Sensitivity of Site Response Analyses to Input Motion Selection: Lessons Learned from Seattle and Boston

Session: Near-Surface Effects: Advances in Site Response Estimation and Its Applications
Type: Oral
Date: 4/28/2020
Time: 11:00 AM
Room: 110 + 140
Description: 

Site response analyses are used to estimate site-specific ground motions as a function of the properties of the soil profile, the assumed constitutive models to represent dynamic soil behavior and the input (bedrock) motion at the base of the soil profile. The objective of this study is to investigate the effects of input motion selection protocols on uncertainties in site response analyses, a topic that has received relatively limited attention to date. We compare current practices for input motion selection, identify shortcomings in these practices and investigate their impact on site-response uncertainty. Three target spectrum definitions for input motion selection are evaluated: the uniform hazard spectrum (UHS), conditional spectrum (CS) conditioned at different spectral periods and risk-targeted maximum considered earthquake (MCE_R) spectrum. We perform analyses at two locations, Seattle, Washington and Boston, Massachusetts, in order to evaluate the influence of different tectonic, geologic and geotechnical conditions. Using the results of probabilistic seismic hazard analyses at each site, we develop suites of input motions using alternative target spectra definitions, perform one-dimensional site response analyses and evaluate the resulting uncertainties in the predicted intensity measures (IMs). When the soil behavior is strongly nonlinear, we find that the influence of input motion selection protocols on site-response uncertainty decreases significantly, and that nonlinear soil behavior can suppress the variability in surface IMs. Analyses using alternative profiles indicate that estimated IMs at stiffer sites are more greatly influenced by input motion selection protocols relative to softer sites. In subduction-zone regions where distinct types of earthquakes contribute to the seismic hazard, we find that the conditional spectrum is advantageous for explicitly incorporating diverse seismic sources and ground-motion databases.

Presenting Author: James Kaklamanos

Authors