| Term | Definition |
|---|
| aftershock | An earthquake that occurs as a consequence of a previous earthquake of larger magnitude. |
| building code | Standards for the design and construction of new buildings that specify the level of shaking a structure must be able to withstand in an earthquake, basd on the maximum intensity expected from the seismic hazard. |
| earthquake | The violent motion of the ground that occurs when rocks being stressed suddenly break along a new or presexisting fault. |
| elastic rebound theory | A theory of fault movement and earthquake generation holding that faults remain locked while strain energy accumulation in the rock formations on both sides, temporarily deforming them until a sudden slip along the fault releases the energy. |
| epicenter | The geographic point on Earth's surface directly above the focus of an earthquake. |
| fault mechanism | The type of fault rupture (normal, thrust, or strike-slip) that produced an earthquake; it is determined by the orientation of the fault rupture and the direction of slip. |
| fault slip | The distance of the displacement between two blocks that occurs during an earthquake when elastic rebound causes the blocks on either side of a fault to spring back to their undeformed state. |
| focus | The point along a fault at which slip initiates an earthquake. |
| foreshock | A small earthquake that occurs in the vicinity of, but before, a main shock. |
| intensity scale | A scale for estimating the intensity of earthquake shaking directly from an event's destructive effects. |
| magnitude scale | The detailed history of Earth's magneticfield reversals going back into geologic time, as determined by measuring the thermoremanent magnetization of rock samples. |
| P wave | The first seismic wave to arrive from the focus of an earthquake; a type of compressional wave. |
| recurrence interval | The average time interval between the occurence of two ruptures on the same fault that produce earthquakes of approximately equal magnitude. |
| S wave | The second seismic wave to arrive from the focus of an earthquake; a type of shear wave. S waves do not exist in liquids or gases. |
| seismic hazard | The intensity of shaking and ground disruption in an earthquake that can be expected over the long term at some specified location, expressed in the form of a seismic hazard map. |
| seismic risk | The earthquake damage that can be expected over the long term for a specified region, such as a county or state, usually measured in terms of average dollar loss per year. |
| seismic wave | A ground vibration produced by earthquakes or explosions. |
| seismicity map | A map that shows the epicenters of earthquakes recorded around the world or in a particular region over a given period of time. |
| seismograph | An instrument that magnifies and records the motions of Earth's surface caused by seismic waves. |
| surface wave | Seismic waves that travel around Earth's surface that arrive later than the S wave. |
| tsunami | A fast-moving, towering sea wave generated by an earthquake that occurs beneath the ocean. |
| compressinal wave | A force that squeezes together or shortens a body. |
| conduction | The mechanical transfer of heat energy (the vibrational energy of thermally agitated atoms and molecules) by the mechanism of atomic or molecular impact. |
| convection | A mechanism of heat transfer in which a heated fluid expands and rises because it has become less dense than the surrounding material. Colder material flows in to take the place of the hot rising fluid, is itself heated, and then rises to continue the cycle. |
| core-mantle boundary | The boundary between the core and lower mantle, about 2890 km below Earth's surface, where S-wave speed drops from about 7.5 km/s to zero, and p-wave speed drops from more than 13 km/s to about 8 km/s. |
| depositional remanent magnetization | A weak magnetization created in sedimentary rocks by the paralles alignment of magnetic particles in the direction of Earth's magnetic field as they settle. |
| dipole | Two oppositely polarized magnetic poles. |
| geotherm | The curve that describes how temperature increases with depth in the Earth. |
| lower mantle | A relatively homogenous region, beginning nearly 660 km below the surface and more than 2000 km thick, where S-wave speed abruptly increases, indicating a major phase change to a more closely packed crystal structure. |
| low-velocity zone | A layer near the base of the lithosphere where the S-wave speed abruptly decreases. |
| magnetic stratigraphy | The time sequence of reversals of Earth's magnetic field as indicated in the fossil magnetic record of layered lava flows. |
| Mohorovicic discontinuity (Moho) | The boundary between the crust and the mantle, at a depth of 5 to 45 km, marked by an abrupt increase in P-wave velocity to more than 8 km/s. |
| paleomagnetism | The remanent magnetization that coalesced in the latest Paleozoic era and comprised all present continents. The breakup of Pangea began in Mesozoic time, as inferred from paleomagnetic and othe data. |
| phase change | A transition in a rock's mineralogy (but probably not its composition) to a denser, more closely packed crystal structure, signaled by a change in seismic-wave velocity. |
| principle of isostasy | The principle that the buoyancy force that pushes a lower-density body (such as a continent or an iceberg) floating in a higher-density medium (such as the asthenosphere or seawater) upward must be balanced by the gravitational force that pulls it downward. |
| seismic tomography | A method that uses the seismic waves from earthquakes recorded on thousands of seismographs all over the world to sweep Earth's interior in many different directions and construct a three-dimensional image of what's inside. |
| shadow zone | 1) A zone beyond 105 degrees from the focus of an earthquake where S waves are absent because the waves are not transmitted through the liquid outer core. 20 A zone at angular distance of 105 to 142 from the focus of an earthquake where P waves are absent because they are refracted downward into the core and emerge at greater distances after the delay caused by their detour through the core. |
| shear wave | A wave that propogates by a side-to-side motion. They cannot propogate through any fluid - air, water, or liquid iron in the Earth's outer core. |
| thermoremanent magnetization | A permanent magnetization acquired by minerals in igneous rocks when groups of atoms of the mineral align themselves in the direction of hte magnetic field that existed when the material was hot. When the material has cooled, these atoms are locked in place and therefore are always magnetized in the same directions. |
| angle of repose | The maximum angle at which a slope of loose material will lie without sliding. |
| consolidated material | Sediment that is compacted and bound together by mineral cements. |
| creep | The slow downhill mass movement of soil or other debris at a rate ranging from about 1 to 10 mm/year. |
| humus | The remains and waste products of the many plants, animals, and bacteria lviving in a soil. |
| liquefaction | The temporary transformation of solid material to a fluid state caused by the pressure of water in the pores of the material. |
| mass movement | A downhill movement of masses of soil or rock under the force of gravity. |
| soil profile | The final composition and appearance of a soil. |
| talus | An accumulation of blocks of fallen rock athe foot of a steep bedrock cliff. |
| unconsolidated material | Sediment that is loose and uncemented. |
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