New Predictive Models for the V max / alpha max Ratio of Strong GroundMotions using Genetic Programming

Although there is enough knowledge indicating on the influence of frequency content of input motion on thedeformation demand of structures, state-of-the- practice seismic studies use the intensity measures such as peak groundacceleration (PGA) which are not frequency dependent. The V max / alpha max ratio of strong ground motions can be used inseismic hazard studies as the representative of frequency content of the motions. This ratio can be indirectly estimatedby the attenuation models of PGA and PGV which are functions of earthquake magnitude, source to site distance, faulting mechanism, and local site conditions. This paper presents new predictive equations for V max / alpha max ratio based on genetic programming (GP) approach. The predictive equations are established using a reliable database released by Pacific Earthquake Engineering Research Center (PEER) for three types of faulting mechanisms including strikeslip,normal and reverse. The proposed models provide reasonable accuracy to estimate the frequency content of site ground motions in practical projects. The results of parametric study demonstrate that V max / alpha max increases throughincreasing earthquake moment magnitude and source to site distance while it decreases with increasing the averageshear-wave velocity over the top 30m of the site.