Vocabulary of Seismology

Asthenosphere—the ductile part of the earth just below the lithosphere is about 180 km thick and is found 100–250 km (60–150 mi) below the Earth’s surface. The asthenosphere yields to persistent stresses more than the rigid crust or the core.

Body Waves—waves that move within the Earth’s interior or within a body of rock. P and S waves are body waves.

Compression—fractional decrease of volume due to pressure.

Crust—the outermost major layer of the earth, ranging from about 10 to 65 km in thickness worldwide. The uppermost 15-35 km of crust is brittle enough to produce earthquakes. Oceanic crust is thinnest; continental crust is thickest.

Focus—the point on the fault at which the first movement or break occurred. Directly beneath the epicenter at 1–50 km depth.

Earthquake—shaking or trembling of the earth that accompanies rock movements extending anywhere from the crust to 680 km below the Earth’s surface. It is the release of stored elastic energy caused by sudden fracture and movement of rocks inside the Earth. Part of the energy released produces seismic waves, like P, S, and surface waves, that travel outward in all directions from the point of initial rupture. These waves shake the ground as they pass by. An earthquake is felt if the shaking is strong enough to cause ground accelerations exceeding approximately 1.0 centimeter/second squared. Types of earthquakes include:

A) 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.

B) Volcanic Earthquakes: earthquakes that result from tectonic forces which occur in conjunction with volcanic activity.

C) Collapse Earthquakes: small earthquakes in underground caverns and mines that are caused by seismic waves produced from the explosion of rock on the surface.

D) Explosion Earthquakes: earthquakes which are the result of the detonation of nuclear and chemical devices 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. See Strike Slip and Subduction

Elastic strain—Earthquakes are caused by the sudden release of energy within some limited region of the rocks of the Earth. The energy can be released by elastic strain, gravity, chemical reactions, or even the motion of massive bodies. Of all these the release of elastic strain is the most important cause, because this form of energy is the only kind...

Epicenter—the point on the Earth’s surface directly above the focus of an earthquake.

Focus—the point on the fault at which the first movement or break occurred. Directly beneath the epicenter at 1–50 km depth.

Frequency—the number of wave cycles per unit of time that pass a given point.

Hazard—a source of unpredictable, unplanned danger.

Intensity—The intensity is 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. See magnitude.

Lithosphere—solid, rocky, outer part of the Earth, ~100 km thick (50 miles) comprised of the crust and the solid portion of the mantle. The thickness is age dependent with older lithosphere is thicker than younger oceanic lithosphere. The lithosphere below the crust is brittle enough at some locations to produce earthquakes by faulting, such as within a subducted oceanic plate.

Magnitude—The magnitude is 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..

Mantle— the layer in Earth’s interior between the crust and the metallic core.

Material Properties of the Earth—the bulk character of the rock, such as composition, density, elastic moduli, mineralogy, and phase (ex. the presence of melt). Elastic waves may propagate through the earth in a manner which depends on the material properties of the earth. The elasticity of the material provides the restoring force of the wave. When they occur in the Earth as the result of an earthquake or other disturbance, elastic waves are usually called seismic waves.

P Wave—the primary body wave; the first seismic wave detected by seismographs; able to move through both liquid and solid rock. Also called compressional or longitudinal waves, they compress and expand (oscillate) the ground back and forth in the direction of travel, like sound waves that move back and forth as the waves travel from source to receiver. P wave is the fastest wave.

Seismic Wave— A seismic wave is an elastic wave generated by an impulse such as an earthquake or an explosion. Seismic waves may travel either through the earth’s interior (P and S waves; the fastest waves) or along or near the earth’s surface (Rayleigh and Love waves). Seismic waves travel at speeds of several kilometers per second.

Seismicity—he geographic and historical distribution (the “where?” and “how often?”) of earthquakes.

Seismic Tomography— Seismic tomography is an imaging technique that uses seismic waves generated by earthquakes and explosions to create computer-generated, three-dimensional images of Earth’s interior.

Seismogram—A real-time record of earthquake ground motion recorded by a seismograph. Seismograms are the records (paper copy or computer image) used to calculate the location and magnitude of an earthquake..

Seismograph—an instrument that records vibrations of the Earth, especially earthquakes. Seismograph generally refers to the seismometer and a recording device as a single unit.

Seismology—science that deals with earthquakes and attendant phenomenon including the study of artificially produced elastic waves in the Earth’s material.

Seismometer—a sensitive instrument that can detect waves emitted by even the smallest earthquakes. (See seismograph.)

Shadow Zone— The shadow zone is the area of the earth from angular distances of 104 to 140 degrees from a given earthquake that does not receive any direct P waves. This zone results from S waves being stopped entirely by the liquid core and P waves being bent (refracted) by the liquid core.

Seismic Wave— A seismic wave is an elastic wave generated by an impulse such as an earthquake or an explosion. Seismic waves may travel either through the earth’s interior (P and S waves; the fastest waves) or along or near the earth’s surface (Rayleigh and Love waves). Seismic waves travel at speeds of several kilometers per second.

Surface Wave—waves that move close to or on the outside surface of the Earth rather than through the deep interior like the faster P or S waves. Two principal types of surface waves, Love and Rayleigh waves, are generated during an earthquakes. Rayleigh waves cause both vertical and horizontal ground motion, and Love waves cause horizontal motion only. They both produce ground shaking at the Earth’s surface but very little motion deep in the Earth. Because the amplitude of surface waves diminishes less rapidly with distance than the amplitude of P or S waves, surface waves are often the most important component of ground shaking far from the earthquake source.

S Waves—secondary body waves that oscillate the ground perpendicular to the direction of wave travel. They travel about 1.7 times slower than P waves. Because liquids will not sustain shear stresses, S waves will not travel through liquids like water, molten rock, or the Earth’s outer core. S waves produce vertical and horizontal motion in the ground surface.

source : IRIS (Incorporated Research Institutions for Seismology)