- Crust - the top layer of the earth, which consists of solid rock. Both the continental crust (land masses) and oceanic crust (the land beneath the ocean) belong to the 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.
- Earthquake - movement of the ground caused by the release of energy from a sudden shift of rocks in Earth's crust.
- Earthquake Hazard - Anything associated with an earthquake that may affect the normal activities of people. This includes surface faulting, ground shaking, landslides, liquefaction, tectonic deformation, tsunamis, and seiches.
- Earthquake Risk - The probable building damage, and number of people that are expected to be hurt or killed if a likely earthquake on a particular fault occurs. Earthquake risk and earthquake hazard are occasionally used interchangeably.
- Epicenter - the point on Earth's surface directly above the focus of an earthquake
- Fault - a break in Earth's crust where movement of rock occurs
- Active Fault - A fault that is likely to have another earthquake sometime in the future. Faults are commonly considered to be active if they have moved one or more times in the last 10,000 years.
- Blind Thrust Fault - A thrust fault that does not rupture all the way up to the surface so there is no evidence of it on the ground. It is "buried" under the uppermost layers of rock in the crust.
- Locked Fault - A fault that is not slipping because frictional resistance on the fault is greater than the shear stress across the fault (it is stuck). Such faults may store strain for extended periods that is eventually released in an earthquake when frictional resistance is overcome.
- Normal fault - a type of fault where forces of tension are pulling rock apart. P (Primary) wave- the fastest moving type of seismic wave, which expands and compresses rock, like the movement of a slinky. Also known as pressure waves.
- Reverse fault - a type of fault where compression pushes rock together. Also known as a thrust fault.
- Strike-slip fault - a type of fault where rocks slide horizontally past each other in opposite directions, with little up or down motion. The San Andreas fault in California and the North Anatolian fault in Turkey are examples of strike-slip faults.
- Focus - the point below Earth's surface where movement of rock produces an earthquake or the point within the earth where an earthquake rupture starts. Also called hypocenter.
- Foreshock - a small earthquake that precedes a larger earthquake
- Ground Motion - The movement of the earth's surface from earthquakes or explosions. Ground motion is produced by waves that are generated by sudden slip on a fault or sudden pressure at the explosive source and travel through the earth and along its surface.
- Intensity - the amount of shaking and type of damage at a particular location. Intensity can be greater or weaker depending on the distance from the epicenter.
- Liquefaction - this happens when loose, moist soil or sand is shaken so hard that individual grains separate, turning the earth into a soft, fluid slurry that can swallow entire buildings.
- Lithosphere - the uppermost layer of the earth, which consists of all solid rock. It includes both the crust and the upper mantle. The lithosphere is about 100 km thick, although its thickness is age dependent (older lithosphere is thicker).The lithosphere below the crust is brittle enough at some locations to produce earthquakes by faulting.
- Magnitude - the measurement of the total strength or amount of energy released by an earthquake based on measurement of the maximum motion recorded by a seismograph.
- Mainshock - The largest earthquake in a sequence, sometimes preceded by one or more foreshocks, and almost always followed by many aftershocks.
- Mantle - the layer beneath the crust. The upper mantle is solid rock; the lower mantle is molten rock.
- Mercalli Scale - a measurement of an earthquake's intensity based on how much damage it causes. The Mercalli scale ranges from Level I (not felt except by very few under favorable conditions) to Level XII, (causing almost total destruction.)
- Moment magnitude scale - a measurement of an earthquake's magnitude based on the amount of movement of the rock along a fault line.
- P (Primary) wave - A seismic body wave that shakes the ground back and forth in the same direction and the opposite direction as the direction the wave is moving. Can travel through both liquids and solids.
- Period - The time interval required for one full cycle of a wave.
- Plates - Large, rigid segments of the earth's crust and part of the mantle below, broken into 12 major and many minor sections that "float" on a plastic, flowing mantle layer.
- Plate Tectonics - A theory supported by a wide range of evidence that considers the earth's crust and upper mantle to be composed of several large, thin, relatively rigid plates that move relative to one another.
- Richter Scale - a measurement of the magnitude of an earthquake based on the readings of a seismograph. The Richter scale is a logarithmic scale ranging from 0 to 9, with each number representing a 10-fold increase in ground motion, and a 30-fold increase in energy released.
- Ring of Fire - The zone of earthquakes surrounding the Pacific Ocean which is called the Circum-Pacific belt where about 90% of the world's earthquakes occur.
- S (Secondary) wave - A seismic body wave that shakes the ground back and forth perpendicular to the direction the wave is moving; the second-fastest moving type of seismic wave, which moves rock horizontally from side to side. Also known as shear waves. S waves cannot pass through liquids, and therefore cannot pass through Earth's liquid outer core.
- Seiche - The sloshing of a closed body of water from earthquake shaking. Swimming pools often have seiches during earthquakes.
- Seismic Moment - A measure of the size of an earthquake based on the area of fault rupture, the average amount of slip, and the force that was required to overcome the friction sticking the rocks together that were offset by faulting. Seismic moment can also be calculated from the amplitude spectra of seismic waves.
- Seismic wave - a vibration that travels through Earth carrying the energy released during an earthquake. They can travel through solids and liquids.
- Seismogram - written recording of the earth's vibrations, produced by a seismograph.
- Seismograph - an instrument that records seismic waves.
- Seismologist - scientist who studies earthquakes.
- Slip - The relative displacement of formerly adjacent points on opposite sides of a fault, measured on the fault surface.
- Surface Faulting - Displacement that reaches the earth's surface during slip along a fault. Commonly occurs with shallow earthquakes, those with an epicenter less than 20 km. Surface faulting also may accompany aseismic creep or natural or man-induced subsidence.
- Surface waves - seismic waves that move along Earth's surface. They can have an up-and-down motion or a horizontal motion. Surface waves travel slower than P or S waves and usually cause the most damage.
- Tectonic Plates - individual sections of the lithosphere of the earth. They fit together in a way similar to a jigsaw puzzle, but are always moving very slowly, floating on the molten rock of the lower mantle.
- Tsunami - a giant, fast-moving wave that is caused by an undersea earthquake. Also known as a seismic sea wave.
- Wavelength - The distance between successive points of equal amplitude and phase on a wave (for example, crest to crest or trough to trough).
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