Correlations between duration and number of effective cycles of earthquake ground motion

An important aspect to include in comprehensive characterization of earthquake ground motion is the duration of the shaking, which can be represented either by an interval of time during which the motion fulfils some specified criteria or else the number of effective cycles of motion. Several different definitions have been proposed for measuring both of these parameters and analysis of empirical predictive equations and a large strong-motion dataset show that durations calculated using different definitions are generally poorly correlated. Numbers of cycles of motion calculated using different definitions can show much better correlations in some cases. In nearly all cases the correlations between calculated durations and numbers of cycles are very poor, with the exception of uniform duration and number of cycles obtained using rainflow range counting, both using absolute rather than relative thresholds. The scatter in this correlation can be reduced by adding an appropriate measure of the period content of the motion as an explanatory variable.