Glossary of Terms

Sources

Allaby, Ailsa and Allaby, Michael. 1990. The Concise Oxford Dictionary of Earth Sciences. Oxford, NY: Oxford University Press.
Leet, L. Don. 1982. Physical Geology, 6th Edition. Englewood Cliffs, NJ: Prentice-Hall.

A

absolute time - Geological time measured in years and determined by the time-dependent decay of radioactive isotopes.
aftershock - a seismic event which occurs, often repeatedly, after an earthquake. Compare with mainshock.
Antelope - system of software modules that implement acquisition, transport, buffering, processing, archiving and distribution of environmental monitoring information. Visit the BRTT website for more information.
aquifer - A body of rock that is water-saturated and is sufficiently permeable to permit ground water to move through it.
aspect ratio - The ratio of width to height, commonly applied tot he shape of volcanoes, sedimentary basins, etc.
asteroid - A small celestial body orbiting the sun, between Mars and Jupiter.
Asthenosphere - The part of the Earth's mantle beneath the lithosphere, which flows (i.e. undergoes plastic deformation) to produce isostatic readjustments as changes in the mass of the lithosphere (above) takes place.
attenuation - A reduction in the amplitude of energy, used frequently for seismic waves.
author - [at this website] the analyst and institution that picked the hypocenter origin time, latitude, longitude, and depth.

B

batholith - A large, generally discordant plutonic igneous intrusion, often elongated, and outcropping over at least 100km² and extending to unknown depth. Batholiths are usually associated with orogenic belts.
Benioff zone - an inclined plane along which earthquake foci cluster at a subduction zone. The Benioff zone is believed to correspond to the position of the subducted slab of oceanic lithosphere.
body wave - push-pull or shake earthquake wave traveling through a body of a medium; distinguished from waves traveling along free surface.
broadband - a system that enables many messages or large amounts of information to be sent at the same time and very quickly between computers or other electronic devices.

C

compressional wave - See P-wave
continental accretion - The outbuilding or vertical thickening of continental crust by the addition of new material. For example, this process may happen tectonically at subduction zones where trench and ocean floor sediment is deformed or scraped onto the continental plate.
continental drift - The lateral movement of continents as a result of sea-floor spreading.
continental shelf - The area of the ocean floor bordering the continental land masses at a depth of 200m or less below sea leavel and physically an integral part of the continent. The gradient of the shelf is approximately 0.10.
convection - Mass movement of a fluid resulting in heat transfer. The heated part expands and rises, cooler material flows in to take its place, and a convection current is established.
core - The central zone of the Earth that lies below the major solid-liquid boundary at 2900 km depth (solid mantle above the liquid outer core). This boundary produces a major seismic discontinuity. It consists of an inner core, which is thought to be solid and a liquid outer core.
creep - Gradual plastic deformation of a rock as a result of continuous stress.
crust - outermost shell of the earth. Continental crust averages 35 km thick, density 2.6 t/m³; oceanic crust, about 5 km thick, density 3 t/m³.

D

dataloggers - instruments that record and store data
density - measure of concentrations of matter, expressed as mass per unit volume.
diffusivity - a measure of the rate at which diffusion occurs through a material.
digitize - to put information into the form of a series of the numbers 0 and 1, usually so that it can be processed by a computer
dyke - a sheet-like igneous body intruded into other rocks. Dykes in sedimentary rocks cross-cut bedding, unlike sills which are parallel to bedding.

E

earthquake - a movement within the Earth's crust or mantle, caused by the sudden rupture or repositioning of underground rocks as they release stress.
earthquake intensity scales - a comparison
epicenter - the point on the earth's surface vertically above an earthquake's focus. It is specified by latitude and longitude. Compare with hypocenter.
epicentral distance - the distance from the epicentral location of an earthquake. It is the shortest distance (great circle path) and is measured either in kilometers or degrees.
Euler's theorem - the theory that describes a displacement of points over a spherical surface in angular coordinates relative to a single fixed point, termed the Euler pole.

