Change in velocity per unit of time. For example, a vehicle is accelerating when its speed increases over time, and decelerating when its speed decreases. The ground also undergoes acceleration due to shaking during an earthquake.

A time-series plot representing the accelerations of the ground recorded during an earthquake.

An instrument that records ground acceleration during an earthquake, commonly known as a seismic accelerometer.

A geological fault that is likely to produce an earthquake sometime in the future. A fault is commonly considered active if it has shown movement or evidence of seismic activity within the last 10,000 years.

An earthquake that follows the mainshock, that is the largest event in a seismic sequence. Aftershocks usually have smaller magnitudes than the mainshock, and occur at distances of roughly 1-2 rupture lengths from it. They can continue for weeks, months, or even years. In general, the larger the magnitude of the mainshock, the larger and more numerous the aftershocks, and the longer they will persist.

Shaking levels at a site may increase, or be amplified, due to the concentration of seismic energy caused by the geometry of sediment velocity structures, such as basin subsurface topography, or by surface topography.

The size of the seismic waves recorded during an earthquake.

Loss of energy experienced by a wave during its propagation. For example, when a pebble is thrown into a pond, waves are generated on the water surface and spread outward from the point where the pebble entered the water. These waves decrease as they travel farther from their point of origin. The reduction in the size (or amplitude) of the waves is called attenuation. Seismic waves also experience attenuation as they move away from the earthquake source.

Inclined plane produced by the interaction of an oceanic plate subducting beneath a continental plate. In the Benioff zone, also known as the Wadati-Benioff zone, earthquakes can be generated by slip along the subduction fault, or by slip resulting from compression or extension of the subducting plate as it sinks into the mantle.

A thrust fault that does not reach the Earth’s surface, and therefore leaves no visible traces on the ground. It is “buried” beneath the uppermost layers of rock in the Earth’s crust.

A seismic wave that propagates from the source through the interior of the Earth in all directions. Body waves differ from surface waves, which travel along the Earth’s surface. P-waves and S-waves are body waves.

Depth within the Earth's crust where the transition occurs from brittle behavior (above the boundary), where the crust tends to break, to ductile behavior (below the boundary), where the crust tends to bend. Most earthquakes occur in the brittle portion of the crust, above the brittle-ductile boundary.

Mechanical stress that occurs when a material is subjected to a force that tends to compress it, reducing its dimensions. This is the component of stress perpendicular to a given surface, such as a fault plane, resulting from forces applied perpendicular to the surface or from remote forces transmitted through the surrounding rock.

The slow and more or less continuous movement of faults caused by ongoing tectonic deformation. Faults that exhibit creep tend not to generate large-magnitude earthquakes.

A change in a material from its original shape. In seismology, deformation is caused by stress and strain.

The angle of inclination that a planar geological surface (such as a fault) makes with the horizontal.

A fault-rupture effect in which ground motion is stronger along the direction of rupture propagation than that in other directions from the seismic source.

Difference between the initial position of a reference point and any later position. In geology, it indicates the permanent shift of a point along a fault due to an earthquake.

The innermost part of the Earth, divided into outer and inner core. The outer core extends from about 2,500 km to 3,500 km below the Earth's surface and is composed of liquid metal. The inner core occupies the central 500 km of the planet and is composed of solid metal.

The outermost layer of the Earth, with a thickness that varies globally between roughly 10 and 70 km. The upper 15–35 km of the crust are sufficiently brittle to generate earthquakes.

A term used to describe both the sudden slip along a fault and the ground shaking caused by the seismic energy released during the slip. An earthquake can be caused by a change in stress in the Earth's crust or by volcanic/magmatic activity.

Measure of the potential dangers of a natural phenomenon associated with an earthquake, such as surface faulting, ground shaking, landslides, liquefaction, tectonic deformation, or tsunamis. Specifically, hazard is a deterministic or, more commonly, probabilistic measure of the occurrence of such events at a given time and location.

Estimate of the probable damage to buildings or people expected in a certain area and over a given time period following an earthquake. Risk is determined by the combination of seismic hazard, vulnerability, and exposure. Hazard and risk are often incorrectly used interchangeably.

The point on the Earth’s surface located vertically above the hypocenter (or focus). It is the point in the crust where an earthquake rupture begins.

A fracture along which the blocks of the crust on either side have moved relative to each other, parallel to the fracture plane, generating an earthquake.

Flat surface along which slip occurs between the two sides of a fault during an earthquake.

Intersection of a fault with the Earth's surface; it is also the line commonly represented on geological maps to indicate the presence of a fault.

Depth at which an earthquake occurs, i.e., the depth of its hypocenter.

