Amateur Geologist Structured Geological Glossary: Geophysics

Structured Geological Glossary: Geophysics

gravity	The force of attraction between to masses pulling them towards the center of each other. The attractive force becomes less with distance. A mountain can attract and deflect a plumb bob from true vertica with the Earth's center.
gravity anomaly	The value of gravity left after subtracting from a gravity measurement the reference value based on latitude, and possibly the free-air and Bouguer corrections.
isostasy	The mechanism whereby areas of the crust rise or subside until the mass of their topography is buoyantly supported or compensated by the thickness of crust below, which floats on the denset mantle. The theory that continent and mountain are supported by low-density crustal roots.
isostatic adjustment	The process whereby lateral transport at the Earth's surface from erosion or deposition is compensated for by movements in a subcrustal layer to maintain equilibrium among units of varying masses and densities. Also called isostatic compensation.
geophysical noise	Fluctuations in a geophysical parameter that are of a quasirandom nature and cannot be attributed to fluctuations in known causative phenomena.
gravity survey	The measurement of gravity at regularly-spaced grid points with repetitions to control instrument drift.
earthquake	The violent oscillatory motion of the ground caused by the passage of seismic wave radiating froma fault along which sudden movement has taken place.
seismicity	" The world-wide or local distribution of earthquake in space and time; a general term for the number of earthquakes in a unit of time. "
microearthquake	An earthquake having a magnitude of 2 or less on the Richter scale.
aseismic	"Not associated with an earthquake as in aseismic slip. Also used to indicate an area with no record of earthquakes; an aseismic zone. "
seismic zone	A region in which earthquake are known to occur.
seismic	Pertaining to earthquake activity.
spasmodic tremor	"Continuous seismic signal with pulsating high-frequency appearance; can be composed of discrete earthquake occurring closely spaced in time or can be phaseless."
seismograph	"An instrument that records seismic wave; that is vibrations of the earth. Used to record and measure earthquake. "
seismic profile	The data collected from a set of seis-toographs arranged in a straight line with an artificial seismic source, especially the times of P-wave arrival.
seismogram	A written record of an earthquake recorded by a seismograph.
accelerometer	A seismograph for measuring ground acceleration as a function of time.
recurrence Interval	The approximate length of time between earthquake in a specific seismically active area.
arrival	The appearance of seismic energy on a seismic record.
phase	The onset of a displacement or oscillation on a seismogram indicating the arrival of a different type of seismic wave.
travel time	The time required for a wave train to travel from its source to a point of observation.
teleseism	An earthquake that is distant (usually more than 20 degrees) from the recording station.
paleoseismology	Study of prehistoric earthquake, often by excavation across an active fault to examine evidence of past surface-rupturing events.
focus (earthquake)	"The point at which the rupture occurs; synonomous with hypocenter. "
hypocenter	"The calculated point below the epicenter at which an earthquake actually begins; the focus. "
epicenter	The point on the Earth's surface directly above the focus (or hypocenter) of an earthquake.
rupture zone	The area of the Earth through which faulting occurred during an earthquake. For very small earthquakes, this zone could be the size of a pinhead, but in the case of a great earthquake the rupture zone may extend several hundred kilometers in length and tens of kilometers in width.
magnitude	A measure of the strength of an earthquake or strain energy released by it as determined by seismographic observations. This is a logarithmic value originally defined by Charles Richter (1935). An increase of one unit of magnitude (for example, from 4.6
major earthquake	An earthquake having a magnitude of 7 to 7.99 on the Richter scale.
great earthquake	An earthquake having a magnitude of 8 or greater on the Richter scale.
Richter scale	"The system used to measure the strength of an earthquake. Developed by Charles Richter in 1935 as a means of categorizing local earthquakes. It is a collection of mathematical formulas; it is not a physical device. "
order of magnitude	"Ten times; thus an increase of two orders of magnitude is 10 x 10 or 100 times."
intensity	A measure of the effects of an earthquake at a particular place on humans, structure and (or) the land itself. The intensity at a point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the earthquake to the point and the local geology at that point.
Modified Mercalli intensity scale	"Proposed in 1902 by Giuseppi Mercalli of Italy; updated in 1931 by H.O. Wood and F. Neumann as Modified Mercalli; further revised in 1956 by C. F. Richter and presented here in abridged form. Earthquake intensity on the MM scale is given as Roman numerals to prevent confusion with Richter magnitude and refers to the earthquake maximum intensity."
T-phase	Tertiary wave, a late-arriving short-period phase often recorded at stations of island or coastal regions, containing energy that travels through a layer of water in the ocean known as the SOFAR channel. Can be caused by earthquake or submarine volcanic eruption.
P wave	Primary, longitudinal, irrotational, push, pressure, dilatational, compressional or push-pull wave. P wave are the fastest body waves and arrive at stations before the S wave or secondary waves. The waves carry energy through the Earth as longitudinal waves, moving particles in the same line as the direction of the wave. P waves can travel through all layer of the Earth. P waves are generally felt by humans as a bang or thump.