F

fault - surface of rock rupture along which has been differential movement.
fault plane - the more or less planar surface of a fault.
felsic - used to describe an igneous rock with abundant light colored minerals like quartz, feldspar and muscovite (e.g. granite).
free oscillation - an oscillation of a body that, once initiated, occurs without an external influence.
focus - source of given set of earthquake waves.

G

Geological Time Scale - time periods of the earth's history, based on stratigraphic, fossil and radiometric analysis. Visit our FAQ page on this topic.
Greenwich Mean Time (GMT) - the time at Greenwich, Greater London, that world time zones are based on.
great circle - a circle on the Earth's surface, the plane of which passes through the center of the Earth.
great circle path - the shortest distance between any two points on the surface of the earth.

H

half life - the time taken for half of the parent atoms of a radioactive isotope to decay.
heat flow - this is the amount of heat leaving the Earth which varies from place to place, measured in mW m-2 (also known as heat flux).
hot spot - a volcanic center 100- 200 km in diameter which is persistent for tens of millions of years and is thought to be the surface expression of a long-lived rising plume of hot mantle.
hypocenter - this is the point in the earth where the earthquake is located. It is specified by latitude, longitude and depth. Compare with epicenter.

I

intensity - measure of effects of earthquake waves on human beings, structures, and earth's surface at particular place. Contrast with magnitude, which is measure of total energy released by an earthquake.
IRIS - Incorporated Research Institutions for Seismology (IRIS). Visit the IRIS homepage for more information.
isostasy - the condition of equilibrium whereby areas of rigid lithosphere tend to ‘float’ in the plastically-deforming weak asthenosphere.
isotherm - a line connecting points of equal temperature.
isotopes - atoms of the same element with different numbers of neutrons in their nuclei, i.e. the same atomic numbers but different mass numbers.

J

julian date - the number of elapsed days since the beginning of a particular year. For example, in this usage, the Julian date for the calendar date of 1998-02-28 would be day 059.

K

L

latitude - the position north or south of the equator measured from 0° to 90°.
lithification - the process of conversion of an unconsolidated sediment to a solid rock.
lithoshpere - the outer part of the Earth down to about 100- 200 km which lies above the asthenosphere and is essentially rigid. It includes the Earth's crust and the upper part of the mantle.
lithostatic pressure - pressure in a rock due to the weight of the overlying rock.
local - an earthquake whose epicenter is less than 1000km from a recording seismometer. Earthquakes whose epicenters are further away than this are regarded as teleseismic events.
longitude - the distance of a place east or west of an imaginary line from the top to the bottom of the Earth, measured in degrees.

M

mafic minerals - dark-colored minerals like pyroxenes and amphiboles. Mafic, literally magnesium-iron bearing, is also used to describe dark colored igneous rocks. Igneous rocks with 44-55% silica are also said to be mafic (or basic).
magma - naturally occurring molten or partially molten rock formed within the Earth from which igneous rocks crystallize. Magma commonly contains dissolved gases which are released on crystallization and/or depressuring.
magma chamber - a reservoir of magma in the crust from which volcanic material is derived.
magnitude - measure of total energy released by an earthquake. Contrast with intensity, which is measure of effects of earthquake waves at particular place. Several different magnitude scales are used: surface wave magnitude (Ms), body wave magnitude (mb), moment magnitude (Mw), local magnitude (Ml) and several others. See magnitude definitions for more information
mainshock - the main earthquake at a specified time and location. Compare with aftershock.
mantle - the part of the interior of the Earth that lies between the Mohorovicic seismic discontinuity at 6 to 90 km depth and the core-mantle boundary at 2900 km depth. The mantle has an overall peridotite (ultramafic) composition, with the types of minerals present varying with depth.
matter - anything that occupies space. Usually defined by describing its states and properties: solid, liquid, or gas; possesses mass, inertia, color, density, melting point, hardness, crystal form, mechanical strength, or chemical properties. Composed of atoms .
mb - body wave magnitude
Mercalli intensity scale - scale to evaluate intensity of earthquake shaking on basis of effects at given place.
meteorite - extraterrestrial material that falls to Earth. Most meteorites are thought to be composed of relatively primitive matter, similar to that which formed the Earth, and to be derived from the asteroid belt between the solar orbits of Mars and Jupiter.
Ml, ML - local magnitude
Milankovitch cycle - cycles of variation in the amount of solar energy falling on the Earth caused by variations in the Earth's orbit around the sun, and the tilt of its axis of rotation.
Mohorovicic discontinuity (Moho) - a seismic discontinuity thought to represent the boundary between the crust and the mantle. Typically 35 km below the surface of the continents and 5-11 km below the ocean floor.
Ms, MS - surface wave magnitude
Mw, MW - moment magnitude

