Empirical Relationships for Frequency Content Parameters of Earthquake Ground Motions

The frequency content of an earthquake ground motion is important because it affects the dynamic response of earth and structural systems. Four scalar parameters that characterize the frequency content of strong ground motions are (1) the mean period (T_m), (2) the average spectral period (T_avg), (3) the smoothed spectral predominant period (T_o), and (4) the predominant spectral period (T_p). T_m and T_avg distinguish the low frequency content of ground motions, while T_o is affected most by the high frequency content. T_p does not adequately describe the frequency content of a strong ground motion and is not recommended. Empirical relationships are developed that predict three parameters (T_m, T_avg, and T_o) as a function of earthquake magnitude, site-to-source distance, site conditions, and rupture directivity. The relationships are developed from a large strong-motion database that includes recorded motions from the recent earthquakes in Turkey and Taiwan. The new relationships update those previously developed by the authors and others. The results indicate that three site classes, which distinguish between rock, shallow soil, and deep soil, provide a better prediction of the frequency content parameters and smaller standard error terms than conventional “rock” and “soil” site classes. Forward directivity significantly increases the frequency content parameters, particularly T_m and T_o, at distances less than 20 km. Each of the frequency content parameters can be predicted with reasonable accuracy, but T_m is the preferred because it best distinguishes the frequency content of strong ground motions.