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  • Acceleration
    The rate of change of velocity. As applied to strong ground motions, the rate of change of earthquake shaking velocity of a reference point. Commonly expressed as a fraction or percentage of the acceleration due to gravity (g), wherein g = 980 centimeters per second squared.
  • Active Fault
    An earthquake fault that is considered to be likely to undergo renewed movement within a period of concern to humans. Faults are commonly considered to be active if they have moved one or more times in the last 10,000-11,000 years, but they may also be considered potentially active when assessing the hazard for some applications even if movement has occurred in the last 500,000 years. See fault.
  • Alluvium
    A soil type consisting of loosely compacted gravel, sand, silt, or clay deposited by streams.
  • Amplification
    An increase in seismic wave amplitude as the waves propagate through certain soils, in sedimentary basins, or in certain topographic configurations (e.g. along ridge lines).
  • Alquist-Priolo (A-P) Special Studies Zone
    More recently known as Earthquake Fault Zone (EFZ). In California, these are defined areas surrounding active faults, as defined by the State Geologist, within which it is necessary to perform fault location studies in order to construct buildings for human occupancy. Buildings for human occupancy may not be constructed within 50 feet of the identified fault rupture trace. Details of the regulations are presented in Special Publication 42, published by the California Geological Survey (CGS).
  • Average Annual Loss
    The long-term loss rate per year due to hazards, calculated as the probabilistic loss contribution of all events.
  • Business Interruption (BI) Loss
    Economic loss associated with loss of function of a commercial enterprise.
  • Damage
    Physical disruption of a structure or equipment item, such as cracking in walls or overturning of equipment.
  • Hazard
    A natural physical manifestation of the earthquake peril, such as ground shaking, soil liquefaction, surface fault rupture, landslide or other ground failures, tsunami, seiche. These hazards can cause damage to man-made structures.
  • Liquefaction
    A ground failure phenomenon in which loose, granular soils below the water table lose shear strength when subjected to many cycles of strong ground shaking.
  • Magnitude (M)
    Magnitude (M) is the most widely used measure of the size of an earthquake. Magnitude scales are logarithmic, found by taking the common logarithm (base 10) of the largest ground motion recorded at the arrival of the type of seismic wave being measured and correcting for the distance to the earthquake's epicenter. A typical seismogram will display separate arrival times for P-waves or compressional waves, and the slower S-waves or shear waves. The difference in arrival times for P- and S-waves indicates site-to-source distance. The logarithmic scale means that an increase in magnitude by one unit corresponds to a tenfold increase in measured wave amplitude. Moreover, the energy released by an earthquake increases by a factor of about 30 for each unit increase in magnitude.
  • Modified Mercalli Intensity (MMI) (abridged)
    A numerical scale ranging from I to XII which describes local ground earthquake intensity in terms of local earthquake effects. In many historical earthquakes (1900 to 1970's), few ground shaking instruments were in use, and ground shaking maps were compiled on the basis of observed effects, using scales like the Modified Mercalli Intensity (MMI) scale. As a result, building damage statistics from older earthquakes are often correlated to the MMI scale.
    • I-V Not significant to structures or equipment.
    • VI Felt by all; many are frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight.
    • VII Everybody runs outdoors. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures; some chimneys broken. Noticed by persons driving motorcars.
    • VIII Damage slight in specially designed structures; considerable in ordinary substantial buildings, with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Chimneys, factory stacks, columns, monuments, and walls fall. Heavy furniture overturned. Disturbs persons driving motorcars.
    • IX Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken.
    • X Some well-built wooden structures destroyed; most masonry and frame structures destroyed, along with foundations; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed (slopped) over banks.
    • XI Few, if any, (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land dips in soft ground. Rails bent greatly.
    • XII Damage total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown upward into the air.
  • Peak Horizontal Acceleration (PHA)
    An instrumental measure of earthquake ground motion intensity, normally taken from a triaxial earthquake accelerogram. The horizontal of the randomly-oriented component maxima may be combined to give a 'geometric mean', or simply taken as the maximum value recorded from the horizontally-oriented axes. The time history may also be processed to instantaneous vectorial maximum value, or rotated to fault-parallel and fault-perpendicular directions. PHA may also be referred to as PGA (peak ground acceleration).
  • Probable Loss
    A level of building damage from earthquake, expressed as a fraction of the building replacement value, having a stated probability of exceedance within a given exposure period. Alternatively, a level of earthquake damage having a stated return period. Probable Loss is found by considering all levels of earthquake hazard that may occur for the site in question, the building damage associated with each hazard level, and the variability of building damage within each hazard state.
  • Probable Maximum Loss
    A term used in the past to characterize the risk of earthquake damage to buildings. Care must be used to avoid ambiguity in definition [ASTM E 2026-07]. PML50 is a term sometimes used interchangeably with Scenario Expected Loss (SEL), and PML90 is sometimes used interchangeably with Scenario Upper Loss (SUL).
  • Probability of Exceedance
    In the context of these risk reports, this is the probability that a specified level of damage will be surpassed within the exposure period (related to building life or term of investment), given the site's seismic environment and the property's seismic vulnerability. Using a Poissonian model, the probability of exceedance and exposure period are related to the average return interval of the loss. For example, a loss level that has a 10% chance of exceedance in a 30-year exposure period may be described as having a 285-year average recurrence interval. A loss level that has a 10% chance of exceedance in a 50-year exposure period has a 475-year average recurrence interval.
  • Risk
    The chance or probability that some undesirable outcome, such as injury, damage or loss, will occur during a specified exposure period.
  • Scenario Loss
    A level of building damage from earthquake, expressed as a fraction of the building replacement value, associated with a stated earthquake hazard scenario. In our reports, probabilistic seismic hazards are used, and the stated scenario is based on the level of ground shaking that has a 10% chance of being exceeded in the exposure period specified by the user. Scenario Loss is further specified as the mean loss (Scenario Expected Loss or SEL) or the 90% nonexceedance loss (Scenario Upper Loss or SUL) for the stated hazard.
  • Vulnerability
    The susceptibility of a building, equipment item or component to damage or loss from a specific hazard.
  • Tsunami
    Seismic seawave
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