S wave	"Secondary wave, a shear or transverse elastic wave, usually the most prominent wave on a seismogram; travels more slowly than P-waves and thus arrives later on seismograms. Shear, secondary, rotational, tangential, equivoluminal, distortional, transverse or shake wave. These waves carry energy through the Earth in very complex patterns of transverse (crosswise) waves. These waves move more slowly than P wave, but in an earthquake they are usually bigger. S wave cannot travel through the outer core because these waves cannot exist in fluids, such as air, water or molten rock."
body wave	A seismic wave that can travel through the interior of the earth. P-waves and S-waves are body waves.
Rayleigh wave	A type of surface wave having a retrograde elliptical motion at the Earth's surface similar to the waves caused when a stone is dropped into a pond. These are the slowest but often the largest and most destructive of the wave types caused by an earthquake. They are usually felt as a rolling or rocking motion and in the case of major earthquake can be seen as they approach. Named after Lord Rayleigh, the English physicist who predicted its existence.
surface waves	Waves that move over the surface of the Earth. Rayleigh wave and Love wave are surface waves.
Lg wave	A surface wave which travels through the continental crust.
Love wave	A major type of surface wave having a horizontal motion that is shear or transverse to the direction of propagation (travel). It is named after A.E.H. Love, the English mathematician who discovered it.
explosion earthquake	Events with ground wave (including P-waves and S-waves) and often air waves, which travel through the air and are transmitted back into the ground in the vicinity of the seismometer.
travel-time curve	A curve on a graph of travel time versus distance for the arrival of seismic wave from distant events. Each type of seismic wave has its own curve.
isoseismal line	A line connecting points on the Earth's surface at which earthquake intensity is the same. It is usually a closed curve around the epicenter.
low-frequency event	General term for discrete events with either enriched low frequencies or deficient high frequencies compared to typical earthquake.
low-velocity zone	A region in the Earth, especially a planar layer, that has lower seismic-wave velocities than the region immediately above it.
B-type earthquake	Events with weak P-waves and no S-waves a low-frequency content and occurring under volcanoes at depths < 1 km.
seismic transition zone	"A seismic discontinuity, found in all parts of the Earth, at which the velocity increases rapidly with depth; especially the one at 300 to 600 kilometers. "
seismic surface wave	A seismic wave that follows the earth's surface only, with a speed less than that of S-waves. There are Raleigh waves (forward and vertical vibrations) and Love wave (transverse vibrations).
seismic discontinuity	Regionally extensive, thin zones in the crust or mantle where seismic velocities change abruptly. May be the result of changes in composition, mineral phase or structural character.
earthquake swarm	A series of minor earthquake, none of which may be identified as the main shock, occurring in a limited area and time.
A-type earthquake	Events with clear P-waves and S-waves occurring under volcanoes at depths of 1-10 km.
seismic belt	An elongated earthquake zone, for example, circum-Pacific, Mediterranean, Rocky Mountain. About 60% of the world's earthquakes occur in the circum-Pacific seismic belt.
leaking mode	A surface seismic wave which is imperfectly trapped so that its energy leaks or escapes across a layer boundary, causing some attenuation or loss of energy.
low-velocity zone	Any layer in the Earth in which seismic wave velocities are lower than in the layers above and below.
long-period event	Discrete events with nearly monotonic, low-frequency (1-5 Hz) waveforms that resonate for many cycles.
ScS	Seismic phase consisting of a shear wave traveling down through the mantle that is reflected at the core-mantle boundary and returns to the earth's surface as a shear wave. ScSn denotes a multiple ScS wave that is further reflected n-1 times at the earth's free surface.
microseism	A weak vibration of the ground that can be detected by seismograph and which is caused by wave, wind, or human activity, but not by an earthquake.
harmonic tremor	A continuous release of seismic energy typically associated with the underground movement of magma. It contrasts distinctly with the sudden release and rapid decrease of seismic energy associated with the more common type of earthquake caused by slippage along a fault. The seismic signal is characterized by a nearly monotonic sinusoidal appearance and low frequency (0.5-5 Hz).
shock wave	Moving wave of strong compression that usually travels faster than sound in the medium.
foreshocks	Smaller earthquake preceding the largest earthquake of a series concentrated in a restricted crustal volume.
aftershock	An earthquake which follows a larger earthquake or main shock and originates in or near the rupture zone of the larger earthquake. Generally, major earthquake are followed by a larger number of aftershock decreasing in frequency with time.
spread	The layout of seismometer or geophone group from which data from a single shot (the explosive charge) are recorded simultaneously.
free oscillation	The ringing or periodic deformation of the whole Earth at characteristic low frequencies after a major earthquake.
refraction (wave)	The departure of a wave from its original direction of travel at the interface with a material of different index of refraction (light) or seismic wave velocity (see also Seismic refraction).