N

nebula - an interstellar diffuse cloud of dust particles and gases.
NEIC - the National Earthquake Information Center, NEIC, broadcasts preliminary information about an earthquake's origin time, epicentral location, depth and magnitude soon after the earthquake occurs. Visit the NEIC.
normal fault - a fault caused by extension, in which the rocks on the upper side of the fracture have moved down relative to the rocks on the under side of the fracture.

O

obduction - the process by which oceanic lithosphere may be thrust onto less dense continental crust at destructive plate margins.
ocean ridge - a raised ridge sometimes taking a median line within an ocean basin along which new ocean lithosphere is formed. The ridge is topographically high because high heat flow causes thermal expansion of the lithosphere.
olivine - group of silicate minerals including forsterite and fayalite with the general formula (Mg, Fe, Mn, Ca)₂SiO₄ and usually dark green. Olivines are common minerals in basalts, peridotites and some other igneous rocks. Olivine is the dominant mineralogical constituent of the upper mantle.
oolite - a rock composed of spherical carbonate particles termed ooliths or ooids which are formed of concentric layers of calcite or aragonite deposited around a nucleus (shell fragment, quartz grain etc.).
ophiolite - a group of basic to ultrabasic igneous rocks and deep sea sediments (cherts) thought to represent oceanic lithosphere (principally crustal) material, which is tectonically emplaced onto an island arc or continental margin.
orogeny - the processes that form structures within mountainous areas, including faulting and folding, metamorphism and igneous activity. Elevantion of mountains is due to uplift (or epeirogeny) of thrusted continental crust during isostatic adjustment.
origin time - the time at which an earthquake begins. Since local times are different all over the world, it is important to "standardize" time. The origin times in this website are always in UTC as determined by NEIC. On a seismogram, the earthquake origin time can be determined by the P wave arrival.

P

Pacific Daylight Time (PDT) - the U.S. standard time zone that is 7 hours earlier than universal time (UTC)
Pacific Standard Time (PST) - the U.S. standard time zone that is 8 hours earlier than universal time (UTC).
palaeomagnetism - the study of the remanent magnetization crystallized at their formation in minerals to determine the history of the Earth's magnetic field and the relative positions of the continents.
phase diagram - a graphical diagram on which the boundaries between different solid phases and between solids, liquids and gases, representing chemical reactions are shown.
plate tectonics - the theory that the outer layer of the Earth consists of rigid plates of lithosphere in motion relative to each other and to the interior of the Earth.
primary (P-) wave - earthquake body wave that travels fastest and advances by push-pull mechanism. Also known as longitudinal, compressional, or P wave.
pyrite - a metallic, yellow mineral composed of iron sulphide: FeS₂. Pyrite is the most abundant sulphide mineral.

Q

Quality-factor - A measure of the losses in a resonance system. The sharper the tuning curve, the higher the Q.
quartz - a common rock forming silica mineral: SiO₂. Quartz is usually white, or clear and glassy (pale grey); internally it has a three -dimensional framework structure and is resistant to weathering. It is a common constituent of many igneous, metamorphic and sedimentary rocks (e.g. quartzites and sandstones).

