Site Response of the Vertical Ground-Motion

In this study we explore, for the first time, the correlation of the vertical site amplification with alternative site-characterization proxies. Specifically, we use both S- and P-wave related proxies, to account for the joint contribution of these wave types within the vertical ground motion. We use surface recordings from large ground-motion databases and supplement the typical site metadata with additional metadata, including P-wave velocity profile and ground-water level (GWL). We find that the most effective predictor for vertical site-response is a combination of VS30, as a first-order proxy for the site-stiffness, together with the depth to ground-water level, which is an independent proxy affecting the P-wave velocity profile. When used together, these two parameters reduce the aleatory variability of ground motions by approximately 0.3 ln units. We further find that the correlation with these proxies greatly depends on magnitude and distance, linking back to the relative contribution of wave-types and its relation to the incidence angle.