Method used to represent the fault and the direction of slip of an earthquake that occurred along it. This representation uses circles with two intersecting curves, resembling a beach ball.

An earthquake that precedes the mainshock, that is the largest event in a seismic sequence. Foreshocks usually have smaller magnitudes than the mainshock. Not all mainshocks are preceded by foreshocks.

The number of occurrences within a given time interval, for example, the number of times the ground shakes up and down or back and forth during an earthquake.

Acceleration due to gravity, equal to 9.8 m/s2, or the strength of the gravitational field (N/kg). Gravity acts on a physical body as a force. Specifically, the force of gravity gives an object its weight according to the formula F=mg on the Earth’s surface, or F=GM/r2 at a distance r from the Earth’s center. G is the proportionality constant in Newton’s law of universal gravitation, equal to 6,67 x 10e-11 Nm2/kg2. During an earthquake, forces generated by ground shaking can be expressed as a percentage of gravity.

The science that studies the size and shape of the Earth, as well as the precise location of points on its surface.

Chronological sequence of geologic events, usually represented in a diagram showing the names of rock layers and indicating the estimated duration of each geologic unit.

The science that studies the Earth, including the materials it is made of, the processes acting on these materials, the resulting products, and the history of the planet and life forms since its origin.

The branch of Earth sciences that uses physical measurements and mathematical models to explore and analyze Earth’s structure, dynamics, and physical properties.

The attractive force between two masses, such as the Earth and an object on its surface, commonly expressed as acceleration of gravity. Variations in the gravity field can provide information on the structure of the Earth’s lithosphere and upper mantle.

Movement of the Earth's surface caused by earthquakes or explosions. Ground motion is generated by waves produced either by sudden slip along a fault or by a sudden pressure at the source of an explosion, which then propagate through the Earth and along its surface.

Physical and geological phenomenon manifested as a series of continuous rhythmic earthquakes recorded by seismographs. Harmonic tremors often precede or accompany volcanic eruptions.

Unit of frequency measured in cycles per second, named after the German physicist Heinrich Hertz, who first produced electromagnetic waves in 1887. 1 Hz corresponds to one complete cycle of rise and fall of a wave per second.

The point within the Earth where an earthquake originates, resulting from movements of lithospheric plates.

A measure of the effects of an earthquake on people, buildings, and the environment.

The ability of a fault located between two tectonic plates to lock and accumulate stress. Strong interplate coupling indicates that the fault is locked and capable of storing a significant amount of stress.

A branch of Newtonian mechanics that describes the patterns and directions of motion of Earth’s rocks that produce deformation in the crust.

When representing data as points on a graph, least-squares regression is the line (or curve) that best fits the data by minimizing the squares of the distances between the points and the curve.

Essential structures for the functioning of a community, such as roads, pipelines, powerlines, sewers, communication networks, and port facilities.

Description of the composition and texture of rocks, i.e., what they are made of and how they appear at a macroscopic or microscopic level.

The solid outer layer of the Earth, including the crust and the uppermost mantle. The lithosphere is approximately 100 km thick, although this varies proportionally with its age. Below the crust,, the lithosphere is sufficiently brittle to produce earthquakes through faulting, as also occurs within an oceanic subduction plate.

A surface wave with horizontal transverse motion (perpendicular to the direction of wave propagation).

A measure of the energy released by an earthquake. Magnitude is based on the maximum ground motion recorded by a seismograph. Several scales have been defined to measure magnitude, the most common being: local (Richter) magnitude (ML), surface-wave magnitude (Ms), body-wave magnitude (Mb), and moment magnitude (Mw). The first three scales have limited range and applicability and cannot adequately measure the strongest earthquakes. Moment magnitude, based on the concept of seismic moment, can be applied to earthquakes of any size, although it is more complex to calculate directly. All magnitude scales, however, should yield approximately equivalent values for the same earthquake.

A measure of the energy released by an earthquake. Magnitude is based on the maximum ground motion recorded by a seismograph. Several scales have been defined to measure magnitude, the most common being: local (Richter) magnitude (ML), surface-wave magnitude (Ms), body-wave magnitude (Mb), and moment magnitude (Mw). The first three scales have limited range and applicability and cannot adequately measure the strongest earthquakes. Moment magnitude, based on the concept of seismic moment, can be applied to earthquakes of any size, although it is more complex to calculate directly. All magnitude scales, however, should yield approximately equivalent values for the same earthquake.

The largest earthquake in a seismic sequence, sometimes preceded by one or more foreshocks, and almost always followed by numerous aftershocks.

The part of the Earth located between the metallic outer core and the crust.

