Acceleration
A change in velocity with time; in seismology and in earthquake engineering, it is expressed as a fraction of gravity (g), with reference to vibrations of the ground or of a structure.

Accelerograph
Instrument for recording acceleration.

Acceptable risk
A risk which everyone impacted is prepared to accept. Action to further reduce such risk is usually not required unless reasonably practicable measures are available at low cost in terms of money, time and effort.

Acid rain
Rain containing dissolved acidic compounds, resulting from chemical pollution of the atmosphere by sulphur and nitrogen compounds. When deposited these increase the acidity of the soil and water causing agricultural and ecological damage.

Aftershock
A smaller earthquake that follows the main shock and originates close to its focus. Aftershocks generally decrease in number and magnitude over time.

Alarm
A visible or audible signal giving warning of danger.

ALARP (As Low As Reasonably Practicable) principle :
That principle which states that risks, lower than the limit of tolerability, are tolerable only if risk reduction is impracticable or if its cost is grossly in disproportion (depending on the level of risk) to the improvement gained.

Alert
Advisory that hazard is approaching and that places emergency forces on standby, pending mobilization, but is less imminent than implied by warning message (see warning).

All-hazards planning
Most hazardous places are threatened by more than one set of hazards and associated risks. Besides the presence of several classes of extreme phenomena, there may be secondary hazards (e.g. earthquake-induced landslides). It is often more efficient to plan for all the major hazards to be expected in a region rather than separately for single ones. This allows economies of scale to be achieved and risks to be tackled comprehensively.

Amplitude
The difference between zero level and peak of any wave such as a seismic wave.

Anemometer
Instrument which measures wind speed or wind speed and direction. (H)

Annual exceedance probability (AEP)
The estimated probability that an event of specified magnitude will be exceeded in any year.

Annual flood
Highest peak discharge in a year.

Antecedent precipitation index (see also antecedent soil moisture)
Weighted summation of past daily precipitation amounts, used as an index of soil moisture.

Anticyclone (area of high pressure, high)
A region where barometric pressure is high or relative to that in the surrounding regions at the same level.

Areal precipitation
The average amount of precipitation which has fallen over a specific area.

Arid zone
An area in which the water resources from ground water and rainfall are insufficient to counterbalance the evaporation.

Aseismic
Nonseismic; used to designate an area free from seismic activity or a tectonic deformation process not accompanied by seismic events.

Ash flow
Pyroclastic flow including a liquid phase and a solid phase composed mainly of ashes.

Assessment
The process of determining the impact of a disaster or events on a society, the needs for immediate, emergency measures to save and sustain the lives of survivors, and the possibilities for expediting recovery and development. Assessment is an interdisciplinary process undertaken in phases and involving on-the-spot surveys and the collation, evaluation and interpretation of information from various sources concerning both direct and indirect losses, short- and long-term effects. It involves determining not only what has happened and what assistance might be needed, but also defining objectives and how relevant assistance can actually be provided to the victims. It requires attention to both short-term needs and long-term implications.

Atmospheric pollution
Contamination of the atmosphere by large quantities of gases, solids and radiation produced by the burning of natural and artificial fuels, chemicals and other industrial processes and nuclear explosions.

Avalanche
The rapid and sudden sliding and flowage of masses of usually incoherent and unsorted mixtures of snow/ice/rock material. (OFDA)

BACK TO GLOSSARY