Glossary of Terms:

Accuracy: a measure of how close the results of an experiment are to the true value; a measure of the correctness of the result.
Antenna: a device for radiating or receiving radio waves.
Amplitude: the maximum disturbance or distance from the constant point.
Asthenosphere: layer of the Earth found 100-500 km below the Earth's surface that yields to persistent stresses more than the rigid crust or the core. Click here to see an image
Atmosphere: the whole mass of air surrounding the earth. Click here to see an image
Body Waves: waves that move within the Earth's interior or within a body of rock.
Collapse Earthquakes: small earthquakes in underground caverns and mines that are caused by seismic waves produced from the explosion of rock on the surface.
Compression: fractional decrease of volume due to pressure.
Conservation: materials involved are neither created nor destroyed.
Convergence: coming together or joining at a common point.
Conversion Factors: ratios that allow variables to be interchanged.
Cretaceous Period: the last period of the Mesazoic Era (approximately 70 million years ago) and its corresponding system of rocks.
Cycle: interval of time during which one sequence of a regular succession of events is completed.
Data: information or material that serves as a basis for discussion and analysis.
Deformation: process where rocks are folded, faulted, sheared or compressed by Earth stresses.
Department of Defense: Home Page
Dip: the angle by which a rock layer or fault plane deviates from the horizontal. The angle is measured in a plane perpendicular to the strike.
Dip-slip Fault: a fault in which the relative displacement is along the direction of dip of the fault plane; the offset is either normal or reverse.
Displacement: overall change in position from start to finish.
Distance: the amount of space between two points.
Divergence: driving or moving apart from a common point.
Earthquake: shaking or trembling of the earth that accompanies rock movements extending anywhere from the crust to 680 km below the Earth's surface.
Elastic Limit: the point at which any additional force to an object will permanently deform its shape.
Elastic Properties: the measure of an objects ability to change shape when a force is applied to it, and return to its original shape when the force on it is released.
Elastic Rebound: an objects ability to return to its original shape after being broken apart.
Electric Current: movement of positive or negative particles accompanied by effects such as heat, a magnetic field, or a chemical transformation.
Epicenter: the point on the Earth's surface directly above the focus of an earthquake.
Explosion Earthquakes: earthquakes which are the result of the detonation of nuclear and chemical devices.
Fault: a fracture or zone of fractures in rock along which the two sides have been displaced relative to each other .
Focus: the point on the fault at which the first movement or break occurred.
Force: a push or pull having both magnitude and direction; equal to a change in an object's momentum.
Frequency: the number of wave cycles per unit of time that pass a given point.
Global Positioning System (GPS): a navigation and positioning tool used commercially, scientifically, and privately to determine exact position and used for Earth-related studies.
Hazard: a source of unpredictable, unplanned danger.
Insoluble: something that does not dissolve in a certain liquid, such as water.
Intensity: the measure, in terms of degrees, of ground shaking and damage to the surface and the effects on humans by earthquakes.
Ionosphere: part of the atmosphere starting at an altitude of 25 miles and extending to 250 miles; area where free particles exist and can be used for the transmission of radio waves. Click here to see an image
Jet Propulsion Laboratory (JPL): Home Page
Latitude: angular distance north or south from the earth's equator measured through 90 degrees.
Lithosphere: solid, rocky, outer part of the Earth, approximately 50 miles thick, comprised of the crust and the solid portion of the mantle. Click here to see an image
Longitude: the arc or portion of the earth's equator intersected between the meridian of a given place and the prime meridian and expressed either in degrees or in time.
Los Angeles Basin: Click here to see an image of the LA Basin
Love Waves: surface waves that move parallel to the Earth's surface and perpendicular to the direction of wave propagation..
Magma: molten rock material that is liquid or pasty which originated within the earth.
Magnitude: greatness or extent of size.
Mantle: the layer in Earth's interior between the crust and the core. Click here to see an image
Mechanical Wave: disturbance of a mechanical medium such as air, water, earth, and space.
Model: a representation or simulation of something that cannot be directly observed
Moment Magnitude: the preferred measure of earthquake size (magnitude) in which the stiffness of the rock, the average slip on the rupture plane, and the area of the rupture plane are taken into account (the "moment" of the earthquake).
National Aeronautics and Space Administration (NASA): Home Page
Network: interconnected or interrelated chain or system.
Orbit: revolution of one body around another.
Oscillatory Motion: motion back and forth about a central point.
Plastic Deformation: an objects ability to change shape under an additional amount of stress beyond its elastic limit before it breaks.
Plate: a large, relatively rigid segment of the Earth's lithosphere that moves in relation to other plates over the asthenosphere.