Identification of distinct areas characterized by homogeneous seismic response, yet exhibiting different potential effects in case of an earthquake.

Numerical value, expressed in Roman numerals, describing the severity (intensity) of an earthquake based on its effects on the Earth’s surface, people, or structures. Although several scales exist to measure intensity, the Modified Mercalli is one of the most widely used. Unlike magnitude, which is a single value for each earthquake, intensity can vary depending on the location.

A mathematical representation of fault movement during an earthquake, composed of nine sets of vector pairs. The tensor depends on the source strength and fault orientation. It is often represented as a “beach ball” similar to the focal mechanism.

Inclined fractures along which rock blocks move primarily in a vertical direction. If the rock mass on the inclined fault moves downward, it is called a normal fault; if it moves upward, it is called a reverse fault. A thrust fault is a type of reverse fault with a dip of 45 degrees or less. Oblique-slip faults exhibit significant components of different slip styles.

Component of stress perpendicular to a given surface. Normal stress can be either compressive or tensional.

Standards regulating construction methods based on seismic hazard.

A body seismic wave that shakes the ground back and forth in the same direction as, or opposite to, the wave propagation.

Earthquakes recorded geologically, most of which are not known through human descriptions or seismogram recordings. Geological records of past earthquakes can include faulted sediment and rock layers, injection of liquefied sand, and landslides.

Time interval required for one full cycle of a wave.

Peak acceleration is the maximum increase in velocity recorded by a specific seismic station during an earthquake.

Model of Earth's dynamics according to which the Earth's crust and upper mantle are composed of several large, thin, rigid sections, called plates, that move relative to each other. Many empirical evidences support the theory of plate tectonics, which is widely accepted by geologists. Plate boundaries are defined by faults: the slip along these faults generally causes earthquakes. Plate boundaries are of three types: (1) Transform boundaries - mainly represented by strike-slip faults, where horizontal motion occurs without creation or destruction of crust; (2) divergent boundaries - characterized by oceanic spreading ridges along which new crustal material is formed; (3) convergent/subduction boundaries - characterized by reverse faults along which material is subducted or consumed into the mantle.

A probability distribution that characterizes the occurrence of discrete events that happen independently of one another in time.

A probability distribution that characterizes the occurrence of discrete events that happen independently of one another in time.

Inverse of attenuation. A higher Q indicates a lower attenuation.

A surface seismic wave that shakes the ground in an elliptical motion, without any transverse or perpendicular displacement relative to the direction of wave propagation.

The average time between two successive seismic events occurring on the same fault or in the same source area.

Energy or wave generated by an earthquake that is reflected at a boundary between two different materials within the Earth, similar to light reflecting off a mirror.

Deviation or bending of a seismic wave’s path caused by passing from one material to another with different elastic properties.

A statistical technique applied to data to determine, for predictive purposes, the degree of correlation between a dependent variable and one or more independent variables.

Difference between measured values and predicted values of a certain quantity.

Magnitude scale that was developed in 1935 by Charles F. Richter (California Institute of Technology) as a mathematical tool to compare the size of earthquakes. Earthquake magnitude is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are also included to account for variations in distance between different seismographs and the earthquake epicenter. On the Richter scale, magnitude is expressed in whole numbers and decimal fractions. Due to the logarithmic nature of the scale, each one-unit increase in magnitude corresponds to a tenfold increase in measured amplitude. In terms of energy, a one-unit increase corresponds to roughly 31 times more energy released than the previous value.

The instantaneous boundary between the part of the fault that is slipping and the part that remains locked during an earthquake. If the rupture propagates in only one direction along the fault, it is called unilateral rupture. If the rupture spreads outward in a circular manner or radiates toward the two ends from an internal point, it is called bilateral rupture.

Speed at which the rupture front propagates along the fault surface during an earthquake.

A body seismic wave that shakes the ground back and forth perpendicular to the direction of wave propagation.

Fragmentation of a fault along its length into smaller faults. This can occur due to intersections with other faults, changes in topography, or bends in the fault orientation. Segmentation can limit the length of fault involved in a single earthquake to a fraction of its total length, thus limiting the maximum possible magnitude.

A section of a fault that has produced earthquakes in the past but is currently inactive. Some seismic gaps have no historically recorded earthquakes, yet the fault segment is still considered capable of generating earthquakes based on other information, such as tectonic plate-motion or strain measurements.

A measure of earthquake size based on the fault rupture area, the average slip, and the force required to overcome friction between the rocks involved in the movement. Seismic moment can also be calculated from the amplitude spectra of seismic waves.

