Glossary

Accelerogram: time history of accclerations.
Accelerograph: instrument which records the signals from an accelerometer.
Accelerometer: sensor with output approximately proportional to the ground accelerations.
Active fault: fault which is known to produce earthquakes based on geologic or seismic evidence.
Aftershocks: additional tremors that occur after the initial earthquake. They are smaller thatn the mainshock and within 1-2 fault lengths distance from the mainshock fault. Aftershocks can continue over a period of weeks, months, or years. In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue.
Amplitude: the size of the wiggles on an earthquake recording.
Anisotropic: showing differences of property or effect in different directions.
Attenuation: decrease in amplitude. often used for the decrease in amplitude of the ground motion with increase in distance from the source. This attenuation is due to two mechanisms, one is the distribution of energy over a larger volume as the distance increases, the other is the loss of energy due to internal damping. The latter effect is frequency dependent and gives higher attenuation of the high frequency motion.
Asthenosphere: zone of the earth's mantle lying beneath the lithosphere, believed to be much hotter and more fluid than the lithosphere.
Basalt: dark igneous rock that is low in silica content and comparatively rich in iron and magnesium.
Body Waves: energy waves travelling within the earth. These waves consist of compressional waves (P-waves) and shear waves (S-waves). Near the source most of the earthquake energy is in the form of body waves.
Capable fault: a fault capable of producing earthquakes. Evaluation of capability is based on geologic or seismic evidence. Capable is used for faults likely, but not certain, to produce earthquakes, often used synonymously with potentially active faults.
Continental plate: a large rigid part of the earth's crust and upper mantle which moves relative to the other continental plates. The speed of movement may be up to 15-20 cm/year. Scandinavia belongs to the Eurasian continental plate.
Core: the innermost part of the Earth. The outer core extends from 2900 to 5100 km below the Earth's surface and is liquid metal. The inner core is the central 1200 km and is solid metal.
Crust: the outer major layer of the earth, separated from the underlying mantle by the Moho discontinuity, and characterized by P-wave velocity less than 8 km/s. The thickness of the crust in the Norwegian Continental Shelf in the range 15-25 km.
Damping: loss of energy, often expressed relative to the critical damping, C(cr) = 2 * SQRT(K*M), where K and M are stiffness and mass of the vibrating system, respectively.
Design earthquake: an earthquake event used for evaluation of the earthquake resistance of structures.
Design motion: description of ground shaking (e.g., time history, response spectrum) at a given site used for evaluation of the earthquake resistance of structures in modern hazard studies usually the result of contributions from all seismic sources surrounding the site and not corresponding to any specific design earthquake.
Dip: inclination of a planar surface (e.g., fault) with a horizontal plane.
Earthquake: shaking of the ground by different types of waves generated by tectonic movements or volcanic activity. By far the largest number of destructive earthquakes are caused by tectonic movements. An earthquake is initiated when the accumulated tectonic stresses at any one point in the ground become greater than the strength at this point. Release of stress at one point may increase the stresses nearby, and result in a progressive rupture which may propagate for several hundred kilometers. The rupture will almost invariably occur along old zones of weakness, faults.
Epicenter: projection of the hypocenter to the surface.
Fault: a zone of fractures in the earth's crust. Earthquakes are caused by a sudden rupture along a fault or fault system; the ruptured area may be up to several thousand square kilometers. Relative movements across a fault may typically be tens of centimeters for magnitude 6.0-6.5 earthquakes, several meters for magnitude 7-8 earthquakes.
Fault plane solution: analysis to determine the focal mechanism for a given earthquake, i.e., the direction of slip and stress conditions of the causative fault. The analyses are usually based on the first motion of P-waves recorded at different stations.
Focal mechanism: mechanism of faulting as obtained through a fault plane solution.
Focus: See hypocenter
Fourier spectra: values of amplitudes versus frequency of the harmonic motions used to describe a given time history (Fourier transformation).
Free field motion: earthquake motion on flat ground unaffected by structures.
GBF: Generalized beamforming. A method for automatic phase association and event location. GBF works from a list of phase detections from each station. For a large set of hypothetical event locations, GBF searches for a pattern of detections that fits the theoretically expected phase arrivals from the hypothetical locations. When a group of matching detections is found, the event location having the best fit to the data is chosen as the most likely epicenter.
Frequency: the number of times something happens in a a certain period of time, such as the ground shaking up and down or back and forth during an earthquake.
GSETT-3: The Group of Scientific Experts Third Technical Test.
Granit: a coarsal-grained igneous crystalline rock, composed of quarz, feldspar and mica.
Hazard: probability of occurrence in a given location of, e.g., destructive earthquakes. Seismic exposure may be used synonymously with seismic hazard.
