Empirical Terrain-Based Topographic Modification Factors for Use in Ground Motion Prediction

We develop a model to predict topographic effects on ground motions using the NGA-West2 data set. Topography at a site is quantified using three parameters: smoothed curvature, smoothed slope, and relative elevation. We examine the relationship between the site residuals, which represent the average modeling error of the reference ground motion prediction equation at a station, and the three topographic parameters. Our analysis shows that there are statistically significant trends in site residuals with respect to relative elevation and smoothed curvature. We propose modification factors to the Chiou and Youngs (2014) model that account for topographic effects on both the median and the standard deviation. These factors are period dependent and are a function of the relative elevation at the site. The factors result in amplification on median by a factor as high as 1.13 at T = 0.5 s for higher values of relative elevation, and deamplification as low as 0.75 between T = 2 s to 3 s for lower values of relative elevations.