Frequency and magnitude of events

Abstract

The frequencies and magnitudes of potentially hazardous geophysical and hydro-meteorological events have an inverse power relationship: the larger and more energetic the event, the rarer it is in time. The relationship is not open-ended but is limited by physical limits dictated by such factors as plate tectonics and climate. The magnitude-frequency relationship can imply that events of intermediate frequency and magnitude may be the most significant in shaping aspects of the earth’s surface. However, rare extreme events can cause permanent changes to fluvial systems and other aspects of the landscape that would not have occurred under the regime of more frequent and less energetic events. The attributes and frequencies of extreme floods are unlikely to be predicted by analysis of more frequent and lower magnitude stream flow data. For natural hazard processes that are influenced by climatic change, the frequency of an event of a given magnitude can vary depending on the length of time and the specific time interval considered with respect to climatic variation.