Ground motion intensity parameters are used to express the relationship between expected structural damage and the seismic forces imposed. The graphical representation of damage probability as a function of ground motion intensity leads to fragility curves that are generally used in loss estimation studies. The most typical parameters used to represent the ground motion intensity are peak ground acceleration, peak ground velocity, spectral acceleration, and spectral displacement. Other parameters obtained from the ground motion trace and response spectra have been recommended in literature, but no consensus on which intensity parameter to use exists because of the various drawbacks of these ground motion intensities. A new spectrum ground motion intensity parameter that relies on the expected elongated period of the structure under seismic forces has been developed. This intensity measure takes into account the approximate yield capacity of the structure and the area between the fundamental and elongated period of the structure under the elastic response spectrum of the given ground motion. The correlation of this intensity measure with the calculated demand parameter, maximum interstory drift in our case, is investigated for a set of 100 ground motion records in order to verify its accuracy. This intensity measure is primarily proposed for the selection of ground motions to be used for the analyses of individual structures that are desired to respond at various levels of nonlinearity. DOI: 10.1061/(ASCE)ST.1943-541X.0000084. (C) 2011 American Society of Civil Engineers.