GLOSSARY OF SEISMIC TERMINOLOGY

Acceleration – The of Center of Resistance – of change, commonly measured in “g” (an resistance provided by walls and frames. of 32 ft/sec/sec or 980 cm/sec/sec = constant on ). Configuration Terms:

Accelerogram – The record from an Building Configuration – , shape and accelerograph showing acceleration as a proportions of the building; size, shape and of time. location of structural elements; and the type, size and location of nonstructural Accelerograph – A strong elements. instrument recording (or ) acceleration. Regular Configuration – Building configurations resisting lateral with Aftershock – One of a of smaller walls, resistant frames or quakes following the main shock of the braced frames - all in simple and near earthquake. symmetrical layout.

Amplification - The period (or ) of Irregular Configuration – Deviation from the ground motion coinciding with the period simple symmetrical building configurations of the building causing significant increase of with repetitive plan and . (See acceleration and damage. WBDG Seismic Design Principles resource page for examples). – Maximum deviation from mean of centerline of a . Structural Configuration – The size, shape and arrangement of the vertical – Reduction of amplitude or carrying the lateral resistance change in wave due to components of a building. over within time. Core – The central part of the earth below a Axial Load – Force coincident with depth of 2,900 kilometers. It is thought to be axis of a member. composed of and and to be molten on the outside with a central inner core. Base Isolation – A method using flexible bearings, whereby a building superstructure is (along a ) - Very slow periodic or detached from its foundation in order to episodic movement along a without reduce earthquake forces. .

Base Shear or Equivalent Lateral Force Crust - The lithosphere, the outer 80 (ELF)– Total shear force acting at the base of kilometers of the earth’s made up of a structure. crustal rocks, sediment and basalt. The general composition is -aluminum-iron. Brittle Failure – Failure in due to limited range; material subject to sudden failure without warning signs.

Center of – in the building plan at which the building would be exactly balanced.

Damping – The rate at which natural Dynamic - The opposite of “static”, when a decays as a result of the of energy. body (building) is in motion. In buildings it is an inherent nature to resonate inefficiently to vibration depending on structural Eccentric Braced Frame – A frame in connections, kinds of and which diagonal bracing is arranged eccentric to nonstructural elements used. “” column/ joints. design measures can reduce the of seismic forces. Effective Peak Acceleration – A coefficient shown on NEHRP maps used to determine Critical Damping – The minimum damping seismic forces. that will allow a displaced system to return to its initial without . – The ability of a material to return to its original form or condition after a – The horizontal or vertical displacing force is removed. Materials have an of a member due to the elastic range. application of external force. Elastoplastic – The total range of – Permanent distortion due to (deformation), including expansion beyond seismic forces. elastic into the plastic range. In the plastic range deformation is permanent. Depth of Focus – the depth of the focus or hypocenter beneath the earth’s Dissipation – Reduction in of commonly classes Earthquakes: Shallow (0- earthquake shock with time and 70 kilometers), intermediate (70-300 distance, or by transmission through kilometers), and deep (300-700 kilometers). discontinuous materials with different absorption capabilities. Design Earthquake – Generally defined as 2/3 of the maximum considered earthquake. Epicenter – The point of the earth’s surface directly above the focus or hypocenter of an Diaphragm – Generally a horizontal member, earthquake. such as a floor or roof slab, which distributes lateral forces to vertical resisting elements. Equivalent Lateral Force (ELF) – The representation of earthquake forces on a Displacement - Lateral movement of the building by a single static force applied at the structure caused by lateral force. base of a building; also referred as Base Shear (V). Drift - Horizontal displacement of building elements due to lateral earthquake Failure – The manner in which a forces. structure fails (column , overturning of structure, etc). – Ability to withstand inelastic without fracturing. Ductility is a material Fault Terms: to fail only after considerable inelastic (permanent) deformation which process Fault – A in the earth’s crust dissipates the energy from the earthquake by across which relative displacement has design. occurred. (Location of slippage between the earth’s plates). Duration – The period of time within which ground acceleration occurs.

Normal Fault – A fault under Eccentric Bracing – The centerlines of where the overlying block moves down the brace, beam and of column and do not dip or of the fault plane. coincide allowing deformation, thereby utilizing ductility. Strike-Slip Fault (or lateral slip) – A fault whose relative displacement is purely Moment Frame – Frames in which horizontal. structural members and joints resist lateral forces by . There are “ordinary”, (Reverse) Fault – A fault under “intermediate” and “special” moment where the overlying block frames. The latter provide the most moves up the dip or slope of the fault resistance. plane. Frequency - The number of wave peaks or Oblique-Slip Fault – A combination of cycles per . The inverse of Period. and slip or thrust and slip faults whose movement is diagonal along the dip Fundamental or Natural Period – The of the fault plane. elapsed time, in , of a single cycle of oscillation. The inverse of Frequency. Faulting – The movement which produces relative displacement of adjacent "g" - see Acceleration. along a fracture. ( valley) - Long, narrow trough Fault Zones – The zone surrounding a major bounded by one or more parallel normal faults. fault, consisting of numerous interlacing small These down-dropped fault blocks are caused faults. by tensional crustal forces.

