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Exploring Earthquakes: On-Line Glossary
Body Wave - Waves that travel through the earth; either compressional / primary (P) or shear / secondary (S).
Compressional Wave / P Wave - A seismic wave in which the oscillation of the material is parallel to the direction in which the wave is traveling.
Epicenter - The point on the earth's surface directly above the focus.
Fault - Fracture within the Earth along which displacement of one side with respect to the other has occurred.
Focal Depth - The depth from the earth's surface to the focus of an earthquake. Earthquakes with focal depths under 70km are termed "shallow." Intermediate depth events range from 70 to 300km, and deep events are from 300 to 700km deep.
Focus - The location on a fault within the Earth where an earthquake's slip began.
Geographic Information System - A system of hardware and software that allows spatially-referenced data to be input, stored, manipulated, analyzed, retrieved, and plotted.
Intensity - A measure of shaking strength of an earthquake at a particular location. Intensity is largest at the epicenter and decreases in all directions from the epicenter. Intensity is measured by the Modified Mercalli scale, from I (not felt except by few) to XII (damage total).
Liquefaction - Loss of strength of loosely-packed, waterlogged sediments in response to strong shaking; a cause for major damage during earthquakes.
Magnitude - A measure of the energy released by an earthquake.
Normal Fault - Breaks in the earth's crust in response to pulling or tension; the overlying block moves down the dip of the fault plane, and the underlying block moves upward.
Richter Scale - Logarithmic scale for measuring earthquake magnitude. A magnitude 7.0 earthquake generates 10 times larger amplitude waves than those of a magnitude 6.0, and releases 32 times more energy.
Seismology - The scientific investigation of earthquakes, and of the structure of the earth, through the study of seismic waves.
Shear Wave / S Wave - A seismic wave that involves oscillation of the material perpendicular to the direction in which the wave is traveling. A shear wave can not travel in a liquid or a gas.
Spreading Zones - Break in the earth's crust where two plates are moving apart. Molten rock rises, cools, pushing two plates apart, and adds new material to the edges of both plates. Currently, ridge push is thought to be smaller than the pull of the subducted slabs.
Strike-Slip Fault - Break in the earth's crust across which two blocks move horizontally past one another.
Subduction Zone - The place where two plates come together. One plates moves downward into the mantle where it becomes the source of intermediate and deep earthquakes.
Surface Wave - Waves traveling along the earth's surface. Along with S waves, these produce the strongest vibrations and are the source of the most damage.
Thrust Fault - Break in the earth's crust from squeezing or compression; the overlying block moves up the dip of the fault plane and the underlying block moves down.
Transform Fault - Break in the earth's crust where two plates slide past each other. This is another name for a strike-slip fault.
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