The average time interval between occurrences of a geologic event, such as a flood or earthquake, of a given or greater magnitude.
Determined from historical records. The average length of time between two events (rain, flooding) of the same size or larger. Recurrence intervals are associated with a probability. (For example, a 25-year flood would have a 4% probability of happening in any given year.)
Average period of time between two episodic events such as earthquakes, floods or volcanic eruptions.