Improved HVSR site classification method for free-field strong motion stations validated with Wenchuan aftershock recordings

Local site conditions play an important role in the effective application of strong motion recordings. In the China National Strong Motion Observation Network System (NSMONS), some of the stations do not provide borehole information, and correspondingly, do not assign the site classes yet. In this paper, site classification methodologies for free-field strong motion stations are reviewed and the limitations and uncertainties of the horizontal-to-vertical spectral ratio (HVSR) methods are discussed. Then, a new method for site classification based on the entropy weight theory is proposed. The proposed method avoids the head or tail joggle phenomenon by providing the objective and subjective weights. The method was applied to aftershock recordings from the 2008 Wenchuan earthquake, and 54 free-field NSMONS stations were selected for site classification and the mean HVSRs were calculated. The results show that the improved HVSR method proposed in this paper has a higher success rate and could be adopted in NSMONS.