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Science Master - Seismological terms Category: Sciences > Seismology Date & country: 27/01/2011, UK Words: 165

recurrence interval
recurrence interval. The average time span between large earthquakes at a particular site. Also termed return period.

reflection
reflection. The energy or wave from an earthquake that has been returned (reflected) from an boundary between two different materials within the earth, just as a mirror reflects light.

refraction
refraction. (1) The deflection, or bending, of the ray path of a seismic wave caused by its passage from one material to another having different elastic properties. (2) Bending of a tsunami wave front owing to variations in the water depth along a coastline.

regression analysis
regression analysis. A statistical technique applied to data to determine, for predictive purposes, the degree of correlation of a dependent variable with one or more independent variables, in other words, to see if there is a strong or weak cause and effect relationship between to things. See least-squares fit.

residual
residual. The difference between the measured and predicted values of some quantity.

return period
return period. See recurrence interval.

right lateral
right-lateral. If you were to stand on the fault and look along its length, this is a type of strike-slip fault where the right block moves toward you and the left block moves away. (See also left-lateral.)

Ring of Fire
Ring of Fire. The zone of earthquakes surrounding the Pacific Ocean which is called the Circum-Pacific belt--about 90% of the world's earthquakes occur there. The next most seismic region (5-6% of earthquakes) is the Alpide belt (extends from Mediterranean region, eastward through Turkey, Iran, and northern India.

rupture front
rupture front. The instantaneous boundary between the slipping and locked parts of a fault during an earthquake. Rupture in one direction on the fault is referred to as unilateral. Rupture may radiate outward in a circular manner or it may radiate toward the two ends of the fault from an interior point, behavior referred to as bilateral.

rupture velocity
rupture velocity. The speed at which a rupture front moves across the surface of the fault during an earthquake.

S wave
S wave. A seismic body wave that shakes the ground back and forth perpendicular to the direction the wave is moving, also called a shear wave.

sand boil
sand boil. Sand and water that come out onto the ground surface during an earthquake as a result of liquefaction at shallow depth.

sea floor magnetic lineaments
sea-floor magnetic lineaments. See magnetic polarity reversals.

secular
secular. Referring to long-term changes that take place slowly and imperceptibly. Commonly used to describe changes in elevation, tilt, and stress or strain rates that are related to long-term tectonic deformation. For example, a mountain that is growing is getting taller so slowly that we cannot see it happen.

segmentation
segmentation. The breaking up of a fault along its length into several smaller faults. This can happen as a result of other faults crossing it, topography changes, or bends in the strike of the faults. Segmentation can limit the length of faulting in a single earthquake to some fraction of the total fault length.

seiche
seiche. The sloshing of a closed body of water from earthquake shaking. Swimming pools often have seiches during earthquakes.

seismic gap
seismic gap. A section of a fault that has produced earthquakes in the past but is now quiet. For some seismic gaps, no earthquakes have been observed historically, but it is believed that the fault segment is capable of producing earthquakes on some other basis, such as plate-motion information or strain measurements.

seismic hazard
seismic hazard. See earthquake hazard and earthquake risk.

seismic moment
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 reflection line
seismic reflection or refraction line. A set of seismographs usually lined up along the earth's surface to record seismic waves generated by an explosion for the purpose of recording reflections and refractions of these waves from velocity discontinuities within the earth.

seismic wave
seismic wave. An elastic wave generated by an impulse such as an earthquake or an explosion. Seismic waves may travel either along or near the earth's surface ( Rayleigh and Love waves) or through the earth's interior (P and S waves).

seismic zone
seismic zone. ?An area of seismicity probably sharing a common cause. Example: "The New Madrid Seismic Zone."

seismicity
seismicity. The geographic and historical distribution of earthquakes.

seismogenic
seismogenic. Capable of generating earthquakes.

seismology
seismology. The study of earthquakes and the structure of the earth, by both naturally and artificially generated seismic waves.

seismometer
seismometer or seismograph. A seismometer is an instrument used to detect and record earthquakes. Generally, it consists of a mass attached to a fixed base. During an earthquake, the base moves and the mass does not. The motion of the base with respect to the mass is commonly transformed into an electrical voltage.

shear stress
shear stress. The stress component parallel to a given surface, such as a fault plane, that results from forces applied parallel to the surface or from reinote forces transmitted through the surrounding rock. If you lean against the edge of the door where the latch is, you are applying shear stress to the door.

shear wave
shear wave. See S wave.

slab
slab. The oceanic crustal plate that underthrusts the continental plate in a subduction zone and is consumed by the earth's mantle.

slip
slip. The relative displacement of formerly adjacent points on opposite sides of a fault, measured on the fault surface.