FEMA – Federal Emergency Management Ground Acceleration - Acceleration of the Agency. Free publications available at: ground due to earthquake forces. http://www.fema.gov/ Ground Displacement - The distance that Flexible System – A structural system that will ground moves from its original position during sustain relatively large displacements without an earthquake. failure. Ground Failure - A situation in which the Focal Depth – Depth of the earthquake (or ground does not hold together such as land hypocenter) below the ground surface. , mud flows and .

Focus (of an earthquake) or Hypocenter – Ground Movement - A general term; includes The point at which the rupture occurs; (It all aspects of motion: acceleration, marks the origin of the kinetic waves of an velocity and displacement. (The plates of the earthquake). earth's crust move slowly relative to one- another accumulating or strain Frame Terms: resulting in slippage and complex vibration inducing forces in a building.) Braced Frame – One having diagonal braces for stability and capacity to resist Ground Velocity - Velocity of the ground lateral forces. during an earthquake.

Concentric Braced Frame – The Hypocenter or Focus - The point below the centerlines of brace, supporting beam and epicenter at which an earthquake actually column coincide. begins; the focus.

Input Motion - A term representing seismic Isolators – Calibrated mountings forces applied to a structure. with springs used to attenuate vibration generated by . For seismic locations Inelastic - Behavior of an element beyond its they are modified in order to absorb lateral elastic limit, having permanent deformation. movement and to keep the machine or equipment upright. These devices are Inertial forces - Earthquake generated available commercially. vibration of the building's mass causing internally generated inertial forces and building Magnification Factor - An increase in lateral damage. Inertial forces are the of forces at a specific site for a specific factor. mass acceleration (F = m a). Magnitude - A of earthquake size Intensity - A subjective measure of the force which describes the amount of energy of an earthquake at a particular place as released. See Richter Scale. determined by its effects on persons, structures and earth materials. Intensity is a Mantle - The main bulk of the earth between measure of energy. The principal scale used the crust and the core. in the United States today is the Modified Mercalli, 1956 version. MM (or Modified Mass – A constant or aggregate of Mercalli) scale is based on observation of the . effects of the earthquake MM-I thru MM-XII (MM-I = not felt, MM-XII = damage nearly MCE: Maximum Credible Earthquake, about total). 50% higher than the Design Base Earthquake (DBE). Isoseismals - Map contours drawn to define limits of estimated intensity of shaking for a Mercalli Scale – See “Intensity”. given earthquake. Microzonation - Seismic zoning, generally by Jacketing – Encasement of existing columns use of maps, for land smaller than with steel or Kevlar to increase resistance. regions shown in typical seismic code maps, but larger than individual building sites. - Earthquake triggering land on a hillside where one land mass - Determination of seismic slides over the other. design forces based upon the theoretical response of a structure in its several modes of Lateral Force Coefficients - Factors applied vibration to excitation. to the of a structure or its parts to determine lateral force for seismic structural Mode - The shape of the vibration . design. Modified Mercalli - See “Intensity”. Liquefaction - Transformation of a () from a solid state into a liquefied Moment Magnitude is the measure of total state as a consequence of increased pore- energy released by an earthquake. It is based pressure induced by vibration. Normally on the of the fault that ruptured in the solid soil suddenly changes to state quake. It is calculated in part by multiplying (usually or granular soil in proximity to the area of the fault’s rupture surface by the water) due to vibration. distance the earth moves along the fault. Macrozones - Large zones of earthquake activity such as zones designated by the Mud Flow - Mass movement of material finer International Building Code map. than sand, lubricated with large amounts of water.

Natural or - The - Induced of maximum constant frequency of a vibrating system in the amplitude produced in a physical state of natural oscillation. when applied oscillatory motion and the natural oscillatory frequency of the system are the NEHERP – National Earthquake Hazard same. When the site and building periods Reduction Program (FEMA). coincide, the buildings resonate with the ground. Then the amplitude of building Nonstructural Components - Those building vibration gradually approaches by time, components, which are not intended primarily resulting in structural failure. The ground may for the structural and bracing of the vibrate at a period of 0.5 to 1.0 sec. Structures building. may vibrate at a period of 0.1 to 6 sec. depending on the type of structure. Oscillation- capable to vibrate. Examples: Out of Phases - The state where a structure in 1 story structure = 0.1 sec. motion is not at the same frequency as the Up to 4 story structure = 0.5 sec. ground motion; or where equipment in a 10-20 story structure = 1 - 2 sec. building is at a different frequency from the Water tank structure = 2.5 - 6 sec. structure. Large = 6 sec.

Period - The elapsed time in seconds of a Response Spectrum - maximum response single cycle of oscillation. The inverse of (generally acceleration) of a site plotted frequency. against increasing periods.