Hertz (Hz): a unit of frequency. Expressed in cycles per second.
Hypocenter: the point where the earthquake started, also called focus. Hypocenter depths are typically 30 km and less for shallow earthquakes, several hundreds of kilometers for earthquakes occurring in subduction zones. Most earthquakes in Fennoscandia originate at depths between 10 and 30 km.
Igneous: produced by solidification of the Earth's internal molten magma.
Inertia: inherent property of a body that makes it oppose any force that would cause a change in its motion. The inertia of a body can be measured by its mass, which governs its resistance to the action of the force.
Intensity: a measure of the ground shaking at a given site based on effects of the earthquake such as how the earthquake was felt, damage to structures, how people reacted, soil or rock slides, etc. Several different intensity scales are presently available, commonly used are modifications of the Mercalli scale
Interplate: between the tectonic plates. Most earthquakes are caused by the relative movements which take place at plate margins, i.e., between plates
Intraplate: within the continental plates. Scandinavia belongs to the Eurasian plate and is well removed from the nearest plate boundary.
Isoseismal: line through sites with equal effect of the earthquake, i.e., equal intensity.
Lifelines: structures that are important or critical for a comunity to function, such as roadways, pipelines, powerlines, sewers, communications, and port facilities.
Lithosphere: the outer solid part of the Earth, including the crust and uppermost 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 eartquakes by faulting, such as within a subducted oceanic plate.
Locked fault: a fault that is not slipping because fricitonal resistance on the fault is greater than the shear stress across the fault (it is stuck). Such faluts may store strain for extended periods that is eventually released in an earthquake whe frictional resistance is overcome.
Love Wave: a type of seismic surface wave having a horizontal motion that is transverse (or perpendicular) to the direction the wave is traveling.
Macroseismic: ground shaking which gives noticeable effects (see intensity).
Magnetic polarity reversal: a change of the Earth's magnetic field to the opposite polarity that has occurred at irregular intervals during geologic time. Polarity reversals can be preserved in sequences of magnetized rocks and compared with standard polarity-change time scales to estimate geologic ages of the rocks. Rocks created along the oceaninc spreading ridges commonly preserve this pattern of polarity reversals as they cool, and this pattern can be used to determine the rate of ocean ridge spreading. The reversal patterns recorded in the rocks are termed sea-floor magnetic lineaments.
Magnitude: a measure of earthquake size. Magnitude was defined by C. Richter in 1935 as: "The logarithm to the trace amplitude in 0.001 mm on a standard Wood-Anderson seismometer located 100 km from the epicenter" The Wood-Anderson instrument measures the responses in the period range near 1 sec. Other magnitude scales have later been devised based on the responses measured in other period ranges, and on maximum amplitudes of specific wave forms Some of the more commonly used magnitude scales are:
ML= local magnitude: similar to the original Richter magnitude. Usually determined from shear wave response in the period range near 1 sec. at relatively close distances from the epicenter (< 600 km).
mb= body wave magnitude is based on the largest amplitude of body waves: usually the compressional component with period near 1 sec. MS= surface wave magnitude is measured in the period range near 20 sec. Mw=moment magnitude is based on the seismic moment and be computed directly from source parameters or from long period components in the earthquake record. Symbol M is also used for this magnitude.
Magnitude scales are also based on other earthquake parameters such as felt area: length of rupture and surface displacement, and area within different intensity zones. A large number of empiric relations between magnitude and other earthquake parameters such as energy, fault movement, fault area, intensity, maximum acceleration, etc., are available. Such relations may differ considerably from one seismic region to another.
Mainshock: the largest earthquake in a sequence, sometimes preceded by one or more foreshocks, and almost always followed by many aftershocks.
Mantle: the part of the Earth's interior between the metallic outer core and the crust
Moho: Mohorovicic discontinuity, a sharp discontinuity in seismic velocities separating the earth's crust from the underlying mantle, also called the crust-mantle boundary.
Natural frequency: the frequency at which a particular object or system vibrates when pushed by a single force or impulse, and not influenced by other external forces or by damping. If you hold a slinky by one end and let it hang down and then give it one push up from the bottom, the rate of up-and-down motion is its natural frequency.
Oceanic spreading ridge: a fracture zone along the ocean bottom where molten mantle material comes to the surface, thus creating new crust. This fracture can be seen beneath the ocean as the line of ridges that form as molten rock reahces the ocean bottom and solidifies.
Oceanic trench: a linear depression of the sea floor caused by the subduction of one plate under another.
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. Slip on faults that define the plate boundaries commonly results in earthquakes.
P-wave: a seismic body wave with particle motion in the direction of propagation, also called compressional wave even though the motion alternates between extension and compressions.
