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Earthquake Education Glossary



Acceleration The acceleration of an object is a magnitude that indicates the rate of change of velocity.
Accelerometer Instrument that measures accelerations in terrain as a function of time
Accelerogram A register of the acceleration of the terrain in a specific place as a function of time. The acceleration is generally registered in three directions: Two orthogonal horizontal components and one vertical
Aftershocks Minor earthquakes that follow a mayor earthquake, concentrated in the same area as the main earthquake.
Aseismic Building designed to resist earthquakes. It is also used to describe zones with little seismic activity (Aseismic Regions)
Aseismic Region Region of the earth that is stable, almost without any earthquakes occurring.
Asthenosphere The layer below the lithosphere, characterized by low seismic velocities
Attenuation Decrease of amplitude of seismic waves due to their transmission through the interior of the earth. Attenuation curves describe the variation of the intensity of the terrain movement as a function of the magnitude and the epicentral distance.
Attenuation Curve Curve that describes the relation between the severity of terrain movement and acceleration, velocity or Mercalli Intensity as a function of the event magnitude and its distance between the epicenter.
Average Recurrence Period Average time between the occurrence of seismic events in a fault or tectonic region
Convection Currents A cooling mechanism of the earth's mantle. It is produced through the movement of a fluid through a hot focal point. The fluid rises which causes it to cool down and become more dense. This causes the fluid to descend again to the hot focal point, repeating the cycle.
Duration of an earthquake Length of a seismogram, measured in seconds where their is a significant amplitude pulse. In an accelerogram the the amplitude width has to be bigger than 0.05g. It is also the amount of time an earthquake is perceived to be felt by people, which is always less than what is registered by the instrument.
Earth's Crust The most external rocky section of the Earth. Has a thickness between 5 to 40 kilometers
Earthquake Sudden and violent movement that originates from the crust or superior mantle of the earth.
Earthquake (Tectonic) Earthquakes that are the result of the liberation of accumulated energy by the deformation of the earth.
Earthquake (Volcanic) Earthquake associated with volcanic activity
Elastic Rebound Theory A theory of the movement of faults and the generation of earthquake that says that faults remain closed while energy is accumulated by both sides of the fault until a sudden displacement occurs throughout the fault, liberating energy.
Epicenter The point in the earth's surface where an earthquake occurred. This point is directly above the earthquakes focal point (hypocenter)
Fault A fracture or zone of fracture in rocks where both sides have been displaced in relation to each other. The displacement can vary from centimeters to kilometers.
Fault (Active) A fault that historically has had displacements or has been a focal point for earthquakes.
Fault (Slip-strike) A fault where the relative displacement is horizontal.
Fault (Inverted or Thrust Fault) Fault where the rocks above the plain of the fault moves above the fault and above those below them. This causes the older stratums to be placed over the newer ones.
Fault (Normal) A vertical fault where the rocks above the plain move down (In relation to the block that was below it)
Fault (Oblique) A fault that combines a vertical and thrust fault
Fault (Transformation) A thrust fault that connects the endings of a oceanic range, an arch of isles or other ranges. Pairs of faults slide between each other all through the transformation fault.
Fault (Vertical) A fault where the relative displacement is in relation to the fault's dip. The displacement can be normal or inverted.
Focal Point (Hypocenter) Place where the earthquake is originated inside the earth.
Geology Science that studies the external and internal structure of the earth, the nature of the materials of which it is composed and the processes by which they change over time.
Intensity (of Earthquakes) A measurement obtained from the effects and damages caused to human infrastructure, changes in the surface of the earth and surveys.
Isoseismic Line Lines in a map that separate places by their degree of intensity.
Liquefaction (of the terrain) Process in which the earth and sand behave as a dense fluid instead of a humid solid during an earthquake
Local Depth Depth of the hypocenter or focal point of the earthquake below the earth's surface.
Love Waves Superficial seismic waves with only horizontal movement in the direction of propagation of the wave.
Magnitude (of an earthquake) Measurement of the size of an earthquake, determined by taking the logarithm (base 10) of the biggest movement that was registered during the arrival of the seismic wave. There are three common measurements: Richter (or local/Ml), p wave volume (Mb) or superficial wave (Ms)
Mantle (of the earth) The section with most volume of the earth between the crust and nucleus of the earth. The depth ranges from 40 to 2,900 kilometers
Microseism Weak seismic waves that can only be detected by seismograms. Are often caused by sea waves, wind or human activity.
Modified Mercalli Scale An intensity scale from grade I to XII. Is used to designate the intensity of an earthquake in a specific location. The intensity is assigned from the perceived effects felt by people (I to VI) and according to the damages caused to buildings (VII to X) and the geological changes that can occur (XI to XII)
Momentum (of an earthquake) Measurement of the size of an earthquake calculated multiplying the rigidity of the rock by the area of the fault and the amount of displacement.
Nucleus (of the earth) Central part of the earth with a depth over 2,900 kilometers that is molten on the outside and solid on the inside.
P Waves The fastest wave, travels from the focal point through the rocks and consists of compressions and dilatations of the material.
Pangaea Name proposed by Alfred Wegener for a supercontinent, composed of all the masses of earth that existed at the end of the Paleozoic era.