An elastic wave generated by an impulse, such as an earthquake or an explosion. Seismic waves can propagate along the Earth’s surface (Rayleigh and Love waves) or through the Earth’s interior (P and S waves).

Division of national territory into areas with different seismic hazard levels (from Zone 1, the most hazardous, to Zone 4, where the hazard is considered lowest).

Geographical and temporal distribution of earthquakes.

Term indicating the capacity to generate earthquakes. The base of the seismogenic zone corresponds to the top of the asthenosphere, which is characterized by greater ductility.

A geological area where earthquakes can originate.

Record produced by a seismograph in response to ground motion caused, for example, by an earthquake or an explosion.

Science that studies earthquakes and the structure of the Earth through the analysis of natural or artificial seismic waves.

Instrument used to detect and record earthquakes. It typically consists of a mass connected to a fixed base. During an earthquake, the base moves while the mass remains stationary. The relative motion between the base and the mass is usually converted into an electrical voltage, which is then recorded on paper, magnetic tape, or other recording media. The resulting record is proportional to the movement of the seismometer mass relative to the Earth, but can be mathematically converted into a record of the absolute ground motion. The term seismograph generally refers to the seismometer and the recording device as a single unit.

Area of the Earth, between angular distances of 104° and 140° from a given earthquake, that does not receive any direct P-waves. Shadow zones form because S-waves are completely stopped by the Earth's liquid core, while P-waves are refracted by it.

Component of stress parallel to a given surface, such as a fault plane, resulting from forces acting parallel to the surface or transmitted from remote forces through the surrounding rock.

Relative displacement of points that were previously adjacent, on opposite sides of a fault, measured along the fault surface. A slip model is a kinematic model that describes the amount, distribution, and timing of fault movement associated with an earthquake.

Velocity at which the two sides of a fault move relative to each other. This quantity is determined through geodetic measurements or based on the displacement of artificial structures or geological features whose age can be estimated. The slip rate is measured parallel to the main direction of slip or estimated from the vertical or horizontal displacement of geological markers.

Term indicating the point or region where the forces that generate acoustic or seismic waves originate.

Approximate measure of the acceleration experienced by a building, modeled as a particle placed on a vertical massless rod, having the same natural vibration period as the building.

Curve showing amplitude and phase as a function of frequency or period, i.e., how much each type of oscillation contributes to the shake generated by an earthquake.

A measure of how much a dataset is different from its expected distribution. It is a statistical measure of dispersion or variability.

Term used to describe random or probabilistic processes.

Small variations in length and volume associated with the deformation of the Earth, caused, for example, by the passage of seismic waves.

Velocity at which the lithosphere is being deformed due to the movement of tectonic plates.

Force per unit area acting on a plane within a body. To fully characterize stress at a point, six values are needed: three normal components and three shear components.

Difference between the stress across a fault before and after an earthquake.

Orientation, relative to north, of the line defined by the intersection between a planar geologic surface (like a fault) and a horizontal surface (like the ground).

A vertical (or nearly vertical) fracture along which rock blocks move primarily in a horizontal direction. If the block opposite the observer moves to the right, the slip style is called right lateral; if it moves to the left, it is called left lateral.

Ground motion with amplitude and duration sufficient to potentially cause damage to buildings or other structures.

Process in which oceanic lithosphere collides with continental lithosphere and descends beneath it. The point where two lithospheric plates meet, with one sliding beneath the other, is called a subduction zone.

Displacement that reaches the Earth’s surface during slip along a fault. This commonly occurs in shallow earthquakes (less than 20 km deep) but can also accompany slow slip (aseismic creep) or natural and human-induced subsidence.

A seismic wave that propagates confined near the Earth’s surface.

Refers to the processes of rock deformation and the resulting structures, affecting large sections of the lithosphere.

Earthquake that occurs at a distance greater than 1,000 km from the measurement site.

Stress that tends to pull and separate a material. This is the component of stress perpendicular to a given surface, such as a fault plane, generated by forces acting perpendicular to the surface or transmitted from remote forces through the surrounding rock.

Sequence of values of a variable quantity over time, measured at fixed time intervals.

A plot showing the arrival times of seismic waves, commonly P or S waves, recorded at different locations as a function of distance from the seismic source. Seismic velocities within the Earth can be inferred from the slope of the resulting curves.

Speed at which a point on the ground is shaking during an earthquake.

Generalized regional model of the Earth's crust representing the crustal structure through layers characterized by different seismic velocities.

The instantaneous boundary between a seismic wave and the Earth material that has not yet been reached by the seismic energy.

The distance between two consecutive points with the same amplitude and phase on a wave.