Earthquake terms

soil

(1) In engineering, all unconsolidated material above bedrock. (2) In soil science, naturally occurring layers of mineral and (or) organic constituents that differ from the underlying parent material in their physical, chemical, mineralogical, and morphological character because of pedogenic processes (3) In other words, dirt

soil profile

All the vertical layers or horizons that make up a soil in a particular place

seismic gap

An area along a fault where there has not been any earthquake activity for a long period of time

shadow zone

An area on Earth's surface where no direct seismic waves from a particular earthquake can be detected.

transform fault

Area where the earth's lithospheric plates move in opposite but parallel directions along a fracture (fault) in the lithosphere.

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

tensional stress

The stress produced by two tectonic plates moving apart

compressional stress

The stress that squeezes something. It is the stress component perpendicular to a given surface, such as a fault plane, that results from forces applied perpendicular to the surface or from remote forces transmitted through the surrounding rock.

Seismology

The study of earthquakes

tectonic plates

a block of lithosphere that consists of the crust and the rigid, outermost part of the mantle

volcanic arc

a chain of volcanoes that sometimes forms on the land when an oceanic plate collides with a continental plate and then slides down underneath it

deformation

a change in the original shape of a material. When we are talking about earthquakes, ________is due to stress and strain

regression analysis

a statistical technique applied to data to determine, for predictive purposes, the degree of correlation of a dependent variable with one or more independent variables, in other words, to see if there is a strong or weak cause and effect relationship between to things

love wave

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

earthquake

a term 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

strike slip

a type of fault where rocks on either side move past each other sideways with little up or down motion

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. Also called a focal-mechanism solution

asperity

an area on a fault that is stuck. The earthquake rupture usually begins at this

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

earthquake hazard

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

spectral acceleration

approximately what is experienced by a building, as modeled by a particle on a massless vertical rod having the same natural period of vibration as the building

seismogenic

capable of generating earthquakes

harmonic tremor

continuous rhythmic earthquakes that can be detected by seismographs. They often precede or accompany volcanic eruptions

fault gouge

crushed and ground-up rock produced by friction between the two sides when a fault moves.

Holocene

denotes the past 10,000 years. It includes most of the time since the end of the most recent ice age. If slip has occurred on a fault during this period, the fault is commonly considered active

Surface Faulting

displacement that reaches the earth's surface during slip along a fault. Commonly occurs with shallow earthquakes, those with an epicenter less than 20 km. Surface faulting also may accompany aseismic creep or natural or man-induced subsidence

aftershocks

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

Strong Motion

ground motion of sufficient amplitude and duration to be potentially damaging to a building or other structure

velocity

how fast a point on the ground is shaking as a result of an earthquake

strain rate

how fast the lithosphere is being deformed from plate tectonic movement

horst

is found together with a graben in an extensional environment. The graben are the downdropped blocks and the horst are the upthrown blocks that lie next to the graben

secular

long-term changes that take place slowly and imperceptibly. Commonly used to describe changes in elevation, tilt, and stress or strain rates that are related to long-term tectonic deformation. For example, a mountain that is growing is getting taller so slowly that we cannot see it happen, but if we were to measure the elevation one year and then the next, we could see that it has grown taller

alluvium

loose gravel, sand, silt, or clay deposited by current or past streams.

moment tensor

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

P wave

or compressional wave, it's 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

Teleseismic

pertaining to earthquakes at distances greater than 1,000 km from the measurement site

pedogenic

pertaining to processes that add, transfer, transform, or remove soil constituents

tectonic

pertaining to the structure or movement of the earth's crust

interplate

pertains to processes between the earth's crustal plates

intraplate

pertains to processes within the plates

Stochastic

processes that have random characteristics

geotechnical

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

tsunamigenic

referring to those earthquakes, commonly along major subduction zone plate boundaries such as those bordering the Pacific Ocean, that can generate tsunamis

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

foreshocks

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

turbidites

sea-bottom deposits formed by massive slope failures. Rivers flowing into the ocean deposit sediments on the continenal shelf and slope. These slopes fail in response to excessive sedimentation load and sometimes earthquake shaking, sending the sediments sliding down to the ocean bottom to create a turbidite. Carbon dating of turbidites has been shown to provide a good method to determine the date of very old earthquakes

surface waves

seismic waves that travel along the Earth's surface

lifelines

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

lateral spread or flow

terms referring to landslides that commonly form on gentle slopes and that have rapid fluid-like flow movement, like water