seismic constant	In building codes dealing with earthquake hazards, an arbitrarily-set acceleration value (in units of gravity) that a building must withstand.
seismic refraction	A mode of seismic prospecting in which the seismic profile is examined for wave that have been refracted upward from seismic discontinuities below the profile. Greater depths may be reached than through seismic reflection.
array	An ordered arrangement of seismometers or geophones the data from which feeds into a central receiver.
borehole geophysics	Science of (seismic or electrical) physical measurement between boreholes or between a borehole and the surface.
seismic reflection	A mode of seismic prospecting in which the seismic profile is examined for wave that have reflected from near-horizontal strata below the surface.
geomagnetic field	Main magnetic field of the earth -- the part generated within the earth.
magnetic north pole	(1) The point where the Earth's surface intersects the axis of the dipole that best approximates the Earth's field. (2) The point where the Earth's magnetic field dips vertically downward.
declination	At any place on Earth, the angle between the magnetic and rotational poles.
magnetometer	An instrument for measuring either one orthogonal component or the entire intensity of the Earth's magnetic field at various points.
inclination	" The angle between a line in the Earth's magnetic field and the horizontal plane; also a synonym for dip. "
arrival time	The time at which a particular wave phase arrives at a detector.
magnetic anomaly	A local increase or decrease in the strength of the earth's magnetic field caused by the magnetism of nearby sediment or rocks.
sferics	Natural atmospheric fluctuations of the electromagnetic fields, caused mainly by thunderstorm activity in the range of 1-10E5 Hz.
polarity chron	Time interval of constant geomagnetic polarity.
paleomagnetism	"The magnetism of an iron-bearing rock imparted to it by the Earth’s magnetic field when the rock formed. Literally, early magnetism; meaning magnetism formed in a past geologic era. The study of paleomagnetism is important for determing the position of plate at various geological times."
dynamo action	Process whereby motions within an electrcally conducting fluid, interacting with a magnetic field, convert their kinetic energy into magnetic energy, thereby sustaining the magnetic field.
secular variation	Slow changes in the orientation of the Earth's magnetic field that appear to be long lasting and internal in origin as opposed to rapid fluctuations, which are external in origin.
polarity zone	Stratigraphic interval in which the rocks or sediment carry a magnetization indicating formation in a field of constant polarity.
depostion remanent magnetization	A weak magnetization created in sedimentary rocks by the rotation of magnetic crystal into line with the ambient field during settling.
chemical remanent magnetization	Magnetization acquired isothermally through chemical change at temperatures below the Curie temperature either by grain growth or by alteration of a magnetic parent to a magnetic daaghter.
magnetic jerk	Change in the slope of the geomagnetic secular variation. The change occurs within a few years and is global in extent.
magnetic reversal	Earth’s magnetic field occassionally flips or reverses polarity. This means that, if a polarity reversal happened today, your compass would point south instead of north! An important clue to plate tectonics.
reversed polarity	Geomagnetic polarity that is opposite to the present polarity.
geomagnetic polarity time scale	Reference for the polarity history of the geomagnetic field, including current correlations of polarity intervals to biozonations, geologic stage and numerical ages.
magnetic anomaly	The value of the local magnetic field remaining after the subtraction of the dipole portion of the Earth's field.
central angle	An angle with the vertex at the center of the Earth with one ray passing through the hypocenter (and also the epicenter) and the other ray passing through the recording station.
seismic sea wave	A tsunami generated by an undersea earthquake.
geodimeter	A surveying instrument to measure the distance between two points on the Earth's surface.
rheology	Science of the deformation and flow of matter.
rheidity	(1) The ability of a substance to yield to viscous flow under large strain. (2) One thousand times the time required for a substance to stop changing shape when stress is no longer applied.
remote sensing	The study of Earth surface conditions and materials from airplanes and satellites by means of photography, spectroscopy, or radar.
pratt isostatic compensation	The mechanism in which variations in crustal density act to counterbalance the varying weight of topographic features. The crust is here assumed to be of approximately uniform thickness, thus a mountain range would be underlain by lighter rocks.
potential energy (gravitational)	The stored energy of a substance. Water has a lot of this if there is an elevation difference. Potential energy can be converted to kinetic energy if the water (or other substance) is allowed to move.
modeling	An investigative technique that uses a mathematical or physical representation of a system or theory that accounts for all or some of its known properties. Models are often used to test the effects of changes of system components on the overall performance of the system.
hertz	The unit of frequency equal to one cycle per second.
geopotential model	Set of coefficients of a series representation of the earth's gravitational field in terms of spherical harmonics.
elastic wave	A wave that is propagated by some kind of elastic deformation, that is, a change in shape that disappears when the forces are removed. A seismic wave is a type of elastic wave.
elastic limit	The maximum stress that can be applied to a body without resulting in permanent strain.
dispersion (wave)	The spreading out of a wave train due to each wavelength traveling with its own velocity.