Challenges in developing vertical hazard for seismic analysis of concrete dams

The seismic response of concrete dams depends on both the horizontal (H) and vertical (V) components of ground motion (GM), and excluding the V components when conducting response history analyses (RHAs) may underestimate the seismic fragility. Although V components of GM time series could be selected to be consistent with the hazard, hazard curves for the V component (or for short, V hazard curves) are rarely available for evaluating such hazard consistency. One of the challenges in developing V hazard curves in the U.S. Geological Survey (USGS) National Seismic Hazard Model (NSHM) is the lack of ground motion models (GMMs) that directly estimate intensity measures (IMs) for the V component of GM. In this study, we compare alternative approaches of developing V hazard curves. Using a case study in California, we compare V hazard curves computed from GMMs for the V component of GM against those from applying GMMs for the V/H ratios and those from selecting multicomponent time series that are consistent with the H hazard. We discuss and invite feedback on issues such as availability of GMMs, applicability of GMMs, and choice of inputs for ground motion selection and modification (GMSM).