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. Normal stress can either be compressional or tensional

interplate coupling

the ability of a fault between two plates to lock and accumulate stress. Strong _________________ 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

dip

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

recurrence interval

the average time between successive geologic events

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. The boundary is between 25 and 60 km deep beneath the continents and between 5 and 8 km deep beneath the ocean floor

geophysics

the branch of earth science which employs physical measurements and mathematical models to explore and analyze the structure and dynamics of the solid Earth and similar bodies and their fluid envelopes

Segmentation

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. 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

Refraction

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

focal depth

the depth of an earthquake hypocenter

lithology

the description of rock composition (what it is made of) and texture

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

residual

the difference between the measured and predicted values of some quantity

stress drop

the difference between the stress across a fault before and after an earthquake

wavelength

the distance between successive points of equal amplitude and phase on a wave (for example, crest to crest or trough to trough)

asthenosphere

the ductile part of the earth just below the lithosphere, including the upper mantle. about 180 km thick.

reflection

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

Strick slip

the fast movement that occurs between two sides of a fault when the two sides of the fault become unstuck. The rock becomes distorted, or bent, but holds its position until the earthquake occurs. When the rock snaps back into an unstrained position it is called elastic rebound. Stick-slip displacement on a fault radiates energy in the form of seismic waves, creating an earthquake

fault scarp

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

stress

the force per unit area acting on a plane within a body. Six values are required to characterize completely the stress at a point: three normal components and three shear components

oceanic spreading ridge

the 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

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

kinematic

the general movement patterns and directions of the earth's rocks that produce rock deformation

Quaternary

the geologic time period comprising about the last 1.65 million years

microzonation

the identification of separate individual areas having different potentials for hazardous earthquake effects

Wavefront

the instantaneous boundary between the seismic waves in the earth material, and the material that the seismic energy has not yet reached

rupture front

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, behavior referred to as bilateral

fault trace

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

Q

the inverse of attenuation. A high Q means a low attenuation

mainshock

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

ground motion

the movement of the earth's surface from earthquakes or explosions. It 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

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

Seismicity

the occurrence or frequency of earthquakes in a region

slab

the oceanic crustal plate that underthrusts the continental plate in a subduction zone and is consumed by the earth's mantle

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__________below the crust is brittle enough at some locations to produce earthquakes by faulting, such as within a subducted oceanic plate

crust

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

mantle

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

Late Quaternary

the past 0.5-1.0 million years. Faults that have slipped during this time are sometimes considered active

subduction zone

the place where two lithospheric plates come together, one riding over the other. Most volcanoes on land occur parallel to and inland from the boundary between the two plates

fault plane

the planar (flat) surface along which there is slip during an earthquake

epicenter

the point on the earth's surface vertically above the hypocenter (or focus), point in the crust where a seismic rupture begins

epicenter

the point within the earth where an earthquake rupture starts. It is the point directly above it at the surface of the Earth. Also commonly termed the focus. See also epicenter

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

radiometric

the process of measuring the absolute age of geologic material by measuring the concentrations of radioactive isotopes and their decay products

Subduction

the process of the oceanic lithosphere colliding with and descending beneath the continental lithosphere

forearc

the region between the subduction zone and the volcanic chain (volcanic arc)

backarc

the region landward of the volcanic chain on the other side from the subduction zone

slip

the relative displacement of formerly adjacent points on opposite sides of a fault, measured on the fault surface

geodesy

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

time history

the sequence of values of any time-varying quantity (such as a ground motion measurement) measured at a set of fixed times. Also termed time series

strain

the small changes in length and volume associated with deformation of the earth by tectonic stresses or by the passage of seismic waves

shear stress

the stress component parallel to a given surface, such as a fault plane, that results from forces applied parallel to the surface or from remote forces transmitted through the surrounding rock

geomorphology

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

geology

the study of the planet earth- the materials it is made of, the processes that act on those materials, the products formed, and the history of the planet and its life forms since its origin

source

the term for the released forces that generate acoustic or seismic waves, also called the earthquake source

plate tectonics

the 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. 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

period

the time interval required for one full cycle of a wave

strike

the trend or bearing, relative to north, of the line defined by the intersection of a planar geologic surface (for example, a fault or a bed) and a horizontal surface such as the ground

Ring of Fire

the zone of earthquakes surrounding the Pacific Ocean- about 90% of the world's earthquakes occur there. The next most seismic region (5-6% of earthquakes) is the Alpide belt (extends from Mediterranean region, eastward through Turkey, Iran, and northern India

seismic waves

vibrations that travel through Earth carrying the energy released during an earthquake

