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I • CONTENTS under given loading conditions; numerically equal to the slope of the tangent of a stress-strva curve. Mohorovitie (M) discontinuity. Seiemie disrnntinuity which sep- arates the earth's crust azd mantle. ' Moment The seismic moment M,=PZiA contains in(ormetian on the rigidity (µ) in the sours region, average dill«ation (u ), end area U) of faulting. It determines the amplitude of the long-period level of the spectrum of ground motion. Monotonic loading. Continuously increasing load in one d'vection. 'Natural frequency. Property of the elastic system in free vibra- tion. Free vibration aeon ntunlly et a discrete frequency when an elastic system vibrates under the action of fortes inherent in the system itself end in the absence of external impressed forces. formal fault Vertical movement along a sloping fault surface in which the bl«k above the fault has moved downward relative to the bl«k below. The Wasatth fault in Utah is an example. formal stress. That stress component normal to a given plane. 'Operating basis earthquake (OBE). A design earthquake used by the U.S. Nuclear fiegulamry Commission in nuclear power plant siting: the largest earthquake that reasonably could be expected to ' eReet the plant site during the operating life of the plant. The powerplent is designed b withstand the OBE end still operate without undue risk to the health and safety of the public. See Sefe shutdown earthquake. 'Oscillation. The variation, ovally with time, ofthe magnitude ofa quantity with respect W a specified reference when the magnitude ie alternately greater end smaller than the reference. Overburden. The generic term applied to uppermost layen of the geologic structure, usually unconsolidated materiels having low seismic velocity overlying r«k P-wave. (See also Compression or Body wave). Body wave in which the direction of the particle motion is the same as the direc- Lion of wave propagation. P-wave vet«ity ie commonly measured in geophysical refraction surveys to define the contact between the competent rock layen (high-vet«ity materials) and the overlying unconsolidated materiels (low-vet«ity materials). Particle acceleration. The time rate of change of particle vet«ity. 'Particle displacement The diRerence between the initial position of a soil particle end any later position. Particle velocity, The time rate of change of particle displacement. Period. The time interval oavpied by one cycle. Permeability. A measure of the ease with which a fluid ran pass through the pore spaces of a formation. Phase. The angle of lag or lead (or the displacement) of a sine wave with respect to a reference; the stage in the count of a rotation or oscillation to which it has advanced, considered in relation to a reference or assumed instant of starting. Phase response. A graph of phase shin venue frequency illus- trates the phase response characteristics of a system. The amplitude-frequency response of a filter to the shape of pulses put through it will be different for diAerent phase characteristin; this response leads to phase distortion. hose velocity, The vel«ity with which any given phase (such as a wave of single frequency) travels; i[ may differ from group veloc- ity because of dispersion. Plane strain (biaxial). A measure of strain that takes place in two direction while remaining zero in the third dimension. ~lastic range. The stress range in which a material will not fail when subjected to the action of a force, but will not recover com- pletely, so that a permanent deformation results when the force is removed. ~Iate tectonics. A theory introduced in 1967 end subsequently re- fined that coniden the earth's ernt and upper mantle W be made up of more than 15 relatively undistorted plates about 60 km thick which more relative to one another. The plates spread from the • XI mid~-eenic ridges where, by means oC the upflow of coal, ,, new lithospheric material is continually added. On the oppos .e mar- gins of the plates, there are usually deep submarine trenches At these trenches, the plates converge from opposite direction (for example, the Nana and South American plate along the Andes Mountain), end one plate is conumed or eubducted beneath the other into the deeper parts of the earth. Earthquake belts or zones mark plate boundaries, the zones along which the lithospheric plate collide, diverge, and elide pest one soother. The San An- dreas fault zone is en example of s boundary between the North American end Pacific plates. Poisson s ratio. The ratio of the tranvene convection to the lon- gitudinal extenion when a rod is stretched. The ratio of the vet«i- ties of P- and Swaves, Ve end Vs, can 6e expressed in terms of Poisson's ratio, v Ve __ 2(1-0) Vs (1-20) Pore water pressure. Pressure or stress transmitted through the pore water filling the voids of the sail. Power spectrum. A graph of power spectral density versus fre- quency. The power spectrum is the square of the emplitude- (requeocy response. Principal stress. Stresses acting normal to three mutually perpen- dicular planes intersecting et a point in a body, on each of which the shearing stresses are zero. Probability of occurrence. The annual rate of occurrence of a hazard. Pulse. A waveform whose duration is short compared to the time scale of interest end whose initial and final values are the same (usually urn). Q. Q, the reciprocal of the specific dissipation funRion,'u en index of the dissipative nature of the earth's trenmission path on prop- agating seismic waves. Q is essentially independent of frequenq or wavelength for a wide range of frequencies. Empirically deter- mined values of Q range from 50 to 500 for crustal materie]s in varion region of the United States. Also called quality fettor. Rayleigh wave. A type of seismic surface wave which propagates along the surface. Particle motion is elliptical and retrograde in the vertical plane containing the direction of propagation, and its amplitude decreases exponentially with depth Raypath A line everywhere perpendicular to wavefronts in iso- tropic media. The path which a seismic body wave takes. Reflection. The energy or wave from a seismic scarce which has been reflected (returned) from an acouetie impedance contrast or aerie of contrasts within the earth. Reflection coefficient The ratio of the amplitude of a reflected wave to that of the incident wave. For normal incidence on an interface which separates media of denities p, and N end vet«i- ties V, and Vz, the reflection ccef5cient for a plane wave is: Pryr - PiV~ PsVa + P,V, Region. A geognphical area surrounding and including the site sufficiently large to contain all the features related to a physical phenomenon or to a particular earthquake hazard. Relative densiq•. The ratio of (1) the diRereoce between the void redo of a coheionless soil in the loosest state and any given void ratio, to (2) the dill'erence between its void ration in the loosest and in the densest state. Resonance. The reinforced response of one of the natural modes of vibration of a body when excited at a frequenry close to the natant frequency of vibration. Resonant frequency. A Crequenq at which rtnonance oavrs.