R

radioactive decay - the spontaneous decay of an unstable atomic nucleus into a lighter nucleus with the emission of a-, b- or g-radiation.
radiogenic isotope - an isotope that was produced by radioactive decay of a radioactive isotope.
radiometric date - the age of a rock in years determined by the relative proportions of radioactive isotopes and their decay products present within the rock.
reflection (seismic) - the bouncing of a wave off a surface in such a way that it obeys the law of reflection, i.e. the angle of incidence is equal to the angle of reflection measured relative to the normal (at right angles) to the surface.
refraction (seismic) - the process by which a wave’s direction of propagation changes as it travels across a boundary between two materials. This bending is caused by the speed of the wave being different in the two materials.
regional - within a particular region (eg. California)
relative time - geological time determined by placing events in a chronological order.
retrograde metamorphism - metamorphism that results in a high-grade mineral or assemblage changing to a lower grade mineral or assemblage. Retrograde metamorphism usually results from the infiltration of water at lower temperatures than the maximum metamorphic temperature attained.
reverse faulting - a steeply inclined fault caused by compression in which rocks on the upper side of the fault move up relative to rocks on the lower side of the fault.
Richter scale/magnitude - a logarithmic scale that measures the amount of energy released during an earthquake on the basis of the amplitude of the highest peak recorded on a seismogra m. Each unit increase in the Richter scale represents a 10-fold increase in the amplitude recorded on the seismogram and a 30-fold increase in energy released by the earthquake. Theoretically the Richter scale has no upper limit, but the yield point of the Earth's rocks imposes an effective limit between 9.0 and 9.5.
rock mechanics - the science of physical behavior of rocks.

S

scarp - a straight line of steep cliff's or slopes. If the steep slope has been formed by a fault it is known as a fault scarp.
schist - a strongly foliated crystalline metamorphic rock. The foliation results from alignment of micas. The term is often qualified by the name of the major mineral present in the rock e.g., biotite-schist, hornblende-schist.
seafloor spreading - the process by which the lithospheric plates either side of an ocean ridge grown by the addition of new material as the plates either side of the ridge move apart.
secondary/surface (S-) wave - earthquake body wave slower than primary wave. Shear, shake, or S wave.
seismic - relating to or caused by an earthquake.
seismic discontinuity - a distinct boundary separating two rock units within which seismic waves travel at different speeds.
seismic gap - a locked fault segment that has not experienced seismic activity for a long time. Because stress tends to accumulate in seismic gaps, they often become the sites of major earthquakes.
seismic moment - a numerical means of measuring an earthquake's total energy release. It is calculated by measuring the total length of fault rupture and then factoring in the depth of rupture, total slip along the rupture, and the strength of the faulted rocks.
seismic stratigraphy - the interpretation of seismic reflection data in terms of inferred stratal relationships and rock types in order to construct a geological history of the region surveyed.
seismogram - record obtained on a seismograph.
seismograph - instrument for recording vibrations, most commonly employed for recording earth vibrations during earthquakes.
seismology - scientific study of earthquakes and other earth vibrations.
shadow zone - a zone at some distance from the epicenter of an earthquake which does not receive direct seismic waves due to refraction at a boundary where the seismic velocity of materials present decreases downwards across the boundary (e.g. in the Earth, the core mantle boundary at 2900 km depth causes a shadow zone from 103 to 142° away from the epicenter).
shake wave - wave that advances by causing particles in path to move from side to side or up and down at right angles to direction of wave's advance, a shake motion. Also called shear wave , or secondary wave.
shear - change of shape without change of volume.
shear wave - wave that advances by shearing displacements (which change shape without changing volume) of medium. This causes particles in path to move from side to side or up and down at right angles to direction of wave's advance. Also called shake wave, or secondary wave.
shearing stress - stress that slices rocks into parallel blocks that slide in opposite directions along their adjacent sides. Shearing stress may be caused by transform motion.
shear zone - an originally planar zone of rock that has been deformed or fractured by shearing motions. Shear zones often act as zones of fluid flow and therefore have associated hydrothermal alteration or mineralization.
sill - an igneous body intruded parallel to the bedding plane of the host sediments.
simple shear - a type of constant volume deformation similar to shearing an deck of cards in one direction.
slumping - the sliding downslope of a mass of sediment or rock by rotary motion along a concave upwards surface. The process can occur on land to produce landslips or beneath the sea to produce submarine slumps.
standing wave - a regular oscillating wave that exists between two fixed (nodal) points.
Stokes' Law - a formula which expresses the rate of settling of a particle in a fluid as a function of its radius, the particle density, the fluid density, the fluid viscosity and the acceleration due to gravity.
strain - when a material is subject to stress, the strain is the deformation produced; for example, longitudinal compressional strain is the change in length of the sample divided by the original length of the sample.
stratigraphy - the study of the sequence and correlation of rock strata in order to interpret the geological history on local and global scales.
stratovolcano - a steep-sided volcano that is composed of alternating layers of lava and pyroclastic deposits.
strike-slip fault- fault in which movement is almost in direction of fault's strike. A high angle or vertical fault on which the movement is parallel to the strike of the fault, sometimes called transverse or transcurrent motion.
subduction - the process by which oceanic lithosphere is conveyed down into the mantle at destructive plate margins. The area in whch this takes place is called a subduction zone, the surface expression
swarm - earthquake swarm. A series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time.
S -Wave - a transverse seismic wave produced by shearing motion. S-waves are so called because they are the second set of waves to arrive from an earthquake.