Seismograph

A device that records ground movements caused by seismic waves as they move through Earth

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

active fault

A fault that is likely to have another earthquake sometime in the future

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

right lateral

A strike slip fault that appears to be moving to the right tune observer standing on either block. ie. The San Andreas 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

S wave

A type of seismic wave in which the shaking is perpendicular to the direction of the wave

aseismic

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

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

oceanic trench

Deep linear depression in the ocean floor where subduction is taking place.

seismic zone

Generally areas defined on a map within which seismic design requirements are constant. In the NEHRP Provisions, seismic zones are defined both by contour lines and county boundaries.

slip rate

Long-term rate of slip (displacement) along a fault; usually measured in millimeters or centimeters per year.

first motion

On a seismogram, the __________ 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

Seiche

Pendulum-like rocking of water in an enclosed area; a form of standing wave that can be caused by meteorological or seismic forces, or that may result from normal resonances excited by tides.

Poisson Distribution

Probability distribution for the number of arrivals during each time period

bedrock

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

Sand boils

Sand and water that come out onto the surface of the ground during an earthquake as a result of liquefaction (water-filled sediment that temporarily loses strength and acts as a fluid). The bubbling mud that looks like it is boiling but it is not.

creep

Slow, more or less continuous movement occurring on faults due to ongoing tectonic deformation. Do not tend to have large earthquakes.

rupture velocity

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

Rayleigh waves

Surface waves that travel in a backward-rotating, elliptical motion, causing both vertical and horizontal ground movement. Can travel through solid, liquid, and gas. Fourth to arrive. Dangerous.

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

core

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

Pleistocene

The most recent epoch of the Late Cenozoic Ice Age, beginning about 1.8 million years ago and marked by as many as 20 glaciations and interglaciations of which the current warm phase, the Holocene epoch, has witnessed the rise of human civilization.

slickensides

The polished surface of a fault caused by slip on the fault; lineated [ ] also have grooves that indicate the direction of fault movement.

sea-floor spreading

The process by which molten material adds new oceanic crust to the ocean floor

great circle

The shortest path between two points on the surface of a sphere lies along a ___________

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

magnetic polarity reversal

a change of the earth's magnetic field to the opposite polarity. This 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. Rocks created along the oceanic spreading ridges commonly preserve this pattern of polarity reversals as they cool, and this pattern can be used to determine the rate of ocean ridge spreading. The reversal patterns recorded in the rocks are termed sea-floor magnetic lineaments.

geologic time scale

a chronological sequence of geologic events usually represented in the form of a chart showing names of various rock layers and indicating the estimated duration of each geologic unit

isoseismal line

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

spectrum

a curve showing amplitude and phase as a function of frequency or period, or how much of each type of shaking there is from an earthquake

graben

a down-dropped block of the earth's crust resulting from extension, or pulling, of the crust

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

fault

a fracture along which the blocks of crust on either side have moved relative to one another parallel to the fracture

ground failure

a general reference to landslides, liquefaction, lateral spreads, and any other consequence of shaking that affects the stability of the ground

velocity structure

a generalized regional model of the earth's crust that represents crustal structure using layers having different assumed seismic velocities

travel time curve

a graph of arrival times, commonly P or S waves, recorded at different points as a function of distance from the seismic source. Seismic velocities within the earth can be computed from the slopes of the resulting curves

slip model

a kinematic model that describes the amount, distribution, and timing of slip associated with an earthquake

halfspace

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

seismic moment

a measure of the size of an earthquake based on the area of fault rupture, the average amount of slip, and the force that was required to overcome the friction sticking the rocks together that were offset by faulting. Can also be calculated from the amplitude spectra of seismic waves

landslide

a movement of surface material down a slope

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. There are many __________ for an earthquake, depending on where you are, unlike the magnitude, which is one number for each earthquake.

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 magnitude (Mb), and (4) moment magnitude (Mw). Scales 1-3 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. All magnitude scales should yield approximately the same value for any given earthquake

Seismogram

a record written by a seismograph in response to ground motions produced by an earthquake, explosion, or other ground-motion sources

thrust fault

a reverse fault in which the hanging wall slides over to the foot wall.

tsunami

a 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

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

seismic reflection line

a set of seismographs usually lined up along the earth's surface to record seismic waves generated by an explosion for the purpose of recording reflections and refractions of these waves from velocity discontinuities within the earth. The data collected can be used to infer the internal structure of the earth