UCLA Civil and Environmental Engineering

In engineering practice, it is common for the ground motion intensity measures used in design to be estimated using a combination of probabilistic and deterministic procedures. Formal probabilistic seismic hazard analyses are performed to estimate intensity measures (IMs) for rock site conditions. This is followed by a deterministic modification of the rock IMs to account for site effects, which is typically done using prescribed site factors available in the literature or in seismic code provisions. In this article we investigate the extent to which ground motions estimated using this semi-probabilistic approach approximate ground motions evaluated in a fully probabilistic context in which the nonlinear site response is integrated into the hazard calculations. Using two existing California sites as examples, we demonstrate that the deterministic application of nonlinear site factors underestimates the ground motions evaluated using a formal probabilistic approach. This misfit arises from multiple sources including different standard deviation terms for rock and soil sites and different controlling earthquakes. In particular, sites having a significant nonlinear site response tend to attract larger contributions from distant earthquakes than do rock sites. Fortunately, a new set of ground motion prediction equations developed through the Next Generation Attenuation project directly incorporate nonlinear site response effects, and hence as those models are integrated into hazard codes, there will be no need to continue in practice the semi-probabilistic approach for ground motion estimation.