Rotation-invariant mean duration of strong ground motion

Strong-motion duration is usually computed separately for three components of recorded ground-motion time series. This results in different values of duration for the three components. Furthermore, the computed duration values are dependent on the sensor orientation. Physically, such dependence is not desirable. In this work, computing duration based on resultant recorded motion instead of individual components is proposed. Such a measure of duration is shown to be rotation-invariant and hence independent of the sensor axes. Furthermore, it is demonstrated that the duration of resultant motion represents the mean duration for all possible arbitrary sensor orientations in three-dimensional space. The results indicate that the apparent difference between duration of horizontal and vertical motion reported in the literature is not universal to all definitions of duration. A set of 462 three-component accelerograms from Europe and the Middle East is used to demonstrate and support the presented findings and arguments.