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Below are definitions of important earthquake terms:

• aftershock: additional tremors that occur after the initial earthquake
• body wave: a vibration from an earthquake that travels deep underground
• complex faults: a series of faults that may tilt rocks in many different directions
• continental drift: when major plates are slowly but steadily moved apart
• crust: the layer of rock at the earth’s surface
• earthquake: a trembling and shaking of the earth’s surface; a vibration or trembling of the ground caused by movement of the subsurface
• epicenter: the position on the earth’s surface above the focus of an earthquake
• fault: great fractures between masses of rock at the earth’s surface; a large crack or break in a series of rocks; where two plates slip past each other
• focus: the place below ground where an earthquake starts
• foreshock: a small tremor before a major earthquake
• geologist: a scientist who studies the origin, history, structure, and processes of the earth
• horst: a block of rock thrown up between normal faults
• hypocenter: the place below ground where an earthquake starts
• inferno: a huge, roaring fire
• landslide: a mass of loose rock and soil that slides down a steep slope
• Love wave: a type of surface wave where motion is horizontal
• magnitude: the size of an earthquake as measured on the Richter scale
• Mercalli Scale: a measure of an earthquake’s intensity, ranging from 1 to 12
• normal faults: a fault in which blocks of rock slip straight down; also called dip-slip faults
• oblique-slip fault: a fault in which blocks of rock slip up or down, and then past each other diagonally; when it happens on a large scale, they are known as transtension or transpression faults
• plates: giant slabs of underground rocks, often bigger than continents
• primary wave: fast-moving body waves that push and pull on rocks laterally; they move at 3 miles (5 km) per second
• Rayleigh wave: a type of surface wave that moves vertically
• reverse fault: a fault in which one block of rock slides up over another
• Richter scale: a scale ranging from 1 to 10 used to show the size of an earthquake
• rift valley: a huge, trough-shaped valley created by faulting
• Ring of Fire: the area around the Pacific Basin where a lot of seismic activity occurs, including the Western coast of North and South America and East Asian islands like Japan
• secondary waves: a type of body wave, slightly slower than a primary wave, that moves rocks vertically or horizontally; they move at 2 miles (3 km) per second
• seismographs: instruments that make an automatic record of the time, duration, direction, and intensity of earthquakes
• seismologists: scientists who study earthquakes and their causes and results
• shear waves: waves that radiate out from an earthquake’s epicenter rocking from side to side at a fast pace
• shock waves: the waves of energy that are produced underground as rocks snap; when they reach the surface they cause violent shaking
• stress: the intensity of two forces working against each other
• surface wave: a vibration from an earthquake that travels at ground level
• tectonic plates: 70-mile-thick blocks of the earth’s crust on which the continents ride
• thrust fault: when a reverse fault has an angle of 45º or less
• tremors: the shaking caused by earthquakes
• tsunami: a large sea wave caused by an earthquake
• vibration: a continuous shaking movement
• wrench fault: a fault in which blocks of rock slip sideways past each other; also known as tear faults, strike-slip faults, or, when especially large, transcurrent faults

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