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Science Master - Seismological terms
Category: Sciences > Seismology
Date & country: 27/01/2011, UK
Words: 167

accelerograph. An instrument that records the acceleration of the ground during an earthquake, also commonly called an accelerometer.

acceleration. When you step on the accelerator in the car or put on the brakes, the car goes faster or slower. When it is changing from one speed to another, it is accelerating (faster) or decelerating (slower). This change from one speed, or velocity, to another is called acceleration. During an earthquake when the ground is shaking.

accelerogram. The recording of the acceleration of the ground during an earthquake.

accretionary wedge
accretionary wedge. Sediments, the top layer of material on a tectonic plate, that accumulate and deform where oceanic and continental plates collide. These sediments are scraped off the top of the downgoing oceanic crustal plate and are appended to the edge of the continental plate.

active fault
active fault. A fault that is likely to have another earthquake sometime in the future. Faults are commonly considered to be active if they have moved one or more times in the last 10,000 years.

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

alluvium. Loose gravel, sand, silt, or clay deposited by streams.

amplification. Most earthquakes are relatively small, in fact, so small that no one feels them. In order for seismologists to see the recording of the movement of the ground from the smaller earthquakes, the recording has to be made larger. It's like looking at the recording through a magnifying glass.

amplitude. The size of the wiggles on an earthquake recording.

arc. A chain of volcanoes (volcanic arc) that sometimes forms on the land when an oceanic plate collides with a continental plate and then slides down underneath it (subduction).

aseismic. This term describes a fault on which no earthquakes have been observed.

asperity. An area on a fault that is stuck.

attenuation. When you throw a pebble in a pond, it makes waves on the surface that move out from the place where the pebble entered the water. The waves are largest where they are formed and gradually get smaller as they move away. This decrease in size, or amplitude, of the waves is called attenuation.

backarc. The region landward of the chain of volcanoes (volcanic arc), in a subduction system.

basement. Harder and usually older igneous and metamorphic rocks that underlie the main sedimentary rock sequences (softer and usually younger) of a region and extend downward to the base of the crust.

bedrock. Relatively hard, solid rock that commonly underlies softer rock, sediment, or soil; a subset of the basement.

Benioff zone
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 in.

blind thrust fault
blind thrust fault. A thrust fault that does not rupture all the way up to the surface so there is no evidence of it on the ground. It is "buried" under the uppermost layers of rock in the crust.

body wave
body wave. 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 examples. Each type of wave shakes the ground in different ways.

brittle ductile boundary
brittle-ductile boundary. The depth in the crust where the crust changes from being brittle (tending to break) above, to being ductile (tending to bend) below. Most earthquakes occur in the brittle portion of the crust above the brittle-ductile boundary.

14C age date. An absolute age obtained for geologic materials containing bits or pieces of carbon using measurements of the proportion of radioactive carbon (14C) to daughter carbon (12C). These dates are independently calibrated with calendar dates. This is used to determine when past earthquakes occurred on a fault.

compressional wave
compressional wave. See P wave.

core. The innermost part of the earth. The outer core extends from 2500 to 3500 miles below the earth's surface and is liquid metal. The inner core is the central 500 miles and is solid metal.

creep. Slow, more or less continuous movement occurring on faults due to ongoing tectonic deformation. Faults that are creeping do not tend to have large earthquakes.

crust. The outermost major layer of the earth, ranging from about 10 to 65 km in thickness worldwide. The uppermost 15-35 km of crust is brittle enough to produce earthquakes.

dip. The angle that a planar geologic surface (for example, a fault) is inclined from the horizontal.

dip slip
dip slip. See fault.

directivity. An effect of a fault rupturing whereby earthquake ground motion in the direction of rupture propagation is more severe than that in other directions from the earthquake source.

displacement. 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. This term is used to describe both sudden slip on a fault, and the resulting ground shaking and radiated seismic energy caused by the slip, or by volcanic or magmatic activity, or other sudden stress changes in the earth.

earthquake hazard
earthquake hazard. Anything associated with an earthquake that may affect the normal activities of people. This includes surface faulting, ground shaking, landslides, liquefaction, tectonic deformation, tsunamis, and seiches.

earthquake risk
earthquake risk. 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 used interchangeably.

epicenter. The point on the earth's surface vertically above the point in the crust where a seismic rupture begins.

fault. A fracture along which the blocks of crust on either side have moved relative to one another parallel to the fracture. Strike-slip faults are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally. If the block opposite an observer looking across the fault moves to the right.

fault creep
fault creep. See creep.

fault plane solution
fault-plane solution. 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.

fault scarp
fault scarp. A feature on the surface of the earth that looks like a step caused by slip on the fault.

fault trace
fault trace. Intersection of a fault with the ground surface; also, the line commonly plotted on geologic maps to represent a fault.

first motion
first motion. On a seismogram, 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
focal depth. A term that refers to the depth of an earthquake hypocenter.

focal mechanism solution
focal-mechanism solution. See fault-plane solution.

focus. See hypocenter.

forearc. The region between the subduction zone and the volcanic chain (volcanic arc).

foreshocks. Foreshocks are relatively smaller earthquakes that precede the largest earthquake in a series, which is termed the mainshock. Not all mainshocks have foreshocks.

frequency. 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.

