fault
A reverse fault in which the upper rocks above the fault plane move up and over the lower rocks at an angle of 30o or less so that older strata are placed over younger. (EQCanada)
Tsunami
An impulsively generated sea wave of local or distant origin that results from large-scale seafloor displacements associated with large earthquakes, major submarine slides, or exploding volcanic islands. (USGS)
Velocity
In reference to earthquake shaking, velocity is the time rate of change of ground displacement of a reference point during the passage of earthquake seismic waves commonly expressed in centimeters per second. (USGS)

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