T

telemetry - the science or process of gathering information about objects which are far away and sending the information somewhere electronically.
teleseismic - an earthquake whose epicenter is more than 1000km from, but detectable at, a recording seismometer. Earthquakes whose epicenters are closer than this are regarded as local events.
thermal conductivity - a measure of the ability of a material to conduct heat, numerically equal to the heat flow divided by the thermal gradient.
thermodynamics - the mathematical treatment of the relation of heat to other forms of energy. In petrology the thermodynamics of minerals is widely used to interpret the equilibrium between different mineral phases and the temperatures and pressures of mineral/mineral assemblage formation.
thrust fault - a low angle reverse (compressional) fault.
thrust sheets - sheet-like units of crustal rocks bounded by a thrust fault
transform fault - point at which strike-slip displacements stop and another structural feature, such as a ridge, develops. A fault laterally displacing ocean ridges. Superficially they appear to be strike-slip faults, but lateral movement in opposite directions to either side of the fault only occurs in the zone between the offset segments of ocean ridge.
trench - a narrow elongate depression of the ocean floor with a crescent shape in plan view and usually parallel to volcanically active continental or island arcs.
TriNet - TriNet is a multifunctional seismic network for earthquake
research, monitoring and computerized alerts. Visit the TriNet homepage
triple junction - a point where three lithospheric plates meet.
tuff - a general term for all consolidated pyroclastic rocks, but often used for the finer grained variation.
turbidity current - a bottom flowing density current of suspended sediment moving swiftly down a subaqueous slope and spreading out at the base of the slope. Such currents occur in lakes and on the continental shelf wher they carry sediment on the ocean floor.

U

UTC - The times of various events, particularly astronomical and weather phenomena, are often given in "Universal Time" (abbreviated UTC) which is sometimes referred to, now colloquially, as "Greenwich Mean Time" (abbreviated GMT). The two terms are often used loosely to refer to time kept on the Greenwich meridian (longitude zero), five hours ahead of Eastern Standard Time. Times given in UT are almost always given in terms of a 24-hour clock. Thus, 14:42 (often written simply 1442) is 2:42 p.m., and 21:17 (2117) is 9:17 p.m. Sometimes a Z is appended to a time to indicate UT, as in 0935Z.

V

viscosity - a measure of the ease with which a fluid will flow.

W

water-table - a surface of an unconfined body of ground water at which the pressure is equal to the atmospheric pressure.
wave - configuration of matter that transmits energy from one point to another.
wavefront - a surface representing the position of a traveling seismic disturbance at any given time.
weathering - the chemical or physical breakdown of rocks at the surface by atmospheric agents and physical processes.

X

Y

Z

URL: http://eqinfo.ucsd.edu/faq/glossary.php [Last updated: 2006-10-26 (299) 22:51:12 UTC]