Percent 'g': g is the force of gravity (an acceleration of 9.78 meters/second²). When thereis an earthquake, the forces caused by the shaking can be measured as a percentage of the force of gravity, or percent g.
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. Slip on faults that define the plate boundaries commonly results in earthquakes. Several styles of faults bound the plates, including thrust faults along which plate material is subducted or consumed in the mantle, oceanic spreading ridges along which new crustal material is produced, and transform faults that accommodate horzontal slip (strike slip) between adjoining plates.
(to be) Polarized: for waves, when the vibrations are restricted to certain directions.
Pseudo acceleration (PSA): see response spectra.
Pseudo velocity (PSV): see response spectra.
Rayleigh wave: a seismic surface wave causing the ground to shake in an elliptical motion, with no transverse, nor perpendicular, motion.
Recurrence interval: average time span between events (e.g. earthquakes)
Reflection: the energy or wave from an earthquake that has been returned (reflected) from a boundary between two different materials within the Earth, just as a mirror reflects light.
Refraction: 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.
Response spectra: describe the maximum response of single-degree-of-freedom systems to given ground motions as a function of the period and the damping of the system. The responses may be pseudo acceleration, pseudo velocity or relative displacement. Pseudo acceleration and pseudo velocity values may be expressed in an approximate way from the relative displacement through the relation: where PSA is pseudo acceleration, PSV is pseudo velocity and RD relative displacement, respectively, and is circular frequency. By using the pseudo values, all three responses can be plotted together in a logarithmic, tripartite nomogram.
Risk: hazard x vulnerability, i.e., probability of occurrence of loading of a certain magnitude times the probability for damage caused by that load. Risk may be expressed in terms of economic costs, loss of lives or environmental damage per unit of time.
Return period: same as recurrence interval, average time period between earthquakes of a given size in a particular region, cycle time.
Seismic: Subject to, or caused by an earthquake or an artificial earth vibration.
Seismic array: A cluster of seismometers distributed over a fairly small area, usually on the order of a few kilometers. Seismic arrays can function like phased array radar receivers, sensitive to waves from a particular direction while excluding waves from other directions. By doing so, arrays can pull small signals out from the surrounding background noise.
Seismic moment: is defined as the rigidity of the faulted rock times the area of the fault times the average amount of slip. Seismic moment can also he determined from the long period components of the seismic record.
Seismicity: same as earthquake activity, i.e., frequency of occurrence and locations of earthquakes.
Seismogram: a record written by a seismograph (seismometer) in respons to ground motions produced by an earthquake, explosion, or other ground-motion sources.
Seismograph: See seismometer.
Seismometer: Instrument for measuring seismic waves by recording ground motion. The principle by which the seismometers work can be thought of as a heavy mass freely supported by a spring from a frame fixed to the Earth. When an earthquake or explosion occurs, sesimc waves traveling through the Earth reach the seismometer. The frame is shaken in response to the motion of the wave. Although the frame is displaced by the ground motion, the heavy mass tends to remain stationary because of its inertia. The displacement of the grounded frame is therefore a measure of the ground motion.
Soil: (1)in engineering, all unconsolidated material above bedrock. (2) in soil science, naturally occuring layers of mineral and (or) organic constituents that differ from the underlying parent material in their physical, chemical, mineralogical, and morphological charecter because of pedogenic processes.
Soil profile: the vertical arrangement of layers of soil down to the bedrock.
Spectrum: a curve showing amplitude and phase as a function of frequency or period, or how much of each type of shaking ther is from an earthquake.
Strain: small changes in length and volume associated with deformation of the rocks by tectonic stresses or by passage of seismic waves.
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: reduction in stress across a fault, e.g. caused by an earthquake. Intraplate earthquakes have in general higher stress drop than interplate earthquakes. Typical values are 1-10 MPa (10-100 bars).
Strike: direction relative to north of a line defined by the intersection of a planar structure (e.g., fault with a horizontal plane.
Subcrop of fault: upper boundary of the fault beneath the surface in cases where the trace of the fault does not reach the surface (fault outcrop).
Subduction: the process of the oceanic lithosphere colliding with and descending beneath the continental lithosphere.
S-wave: a seismic body wave with particle motion perpendicular to the direction of propagation, also called shear wave. The passage of an S-wave involves a pure shear of the medium.
Tectonic: rock deforming processes (e.g., faults and flexures) in the earth's crust which involve relatively large areas.
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.
Travel time 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: 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.
Universal time (UT): mean (solar) time of the Greenwich meridian.
Vulnerability: degree of damage caused by various levels of loading. The vulnerability may be calculated in a probabilistic or deterministic way for a single structure or groups of structures.
Wavelength the distance between successive points of equal amplitude and phase on a wave (for example, crest to crest or trough to trough)