Duration is the length of time over which significant ground shaking occurs during an earthquake. It is a critical intensity measure because cyclic loading, energy accumulation, and fatigue-related damage all depend on how long strong motion persists, not just on peak amplitude.
Duration refers to the time span over which significant ground shaking occurs during an earthquake. Unlike peak measures such as PGA or PGV, which describe a single instant of maximum intensity, duration characterizes the overall temporal extent of the motion. It is a critical parameter for phenomena that depend on accumulated damage, including liquefaction, cyclic softening, low-cycle fatigue, and progressive structural degradation.
Several definitions of duration exist, each suited to different applications. Bracketed duration is the time between the first and last exceedances of a specified acceleration threshold (commonly 0.05g). Significant duration is the time interval over which a specified fraction (typically 5% to 95%) of the total Arias intensity accumulates. Effective duration and uniform duration are related variants used in specific contexts. The choice of definition affects the numerical value of duration significantly β a single ground motion can have a bracketed duration twice its significant duration. Modern ground motion databases typically report multiple duration measures for each record.
Duration is particularly important for long-duration events. Subduction zone earthquakes β such as the 2011 Tohoku earthquake β can produce strong shaking lasting 2 to 3 minutes, far longer than typical crustal events. For structures that degrade under cyclic loading, the number of significant cycles matters more than the peak amplitude. This is why modern ground motion selection procedures often match not just the target spectrum but also the duration and Arias intensity of the design scenario. In performance-based earthquake engineering, duration-sensitive intensity measures β such as Arias intensity or cumulative absolute velocity (CAV) β are increasingly preferred over peak-only measures for structures whose damage is dominated by cyclic degradation.