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Following is a list of terms and definitions provided for the purposes of understanding the content of this site and its links and enabling visitors and users of this site to search for other materials using related terminology. All terms and definitions are from the United States Geological Survey website. 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 earthquakes are followed by a larger number of aftershocks, decreasing in frequency with time. Amplitude - The maximum height of a wave crest or depth of a trough. Array - An ordered arrangement of seismometers or geophones, the data from which feeds into a central receiver. Arrival - The appearance of seismic energy on a seismic record. Arrival Time - The time at which a particular wave phase arrives at a detector. Aseismic - Not associated with an earthquake, as in aseismic slip. Also used to indicate an area with no record of earthquakes; an aseismic zone. Body Wave - A seismic wave that can travel through the interior of the earth. P-waves and S-waves are body waves. 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. Continental Drift - The theory, first advanced by Alfred Wegener, that Earth's continents were originally one land mass. Pieces of the land mass split off and migrated to form the continents. Earthquake - Sudden motion in Earth caused by abrupt release of slowly accumulated strain. Shaking of the Earth caused by a sudden movement of rock beneath its surface. Earthquake Swarm - A series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time. 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. Epicenter - The point on the Earth's surface directly above the focus of an earthquake. The epicenter is where the earthquake begins. Fault - A fracture or break in the Earth's crust along which one side of the break is pushed up, down, or sideways. First Arrival - The first recorded signal attributed to seismic wave travel from a source. Focus - The center of an earthquake where rupture begins. That point within the Earth from which originates the first motion of an earthquake and its elastic waves. Foreshock - A small tremor that commonly precedes a larger earthquake or main shock by seconds to weeks and that originates in or near the rupture zone of the larger earthquake. Geologic Hazard - A naturally occurring or man-made geologic condition that presents a risk or is a potential danger to life and property. Geologic Map - A map on which is recorded geologic information, such as the distribution, nature, and age relationships of rock units, and the occurrence of structural features, mineral deposits, and fossil localities. Geology - The scientific study of the origin, history, behavior, and structure of the Earth. Great Earthquake - An earthquake having a magnitude of 8 or greater on the Richter scale. Hazards - Hazards are unpreventable natural events that, by their nature, may expose our Nation's population to the risk of death or injury and may damage or destroy private property, societal infrastructure, and agricultural or other developed land. Hypocenter - The calculated location of the focus of an earthquake. Intensity - A measure of the effects of an earthquake at a particular place on humans, structures 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. 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. 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. Liquefaction - Process where water-saturated sediment (sandy material) temporarily looses strength, usually because of an earthquake, and behaves like a fluid. Soil or sand changes from solid ground and behaves like a liquid, which can cause the ground above the liquified sediment to break into small blocks. The movement of the land surface, known as lateral spreading, can cause serious damage to buildings, bridges, underground pipes, water lines, and sewers. 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. Low-Velocity Zone - Any layer in the Earth in which seismic wave velocities are lower than in the layers above and below. 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 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released. In other words, a magnitude 6.7 earthquake releases over 900 times (30 times 30) the energy of a 4.7 earthquake - or it takes about 900 magnitude 4.7 eearthquakes to equal the energy released in a single 6.7 earthquake! There is no beginning nor end to this scale. However, rock mechanics seems to preclude earthquakes smaller than about -1 or larger than about 9.5. A magnitude -1.0 event release about 900 times less energy than a magnitude 1.0 quake. Except in special circumstances, earthquakes below magnitude 2.5 are not generally felt by humans. Major Earthquake - An earthquake having a magnitude of 7 to 7.99 on the Richter scale. Microearthquake - An earthquake having a magnitude of 2 or less on the Richter scale. Microseism - A more or less continuous motion in the Earth that is unrelated to an earthquake and that has a period of 1.0 to 9.0 seconds. It is caused by a variety of natural and artificial agents. Modified Mercalli Scale - Mercalli intensity scale modified for North American conditions. A scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, that is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects. P Wave - Primary, longitudinal, irrotational, push, pressure, dilatational, compressional, or push-pull wave. P waves are the fastest body waves and arrive at stations before the S waves, 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 layers of the Earth. P waves are generally felt by humans as a bang or thump. Paleoseismology - The study of ancient (prehistoric) earthquakes. Plate - Rigid slabs made of the Earth's crust that move relative to one another. Movement along plate boundaries can result in an earthquake or volcano. There are about a dozen or so plates that cover the Earth. An entire continent may lie on top of a plate, for example North America is part of the North American plate. Plate Boundary - The place where two or more plates in the Earth's crust meet. Plate Tectonics - The theory that the Earth's crust and upper mantle (the lithosphere) is broken into a number of more or less rigid, but constantly moving, segments or plates. 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 earthquakes, can be seen as they approach. Named after Lord Rayleigh, the English physicist who predicted its existence. Recurrence Interval - The approximate length of time between earthquakes in a specific seismically active area. 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. 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. S Wave - 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 waves, but in an earthquake they are usually bigger. S waves cannot travel through the outer core because these waves cannot exist in fluids, such as air, water or molten rock. Seiche - A free or standing wave oscillation of the surface of water in an enclosed basin that is initiated by local atmospheric changes, tidal currents, or earthquakes. Similar to water sloshing in a bathtub. Seismic - Of or having to do with earthquakes. 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. Seismic Waves - Seismic or earthquake waves are caused by the release of energy in the Earth's crust. Seismic Zone - A region in which earthquakes are known to occur. Seismicity - Earthquake activity. Seismologist - A scientist who studies earthquakes. Seismogram - The paper record of an earthquake's motion. Today, digital files are replacing the traditional paper seismograms. Seismograph - An instrument used for recording the intensity and duration of an earthquake. Seismology - The study of earthquakes and of the structure of the Earth by natural and artificial seismic waves. Seismometer - The sensor part of an instrument that detects motions of the Earth's surface caused by seismic waves produced during an earthquake. A seismograph records the ground motion. Seismometry - The instrumental aspects of seismology. Signal-to-Noise Ratio - The comparison between the amplitude of the seismic signal and the amplitude of noise caused by seismic unrest and (or) the seismic instruments. Spreading Center - An elongated region where two plates are being pulled away from each other. New crust is formed as molten rock is forced upward into the gap. Examples of spreading centers include the Mid-Atlantic Ridge and the East African Rift. Subduction - The process in which one lithospheric plate collides with and is forced down under another plate and drawn back into the Earth's mantle. Subduction Zone - An elongated region along which a plate descends relative to another plate, for example, the descent of the Nazca plate beneath the South American plate along the Peru-Chile Trench. Surface Waves - Waves that move over the surface of the Earth. Rayleigh waves and Love waves are surface waves. Topography - The physical features of a place; or the study and depiction of physical features, including land elevations. Tsunami - One or a series of huge sea waves caused by earthquakes or other large-scale disturbance of the ocean floor. (Referred to incorrectly by many as a tidal wave, but these waves have nothing to do with tides.) The word tsunami is Japanese, meaning "harbor wave." For more definitions: |