Ground Motion Models

The current state-of-the-art ground motion models for application to the United States is summarized. The recent NGA ground motion models for shallow crustal earthquakes are in much more advanced than the currently available models for subduction earthquakes and for stable continental regions. The models for the subduction and stable continental regions will likely change significantly in the next several years as major studies are being started to improve these models. There is a move to using the larger horizontal component as compared to the average horiontal component in US building codes to define the design ground motion. A key issue for all ground motion models is the standard deviation. In particular, the standard use of the ergodic assumption tends to overestimate the standard deviation by about 40%, but removing the ergodic assumption leads to increased complexity in the models for the median ground motion as increased epistemic uncertainty in the median for a specific source/site combination is needed. In the future, there will be a greater reliance of finite-fault numerical simulations to estimate the ground motion, but there is a large range of results from different models that needs to be resolved before the can be wide-spread application of these approaches.