slip model
slip model. A kinematic model that describes the amount, distribution, and timing of slip associated with an earthquake.

slip rate
slip rate. How fast the two sides of a fault are slipping relative to one another, as determined from geodetic measurements, from offset man-made structures, or from offset geologic features whose age can be estimated. It is measured parallel to the predominant slip direction or estimated from the vertical or horizontal offset of geologic markers.

soil
soil. (1) In engineering, all unconsolidated material above bedrock. (2) In soil science, naturally occurring layers of mineral and (or) organic constituents that differ from the underlying parent material in their physical, chemical, mineralogical, and morphological character because of pedogenic processes (3) In other words, dirt.

soil profile
soil profile. The vertical arrangement of layers of soil down to the bedrock.

source
source. The released forces that generate acoustic or seismic waves, also called the earthquake source.

spectrum
spectrum. A curve showing amplitude and phase as a function of frequency or period, or how much of each type of shaking there is from an earthquake.

standard deviation
standard deviation. How much a set of data is different from the curve it should make when plotted on a graph. Or, the square root of the average of the squares of deviations about the mean of a set of data. Standard deviation is a statistical measure of spread or variability.

station
station. Refers to the place where a geophysical instrument is located.

stick slip
stick-slip The fast movement that occurs between two sides of a fault when the two sides of the fault become unstuck. Stick-slip displacement on a fault radiates energy in the form of seismic waves, creating an earthquake.

stochastic
stochastic. Applied to processes that have random characteristics.

strain
strain. Small changes in length and volume associated with deformation of the earth by tectonic stresses or by the passage of seismic waves.

strain rate
strain rate. How fast the lithosphere is being deformed from plate tectonic movement.

stress
stress. Force per unit area acting on a plane within a body. Six values are required to characterize completely the stress at a point: three normal components and three shear components.

stress drop
stress drop. The difference between the stress across a fault before and after an earthquake.

strike
strike. Trend or bearing, relative to north, of the line defined by the intersection of a planar geologic surface (for example, a fault or a bed) and a horizontal surface such as the ground.

strike slip
strike-slip. See fault.

strong motion
strong motion. Ground motion of sufficient amplitude and duration to be potentially damaging to a building or other structure.

subduction
subduction. The process of the oceanic lithosphere colliding with and descending beneath the continental lithosphere.

subduction zone
subduction zone. The place where two lithosphere plates come together, one riding over the other. Most volcanoes on land occur parallel to and inland from the boundary between the two plates.

surface faulting
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 wave
surface wave. Seismic wave that travels along the earth's surface. Love and Rayleigh waves are the most common.

surface wave magnitude
Surface-wave magnitude. See magnitude.

tectonic
tectonic. Refers to rock-deforming processes and resulting structures that occur over large sections of the lithosphere.

teleseismic
teleseismic. Pertaining to earthquakes at distances greater than 1,000 km from the measurement site.

time history
time history. The sequence of values of any time-varying quantity (such as a ground motion measurement) measured at a set of fixed times. Also termed time series.

transform fault
transform fault. A special variety of strike-slip fault that accommodates relative horizontal slip between other tectonic elements, such as oceanic crustal plates. Often extend from oceanic ridges.

traveltime curve
traveltime curve. A graph of arrival times, commonly P or S waves, recorded at different points as a function of distance from the seismic source. Seismic velocities within the earth can be computed from the slopes of the resulting curves.

tsunami
tsunami. A sea wave of local or distant origin that results from large-scale seafloor displacements associated with large earthquakes, major submarine slides, or exploding volcanic islands.

tsunami magnitude
tsunami magnitude (Mt). A number used to compare sizes of tsunamis generated by different earthquakes and calculated from the logarithm of the maximum amplitude of the tsunami wave measured by a tide gauge distant from the tsunami source.

tsunamigenic
tsunamigenic. Referring to those earthquakes, commonly along major subduction-zone plate boundaries such as those bordering the Pacific Ocean, that can generate tsunamis.

turbidites
turbidites. Sea-bottom deposits formed by massive slope failures where rivers have deposited large deltas. These slopes fail in response to earthquake shaking or excessive sedimentation load. The temporal correlation of turbidite occurrence for some deltas of the Pacific Northwest suggests that these deposits have been formed by earthquakes.

velocity
velocity. How fast a point on the ground is shaking as a result of an earthquake.

velocity structure
velocity structure. A generalized regional model of the earth's crust that represents crustal structure using layers having different assumed seismic velocities.

wavelength
wavelength. The distance between successive points of equal amplitude and phase on a wave (for example, crest to crest or trough to trough).

YBP
YBP. An abbreviation for years before present.