We have developed a modification to the ground motion model of Abrahamson [1] that takes into account the effects of basin generated surface waves. The main feature of our model is that for response spectral accelerations at periods of 4 and 5 seconds, the Abrahamson [1] model for soil sites should be scaled by a factor of 1.65 in order to represent the ground motions on soil sites located within sedimentary basins. Our finding that no scaling of the Abrahamson [1] model is required for periods shorter than 4 seconds reflects the fact that most of the deep soil recordings on which that model is based come from sedimentary basins. The fact that our first order model does not have a dependence on distance to the basin edge or the depth of the basin makes it easily applicable in ground motion calculations, especially those for probabilistic seismic hazard analyses, which typically involve the calculation of ground motions from many different scenario earthquakes occurring on a variety of faults surrounding the site. We have identified many second order basin effects that are not quantified in our first order model of basin effects.
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