Seismic-hazard analysis (SHA) is typically performed using ergodic ground-motion models (GMMs), wherein the site response component is derived from global data and conditioned on the time-averaged shear-wave velocity in the upper 30 meters (VS30) and a “basin depth” term (e.g., Z1.0 or Z2.5). In the ergodic GMMs, for a given VS30, there is an implicit shear-wave velocity (VS) profile associated with the site response prediction that has smooth increases of velocity with depth. When a site-specific VS profile is characterized by abrupt velocity contrasts, for example at the rock-soil interface, the site response is likely to differ significantly from ergodic model predictions. This limitation of the ergodic models can be overcome by incorporating non-ergodic site response in the SHA. This approach involves customizing the site response for site-specific conditions, which has the effect of decreasing overall model uncertainty.
In this paper, we describe results from ergodic SHA and SHA that incorporates non-ergodic site response at two sites in the San Francisco Bay Area. Both sites are characterized by a strong impedance contrast at the top of competent bedrock. Depth to bedrock at these sites varies, ranging from 75 meters to more than 400 meters. At each of the sites, nearby ground-motion records indicate that the ergodic GMMs tend to underestimate spectral accelerations at oscillator periods that are close to the fundamental site period. Conversely, there are typically broad period ranges where the ergodic GMMs overestimate spectral acceleration. Since the non-ergodic site response considers these local ground-motion data, these differences are reflected in the non-ergodic results. The findings from these two sites underscore the importance of estimating the fundamental site period, the limitations of ergodic models at sites with strong impedance contrasts, and the benefits of implementing non-ergodic site response into SHA
