Earthquake Engineering Terms

• Accelerograph


  A type of seismograph used to measure ground acceleration as a function of time
• Active fault


  A fault along which slip has occurred in recent geological time, or where earthquake foci are located
• Active margin


  Continental margin characterized by earthquakes and volcanic activity (i.e. location of transform fault or subduction zone)
• Aftershock


  An earthquake that occurs after a "mainshock" (or larger earthquake). Aftershocks occur in the same general region as the "mainshock" and result from readjustments of stress at places along the fault zone. For great earthquakes (M=8), aftershocks may occur over hundreds of kilometres. Depending on the size and depth of the earthquake, aftershocks may occur for many months after the mainshock; however, the rate of aftershock activity dies off quickly with time
• Amplitude (wave)


  The maximum height of a wave crest or depth of a trough
• Array


  An ordered arrangement of seismometers or geophones, the data from which feeds into a central receiver
• Arrival


  The appearance of seismic energy on a seismic record
• Arrival time

  The time at which a particular wave phase arrives at a detector
• Aseismic


  Not associated with an earthquake, as in aseismic slip. Also used to indicate an area with no record of earthquakes: an aseismic zone
• Asperities (fault)


  Roughness on the fault surface posing local resistance to slip
• Asthenosphere


  The layer below the lithosphere that is marked by low seismic-wave velocities and high seismic-wave attenuation
• Attenuation


  The reduction in amplitude of a wave with time or distance travelled
• Auxiliary fault-plane


  A plane orthogonal to the fault plane
• Barrier (fault)


  An area of fault surface that is resistant to slip because of geometrical or structural changes
• Benioff zone


  A narrow zone, defined by earthquake foci, that is tens of kilometers thick dipping from the surface under the Earth's crust to depths of up to 700 kilometers. (Also Wadati-Benioff zone.)
• Blind thrust


  A deep crustal thrust-fault with no or only indirect surface expression such as a fold structure
• Body wave


  A seismic wave that can travel through the interior of the earth. P-waves and S-waves are body waves
• Body-wave magnitude


  Magnitude of an earthquake as estimated from the amplitude of body waves
• Capable fault


  A fault along which it is mechanically feasible for sudden slip to occur
• Characteristic earthquake


  An earthquake with a size and generating mechanism typical for a particular fault source
• Coda


  The concluding train of seismic waves that follows the principal waves from an earthquake
• Consolidated


  Tightly packed. Composed of particles that are not easily separated
• Continental Drift

  The theory, first advanced by Alfred Wegener, that Earth's continents were originally one land mass. Pieces of the land mass split off and migrated to form the continents
• Continental shelf

  Part of the continental margin between the coast and the continental slope
• Core

  The innermost layers of the Earth. The inner core is solid and has a radius of about 1300 kilometres. (The radius of the Earth is about 6371 kilometres.) The outer core is fluid and is about 2300 kilometres thick. S-waves cannot travel through the outer core
• Crust


  The layer of rock located immediately below the earth's surface. Beneath continents, it is typically about 35 km thick, and composed of granite. Under the ocean, the crust is about 5-10 kilometres thick and composed mainly of basalt
• Damping


  Loss of energy in wave motion due to transfer into heat by frictional forces
• Density

  The mass per unit volume of a substance, commonly expressed in grams per cubic centimetre
• Depth of an earthquake

  The value given is the depth below the surface of the mean spheroid
• Dilatancy (of rocks)

  The increase in the volume of rocks mainly due to pervasive microcracking
• Dip

  The angle by which a rock layer or fault plane deviates from the horizontal. The angle is measured in a plane perpendicular to the strike
• Dip-slip fault

  A fault in which the relative displacement is along the direction of dip of the fault plane; the offset is either normal or reverse
• Dispersion (wave)


  The spreading out of a wave train due to each wavelength travelling with its own velocity
• Duration (of strong shaking)

  Time interval between the first and last peaks of strong ground motion above a specified amplitude
• Earthquake

  The sudden release of stored elastic energy caused by the sudden fracture and movement of rocks along a fault. Some of the energy released is in the form of seismic waves, that cause the ground to shake
• Earthquake occurrence (recurrence) interval

  The average interval of time between the occurrence of earthquakes in a particular region
• Earthquake swarm

  A series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time
• Elastic wave

  A wave that is propagated by some kind of elastic deformation, that is, a change in shape that disappears when the stresses are removed. A seismic wave is a type of elastic wave
• Epicentre

  The point on the earth's surface directly above the focus (hypocentre) of an earthquake
• Fault

  A zone of fractures or breaks in rocks, where movements occur. Earthquakes often occur along faults because they are weak zones in the rock