G or g. See percent "g".

geodesy. The science of determining the size and shape of the earth and the precise location of points on its surface.

geodetic. Referring to the determination of the size and shape of the earth and the precise location of points on its surface.

geomorphology. The study of the character and origin of landforms, such as mountains, valleys, etc.

geophysics. The study of the earth by physical methods.

geotechnical. Referring to the use of scientific methods and engineering principles to acquire, interpret, and apply knowledge of earth materials for solving engineering problems.

gravity. The attraction between two masses, such as the earth and an object on its surface. Commonly referred to as the acceleration of gravity. Changes in the gravity field can be used to infer information about the structure of the earth's lithosphere and upper mantle.

ground failure
ground failure. 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 (shaking). 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.

halfspace. A mathematical model used to approximate the earth when performing some calculations in seismology. The model is much simpler than the real earth.

harmonic tremor
harmonic tremor. Continuous rhythmic earthquakes that can be detected by seimographs. Harmonic tremors often precede or accompany volcanic eruptions.

hazard. See earthquake hazard.

Hertz (Hz). A unit of frequency. Expressed in cycles per second.

Holocene. Refers to a period of time between the present and 10,000 years before present. Applied to rocks or faults, this term indicates the period of rock formation or the time of most recent fault slip. Faults of this age are commonly considered active.

hypocenter. The point within the earth where an earthquake rupture starts. Also commonly termed the focus.

intensity. A number (written as a Roman numeral) 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.

interplate coupling
interplate coupling. 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 interplate
intraplate and interplate. Intraplate pertains to processes within the earth's crustal plates. Interplate pertains to processes between the plates.

isoseismal. A contour or line on a map bounding points of equal intensity for a particular earthquake.

kinematic. Referring to the general movement patterns and directions of the earth's rocks that produce rock deformation.

landslide. The downslope movement of soil and/or rock.

Late Quaternary
Late Quaternary. The age between the present and 500,000 years before the present. Faults of this age are sometimes considered active based on the observation of historical activity on faults of this age in some locales.

lateral spread
lateral spread and flow. Terms referring to landslides that commonly form on gentle slopes and that have rapid fluid-like flow movement, like water.

least-squares fit. When plotting data points on a graph, the least-squares-fit is the line or curve that comes closest to going through all the points.

left lateral
left-lateral. If you were to stand on the fault and look along its length, this is a type of strike-slip fault where the left block moves toward you and the right block moves away. (See also right-lateral.)

lifelines. Structures that are important or critical for a community to function, such as roadways, pipelines, powerlines, sewers, communications, and port facilities.

liquefaction. A process by which water-saturated sediment temporarily loses strength and acts as a fluid, like when you wiggle your toes in the wet sand near the water at the beach. This effect can be caused by earthquake shaking.

lithology. The description of rock composition (what it's made of) and texture.

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 (older lithosphere is thicker).The lithosphere below the crust is brittle enough at some locations to produce earthquakes by faulting, such as within a subducted oceanic plate.

locked fault
locked fault. 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
Love wave. A type of seismic surface wave having a horizontal motion that is transverse (or perpendicular) to the direction the wave is traveling.

Ma. An abbreviation for one million years ago (Megannum).

magnetic polarity reversal
magnetic polarity reversal. A change of the earth's magnetic field to the opposite polarity that has occurred at irregular intervals during geologic time. Polarity reversals can be preserved in sequences of magnetized rocks and compared with standard polarity-change time scales to estimate geologic ages of the rocks.

magnitude. A number that characterizes the relative size of an earthquake. Magnitude is based on measurement of the maximum motion recorded by a seismograph. Several scales have been defined, but the most commonly used are (1) local magnitude (ML), commonly referred to as "Richter magnitude," (2) surface-wave magnitude (Ms), (3) body-wave

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

mantle. The part of the earth's interior between the metallic outer core and the crust.

Moho. 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.

moment magnitude
moment magnitude. See magnitude.

natural frequency
natural frequency. The frequency at which a particular object or system vibrates when pushed by a single force or impulse, and not influenced by other external forces or by damping. If you hold a slinky by one end and let it hang down and then give it one push up from the bottom, the rate of up-and-down motion is its natural frequency.

normal fault
normal fault. (See fault.)

normal stress
normal stress. 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. ( See also shear stress.)

ocean trench
oceanic trench. A linear depression of the sea floor caused by the subduction of one plate under another.

oceanic spreading ridge
oceanic spreading ridge. A fracture zone along the ocean bottom where molten mantle material comes to the surface, thus creating new crust. This fracture can be seen beneath the ocean as a line of ridges that form as molten rock reaches the ocean bottom and solidifies.

P wave
P wave. 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.

paleoseismic. Referring to the history of seismic events that is determined by looking at the layers of rock beneath the surface and how they have been shifted by earthquakes in the past.

peak acceleration
peak acceleration. See acceleration.

pedogenic. Pertaining to processes that add, transfer, transform, or remove soil constituents.

percent g
percent "g". G or g is the force of gravity (an acceleration of 9.78 meters/second2). When there is an earthquake, the forces caused by the shaking can be measured as a percentage of the force of gravity, or percent g.

period. The time interval required for one full cycle of a wave.

plate tectonics
plate tectonics. A theory supported by a wide range of evidence that considers the earth's crust and upper mantle 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.

Pleistocene. The time period between about 10,000 years before present and about 1,650,000 years before present. As a descriptive term applied to rocks or faults, it marks the period of rock formation or the time of most recent fault slip, respectively.

Poisson distribution
Poisson distribution. A probability distribution that characterizes discrete events occurring independently of one another in time.

Q. See attenuation.

Quaternary. The geologic time period comprising about the last 1.65 million years.

radiometric. Pertaining to the measurement of geologic time by the analysis of certain radioisotopes in rocks and their known rates of decay.