Directivity Correction for the Next Generation Attenuation (NGA) Relations

Using ground motion residuals of four NGA models, directivity effect and the dependence of directivity on various seismological and geometrical parameters were studied and correction terms for the four models were obtained. Directivity was clearly seen in the ground motion residuals of most earthquakes in the NGA database. Specifically, this phenomenon was found to be significant for large earthquakes, but it was not consistently observed for small events. It was seen to strongly depend on period of ground motion and on source distance, having its largest values in the very near-source region and for long periods. This study further demonstrated that the intensity of directivity depends on the geometry of the fault, orientation of fault surface with respect to the site, and the location of hypocenter. Applying the correction terms to the NGA models resulted in considerable reductions in the uncertainty term sigma, especially at long periods and at short distances.