Seismic hazard

Seismic hazard is the probability and severity of earthquake-related ground effects at a location over a specified period. It describes the physical shaking and related effects, not the resulting damage or loss.

Probabilistic seismic hazard analysis: A method that estimates the likelihood of specified ground motions at a site by combining earthquake sources, recurrence rates, and ground-motion models. It quantifies how likely different shaking levels are at a location during a chosen time span.
Peak ground acceleration: The largest acceleration recorded at a point during earthquake shaking. It is one common measure used to express the intensity of shaking in hazard maps.
Seismic hazard map: A map showing estimated earthquake ground-motion levels or exceedance probabilities across a region. Maps display how hazard varies geographically and support planning and design.
Earthquake intensity: A measure of the effects and shaking experienced at a particular place during an earthquake. Intensity describes an event's realized local effects; hazard concerns probabilities across possible events.
Ground-motion prediction equation: An empirical or theoretical model that estimates earthquake ground motion from source, path, and site characteristics. These equations translate possible earthquakes into estimates of shaking at the site.
Spectral acceleration: A measure of earthquake motion representing the maximum response of an idealized oscillator at a specified period and damping. It captures shaking demands across structural periods more directly than a single peak value.
Seismic building code: A set of requirements intended to make buildings perform acceptably under earthquake effects. Codes use hazard estimates to set design shaking levels for structures.
Earthquake magnitude: A logarithmic measure of an earthquake's size based on seismic waves or fault rupture. Magnitude characterizes an earthquake source, not the probability or ground motion at a specific site.
Earthquake recurrence interval: The average time between earthquakes of a specified size on a fault or within a region. Recurrence estimates help assign probabilities to future earthquakes in hazard calculations.
Return period: The long-term average interval between events that meet or exceed a specified level, under a stated probability model. Hazard levels are often summarized by their annual exceedance probability or equivalent return period.

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