Plate Tectonics: a geological model in which the Earth's lithosphere (crust and upper most mantle) is divided into a number of more-or-less rigid segments which move in relation to one another.
Period: the time for one complete wave cycle to be completed.
Precision: a measure of how well a result has been determined, without reference to its agreement with the true value; a measurement of the reproducibility of the result.
P Wave: the primary body wave; the first seismic wave detected by seismographs; able to move through both liquid and solid rock; compressional waves, like sound waves, which compress and expand matter as they move through it.
Random Errors: errors in precision that effect the reproducibility of results from test to test. These are usually a result of errors that occur in the instrument itself and the experiment surroundings.
Rate: an expression that describes a change in position or velocity with respect to time.
Rayleigh Waves: surface waves that move in an elliptical motion, producing both a vertical and horizontal component of motion in the direction of wave propagation.
Receiver: a piece of equipment that receives information, such as a radio signal.
Recurrence Interval: the average period of time between earthquakes in a seismic region.
Repeatability:the average change in the position over the total time.
Rigid Body: objects whose constituent particles remain in fixed orientations relative to each other, as well as systems whose constituent parts may move relative to one another.
Sample Rate: number of times data are collected in a given period of time (i.e. a 30-second sample rate means that data are collected every 30 seconds).
Scalar: a quantity that only has a magnitude, and is represented by a single number, such as time and mass.
Scripps Institute of Oceanography (Scripps): Home Page
Seismic Moment: a parameter related to the angular leverage of the forces that produce slip on a fault; determined from the seismic waves and field measurements that describe the fault area.
Seismicity: relative frequency and distribution of earthquakes.
Seismogram: real-time record, made of metal tape, of seismic waves.
Seismograph: an instrument used to record seismic waves. Click here to see an animation
Seismology: science that deals with earthquakes and attendant phenomenon including the study of artificially produced elastic waves in the Earth's material.
Shear: a type of strain in which the shape of a material is displaced laterally with no corresponding change in volume. Click here to learn more!
Signal: detectable physical quantity.
Silicate: insoluble metal salt with silicon and oxygen in its anion; silicates with the addition of quartz describe the chemical group of minerals which make up most of the Earth's crust.
Slip Fault: the relative motion of one face of a fault relative to the other.
Solution: the final position of the GPS station determined after data are collected and processed.
Solution interval: amount of time between GPS data solutions (i.e. daily, monthly, etc.)
Southern California Earthquake Center (SCEC):Home Page
Southern California Integrated GPS Network (SCIGN): Visit the Home Page or the SCIGN section of our web page.
Strain: change in the shape or volume of a material, often recorded in three-dimensions. Click here to learn more!
Stress: a measure of forces acting on a body. Click here to learn more!
Surface Wave: waves that move close to or on the outside surface of the Earth.
S Waves: secondary body waves that shear, or cut the rock they travel through sideways at right angles to the direction of motion; cannot travel through liquid; produce vertical and horizontal motion in the ground surface.
System: a group of devices forming a network or used for a common purpose.
Systematic Errors: errors in accuracy that make results differ from their "true" value, but are reproducible; usually a result of operator error.
Tectonic Earthquake: earthquake that occurs when the earth's crust breaks due to geological forces on rocks and adjoining plates that cause physical and chemical changes.
Tectonics: large-scale deformation of the outer part of the Earth resulting from forces in the Earth.
Tension: the stretching or extension of a material.
Theory: a hypothetical entity or structure explaining or relating an observed set of facts.
Time Series: a graph that shows receiver position, in three dimensions, over time, for each individual GPS site. Click here to see some!
Troposphere: found below the stratosphere, 7-10 miles from Earth's surface; area where clouds form, temperature changes, and convection is active. Click here to see an image
Uncertainty: error; an estimate of the differences in values from test to test that are divided into two types, systematic and random, depending on their origin.
Units: tells others what is being described and compared.
United States Geological Survey (USGS): Home Page
Value of Proportionality: a constant value that relates strain to stress.
Vector: a quantity that represents both a magnitude and direction; often described by an arrow whose length represents a magnitude and its orientation describes its direction.
Velocity Map: a map that uses vectors to show how fast and in what direction ground GPS stations are moving relative to one another. Click here to see one!
Volcanic Earthquakes: earthquakes that result from tectonic forces which occur in conjunction with volcanic activity.
Volume: space occupied or enclosed by cubic units.
Wave: a disturbance that moves through a system.
Wavelength: the distance over which a wave pattern repeats.

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Last modified on 8/13/98 by Maggi Glasscoe (scignedu@jpl.nasa.gov)