Performance Based Design – New concept Return Period of Earthquakes - The time of designing a project for optimum period (years) in which probability is 63 percent performance within a given life cycle (usually that an earthquake of a certain magnitude will 50 years for institutional use). By definition the recur. building program is to include the careful analysis of all physical, economical, Richter Magnitude Scale - A measure of environmental, aesthetic, and sociological earthquake size which describes the amount of factors that will result in the desirable energy released. The measure is determined functioning of the project. This, of course, by taking the common (base 10) of includes hazard mitigation (natural or man- the largest ground motion observed during the made) and the agreed upon level thereof. arrival of a P-wave or seismic and applying a standard correction for distance Plate - The theory and study of to the epicenter. (Each unit of the Richter plate formation, movement, interaction and Scale represents a 10 times increase in wave destruction; the theory which explains amplitude. This corresponds to approx. 31 seismicity, volcanism, mountain building and times increase of energy discharge for each paleomagnetic evidence in terms of plate unit on the Richter Scale.) – See Moment . Magnitude Scale an alternative.

P-Wave – See “Waves”. Rift - A fault trough formed in a zone or in other areas in tension. (See Relative Rigidity - The comparative Graben) of interconnected structural members in view of relative distribution of the horizontal force. (Only identical stiffness of interconnected members can share the total load equally.)

Rigidity - Relative stiffness of a structure or Soil Structure Interaction - The effects of the element. In numerical terms, equal to the properties of both soil and structure upon reciprocal of displacement caused by a unit response of the structure. force. Spectra - A indicating maximum Scarp - A cliff, escarpment, or steep slope of earthquake response with respect to natural some extent formed by a fault or a cliff or steep period or frequency of the structure or element. slope along the margin of a plateau, mesa or Response can show acceleration, velocity, terrace. displacement, shear or other properties of response. Seiche - A on the surface of water in an enclosed or semi-enclosed basin Stability - Resistance to displacement or (lake, bay or harbor). overturning.

Seismic - Pertaining to earthquake activities. Stiffness - Rigidity, or resistance to deflection or drift. A measure of deflection or of staying Seismic Zone – Areas defined on a map in alignment within a certain stress. within which seismic design requirements are constant. Strain – Deformation per unit of material of the original . Seismicity - The worldwide or local distribution of earthquakes in and time; Strain Release - Movement along a fault a general term for the number of earthquakes plane; can be gradual or abrupt. in a , or for relative earthquake activity. Strength - A measure of load without exceeding a certain stress. Seismograph - A device, which writes or tapes a permanent, continuous record of earth Stress – Internal resistance within a material motion, a seismogram. opposing a force to deform it.

Shear Distribution - Distribution of lateral Subduction - The sinking of a plate under an forces along the height or width of a building. overriding plate in a convergence zone.

Shear Strain - The obtained by dividing S-Wave – See “Waves”. shear displacement by the thickness of the rubber layer in shear. Time Dependent Response Analysis - Study of the behavior of a structure as it responds to - The stress at which a a specific ground motion. material fails in shear. – The of a force that tends to Shear Wall - A wall designed to resist lateral produce . The product of a force and a forces parallel to the wall. A shear wall is arm. normally vertical, although not necessarily so. Torsion - Twisting around an axis. (The center Simple Motion - Oscillatory motion of the mass does not coincide with the center of a wave, single frequency. Essentially a of resultant force of the resisting building vibratory displacement such as that described elements causing or twisting action in by a weight, which is attached to one of a plans and stress concentrations. in and allowed to vibrate freely. general reduces torsion.)

Trench - A long and narrow deep trough in the through the earth's crust, and consisting of sea floor; interpreted as marking the along a train of compressions and dilatations of which a plate bends down into a subduction the material (push and pull). zone. S-Wave - Shear wave, produced - A sea wave produced by large area essentially by the or tearing displacements of the bottom, the result motions of earthquakes at right to of earthquakes or volcanic activity. (Tidal wave the direction of . caused by ground motion.) Seismic Surface Wave - A Tuning - To modify the period of the building that follows the earth's surface only, with a beyond the range of the site period to avoid less than that of S-waves. resonance. Examples of "tuning" include lowering the height of a building; lowering the Wave - The distance between position of weight in a building; changing successive similar points on two wave cycles. materials; changing fixity of base, etc. The longer the period, the less inertial forces can be expected. Short periods close to the fault and long periods far from the fault are usual.

Velocity – Rate of change of distance traveled with time in a given direction in centimeters/second.

Vibration - A periodic motion that repeats itself after a definite interval of time.

Wave Terms:

Body Wave – Seismic waves within the earth.

Longitudinal Wave - Pure compressional wave with volume changes.

Love Wave – Surface waves that produce a sideways motion.

Rayleigh Wave - Forward and elliptical vertical seismic surface waves.

P-Wave - The primary or fastest waves traveling away from a seismic event