Passive Margin Continental margin far away from the border of the plate, without active volcanos and low earthquake activity.
Plate Tectonics Theory of movement and interaction between plates. Tries to explain earthquakes, volcanoes and the formation of mountains due to big movements of the earth's surface.
Precursor Earthquake A small earthquake that precede a bigger one in a concentrated series.
Probability of Exceedance Probability that a determined acceleration or intensity value produced by an earthquake are exceeded during a specific time period.
Probability of Occurrence Probability that an earthquake occurs during a specific time period
Rayleigh Waves Superficial seismic waves with only vertical movement in the direction of propagation of the wave.
Return Period The average time between the ocurrence of events in a specific place with a specific set of characteristics. For example the maximum acceleration greater than .20g.
Rift Valley A lowland region limited laterally by faults.
Rifting Fragmentation of a continent.
S Waves Secondary seismic waves, traveling slower than P waves and consists of elastic vibrations in the direction of propagation. Cannot propagate through liquids.
Seismic Breach (Seismic Gap) The zone of a fault or segment between tectonic plates known to have a prolonged time of calm where large amounts of elastic energy has accumulated, thus presenting a high probability of a seismic event occurring.
Seismic Energy Part of the elastic energy released by the rupture that occurs in a fault. This energy is radiated as elastic and seismic waves. Most of the elastic energy that is released is dissipated in the form of heat.
Seismic Geomorphology Science that is focused on detecting anomalies in the earth's surface that are attributable to earthquakes in the past and tries to quantify it.
Seismic Regionalization Process of determining the danger or seismic threat in an area with the purpose of delimiting zones from the same seismic threat.
Seismic Source Volume of rock that is fractured during the earthquake
Seismic Swarm A series of earthquakes with a similar magnitude that occur in the same place.
Seismic Waves Seismic Waves in the earth, normally generated by an earthquake or an explosion.
Seismic Sequence Consists of a main earthquake followed by a series of minor aftershocks
Seismic Threat The relative seismic threat of an earthquake in a location relative to another. The probability of an earthquake occurring in an interval of time.
Seismic Tomography Method that utilizes the seismic waves registered on thousands of seismographs around the world to study the interior of the earth and construct a three dimensional image of what is inside it.
Seismic Vulnerability Parameter that indicates how prone a structure is to suffer from damage caused by an earthquake. It is measured in the degree of damage that can be caused to the structure from an earthquake of a specific intensity. It is also determined based on the type of construction and degradation of the structure
Seismic Zonification Process of determining the seismic threat of various places with the purpose of delimiting zones subject to a similar seismic threat level.
Seismicity Describes the seismic activity in a specific geographic area. Generally refers to the frequency of occurrence and the magnitude of previous earthquakes. Can be given by the average amount of earthquakes or by earthquakes of a specific characteristic, for example magnitudes higher than 6 in a specific region.
Seismograph Instrument that registers the movement of the earth's surface as a function of time.
Seismology The study of earthquakes, seismic sources and seismic wave propagation throughout the earth.
Seismologist Person that applies the principles and procedures of Seismology.
Seismometer Sensor part of a Seismograph, normally a suspended pendulum
Seismicity Map Map that shows registers of the epicenters of earthquakes in a specific region or around the world for a specific time period.
Superficial Waves Seismic waves that only follow the earth's surface and are slower than S waves. There are two types of superficial waves: Rayleigh and Love Waves
Tectonic Accretion A process of continental growth where buoyant fragments of the earth's crust are added to continents during tectonic plate movements, causing an extension of the continental lithosphere.
Tectonic Plate A part of the lithosphere of the earth, large and rigid that moves in relation to other parts of the lithosphere over deeper parts of the earth's interior. The plates clash in convergence zones and are separated in divergence zones.
Teleseism An earthquake that is far away from an observation point by over ten degrees of latitude or longitude.
Tsunami Series of waves generated by seabed movement caused by earthquakes
Wave Frequency The number of times that a cyclical process is repeated in a wave per unit of time; Number of cycles per unit of time in an oscillatory process. The unit of frequency is Hertz and is measured as cycles per second; The frequency is the inverse of the wave period.
Wave Period Interval of time between two consecutive peaks in a sinusoidal wave; Interval between maximum amplitudes of seismic waves. The period is measured in seconds and is the inverse of the wave frequency.
Zone (Benioff) Narrow zone defined by the earthquake's focal point with a thickness of around a dozen kilometers, that descends from the earth's crust.
Zone (Subduction) Oceanic plate that descends to the interior of the earth moving away from the ocean floor. Normally is a place where intermediate and deep earthquakes occur that define the Benioff Zones.
Zone (Shadow) Zone that is over 105° from the focal point of an earthquake where the S waves are missing due to the S waves not transmitting through the molten exterior of the earth's nucleus. Zone in an angular distance between 105° and 142° from the docal point of an earthquake where the P waves are missing since they are refracted below the nucleus and emerge at bigger distances after the delay caused by their movement through the nucleus.
Zone (Seismic) Geographic area inside a seismic region in which the requirements for the design of seismic resistant structure is the same