An empirical model for the mean period (Tm) of ground motions using the NGA-West2 database

Frequency content is an important characteristic of earthquake shakings. The mean period (T-m) of a ground motion has been regarded as the preferred frequency content parameter. In this paper, a new predictive model for T-m is developed based on the horizontal components of ground motions selected from the expanded NGA-West2 database. The new model includes a piece-wise linear moment magnitude (M-w) term with a breaking M-w value as 5 and a cutoff value as 7.3, which is adopted based on the distribution of empirical data as well as physical considerations. A trilinear rupture distance term with breaking points at 100 km and 200 km is also adopted in the model. The average shear wave velocity at top 30 meters (V-s30) and a basin depth parameter (Z(1)) are used to represent the effect of local site conditions on T-m. A forward directivity term that takes effect at rupture distances within 40 km is included in the functional form. Besides, we derive a magnitude-dependent aleatory variability model to capture the varying within-event standard deviations versus M-w. The proposed model can be used to estimate Tm for earthquake scenarios with magnitudes in the range of 3-7.9 and rupture distance up to 300 km.