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VIII co~ITENTs
<br />Quaternary or more recent movement, or (2) macro-ceimic activity
<br />has been assotieted with it, or (3) it has a demonstrated sWcturel
<br />relation to a knows capable fault such that movement of the one
<br />may cause movement of the other, especially during the lifetime of
<br />the project under consideration.
<br />Compresdon wave. A wave in which an element of the medium '
<br />changes volume without rotation.
<br />Conver4d wave. A wave which is converted from longitudinal to
<br />transverse, or vice verse, upon reflection or refraction at oblique
<br />incidence from m interface.
<br />Convolutlon. The change of wave shape as a result of passing a
<br />signal through a linear filter.
<br />Corner frequency. A spectrum's corner frequency is that frr
<br />quenq where the high- end low•frequency trends intersect The
<br />location of the cornerfrequency,/,', is related to the radius, r, of the
<br />equivalent circular fault through the relation r =
<br />~- where u the shear-wave vet«it at the source.
<br />x+I. ~ ~ y
<br />Covariance. A statistical regression analysis technique that allows
<br />one W enalyu subsets of data having a common characteristic
<br />property.
<br />Critical angle. Angle of incidence, Br, for which the refracted ray
<br />grazes the surface of contact between two media in which the seis-
<br />mic velocities ere µ end ur.
<br />Crust The outermost portion of the earth, averaging about 30 km,
<br />that overlie the Mohorovicie dixontinuity.
<br />Design earthquake. The largest earthquake that bas such a high
<br />probability of oaurrence based on studies of historic seismicity
<br />and sWctural geology Chat it is appropriate to design a structure
<br />to withstand it Ground shaking of the design earthquake might be
<br />exceeded, but the probability of thisheppening v considered to be
<br />small.
<br />Design spectra. Spectra appropriate for earthquakrresistant de-
<br />sign purposes. Design spectra ere typically smooth curves that
<br />have been modified from a family of spectre of historic earthquakes
<br />to take atxrount of features peculiar to a geographic region and a
<br />particular site. Design spectra do no[ include the effect of sail-
<br />structure interaction.
<br />Design time history. One oC a family of time histories which pro-
<br />dares a response spectrum that envelopes the smooth design
<br />spectrum, for a selected value of damping, at all periods.
<br />Earthquake hazards. The probability that natural events accom-
<br />panying an earthquake such as ground shaking, ground failure,
<br />surface faulting, tectonic deformation, and inundation, which may
<br />cause damage and loss of life, will «cur et a site during a specified
<br />exposure time. See Earthquake risk.
<br />Earthquake risk. The probability that s«ial or economic conse-
<br />quences of earthquakes, expressed in dollars or wualties, will
<br />equal or ezceed specified values at a site during a specified expo-
<br />sure time.
<br />Earthquake waves. Elastic waves propagating in the earth, set in
<br />motion by a sudden change such as faulting o(a portion of the
<br />earth.
<br />Effective peak acceleration. The peak ground acceleration aRer
<br />the ground-motion record has been filtered to remove the very high
<br />frequencies that have little influence upon structural response.
<br />Effective peek velocity. The peak ground vet«ity alter the ground
<br />motion record has been filtered [o remove high frequencies.
<br />Epicenter. The point on the Earth's surface vertically above the
<br />point where the lust rupture end the first earthquake motion «-
<br />cur.
<br />Exceedaneeprobability. The probability )for example, IO percent)
<br />over some period bf time that an earthquake will generate a level
<br />of ground shaking greater than some specified level.
<br />~xpos~~re tune. The period of time (for example, 50 years) that a
<br />str rte is exposed to the earthquake threat. The exposure time is
<br />sometimes chosen to be equal to the design lifetime of the actor
<br />lure.
<br />Failure. A Condition in which movement caused by shearing stress-
<br />es in a structure or soil mw is of sufficient magnitude to destCOy
<br />or seriously damage it
<br />Fault A fracture or Credure cone along which displacement of the
<br />two aides relative to one another has amrmd parallel to the fra~-
<br />ture. See Active, Capable, Normal, Throat and Stsdke•alip
<br />faults.
<br />Filter. That part of a system which dixriminates against some oC
<br />the information entering it The discrimination is usually on the
<br />basis of frequency, although other bases such as wavelength may
<br />be used. Linear filteringu Called convolution.
<br />Finite element analysis. An analysis which uses en assembly of
<br />finite elements which are corrected at a finite number of nodal
<br />points to represent a structure or a soil continum.
<br />Firm soil. A general term for soil characterized by a shear wave
<br />vet«ity of 600 to 765 m ~. See Soft soil, Bedrock.
<br />Focal depth. The vertical ditanre between the hyp«enter and the
<br />epicenter in an earthquake.
<br />Focus. The point within the earth which marks the origin of the
<br />elastic waves of an earthquake.
<br />Forced vibration. Vibration that neon if the response is imposed
<br />by the excitation. If the excitation is periodic and Continuing, the
<br />oxilletion issteady-state.
<br />Fourier spectrum. See Amplitude spectrum, Phase response,
<br />and Fourier transform.
<br />Fourier transform. The mathematical formulas that convert a
<br />time function (waveform, seismogram, etc) G(t) into a function of
<br />frequency S(/') and vice versa
<br />Free field. The regions of the medium that are not influenced by
<br />manmade structures, or a medium that contains no such etrvc-
<br />tures. It elsorefers to that region in which boundary effects do not
<br />significantly influence the behavior of Ute medium.
<br />Free vibration. Vibration that accun in the absence of forced
<br />vibration.
<br />Frequency. Number of q~cles «curring in unit time. Hertz (hz) is
<br />the unit of frequency.
<br />Gaussian distribution. Equals normal distribution lbell-shaped
<br />curve): A quantity or set of values so distributed about a mean
<br />value, m, that the probabiliry•, e1.1a1, of a value lying within an
<br />interval, ~a, centered at the point, a, is:
<br />c(Ga)= 1 e -la -colt Da
<br />2-v 2a+
<br />where o is the standard emr of estimate.
<br />Geometrical damping. That component of damping due to the ra-
<br />dial spreading of energy with distance from a given source.
<br />Geophone. Sensing device used to measure electronically the rate
<br />of travel of sound or force waves transmitted through the earth
<br />from a known source.
<br />Geophysics. The study of t}e physical characteristics and prop-
<br />erties of the earth.
<br />Geotechnical. Related to soil mechanics.
<br />Grain size. A term relating m the siu of mineral particles that
<br />make up a soil deposit.
<br />Ground response, ground motion, seismic response. A general
<br />term, includes all aspects of ground motion, namely, particle accel-
<br />eration, vet«ity, or displacement; stress and strain from a nuclear
<br />explosion, an earthquake, or another energy source.
<br />Croup velocity. The vet«ity with which most of the energy in a
<br />wave train travels. In dispersive media where vet«ity varies with
<br />frequency, the wave train changes shape a, it progresses so that
<br />individual wave cresU appear to travel at a diRerent vet«ity (the
<br />phase vet«ity) than the overall energy as approximately enclosed
<br />by the envelope of the wave [rain.
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