SCEC Award Number 19097 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Region-Specific Fourier-Based Site Amplification Modeling
Name Organization
Jeff Bayless AECOM Andreas Skarlatoudis AECOM Paul Somerville AECOM Jonathan Stewart University of California, Los Angeles
Other Participants
SCEC Priorities 4a, 4b, 4c SCEC Groups EEII, GM
Report Due Date 03/15/2020 Date Report Submitted 03/13/2020
Project Abstract
This study addresses the need for improved nonlinear site amplification models on the SCEC BBP. We infer site response in northern and southern California from earthquake recordings and develop appropriate models for frequency-dependent amplification of horizontal-component Fourier amplitude ordinates as a function of site parameter Vs30 and the peak ground acceleration on reference rock. The bulk of this study follows the conceptual framework of Seyhan and Stewart (2014), in which a semi-empirical site amplification model was developed for response spectra (PSA). The model is applicable over the frequency range 0.1-100 Hz and the V_s30 range 180-1500 m/s. The model does not have magnitude or distance dependence.

The nonlinear term decreases the overall amplification for strong shaking levels (quantified through the PGA_r). The FAS data analysis implies slightly weaker V_s30 dependence of the nonlinear reduction in FAS amplification than two other similar models: Seyhan and Stewart (2014) and Hasash et al. (2018). Overall, the linear V_s30 scaling models compare favorably with other empirically-based models (e.g. Bora et al. 2019; Seyhan and Stewart, 2014); especially at low frequencies. The Hashash et al. (2019) model is most different because it features peaks in amplification over distinct frequency bands which correspond to resonance with the modal frequencies of the V_s profile. The data analysis implies considerable regional variations in the linear V_s30 scaling.
Intellectual Merit This research is directly related to the Ground-Motion prediction focus group and to refining physics-based simulation methodologies. There is increasing recognition that simulations can be utilized in engineering applications, but the simulations require validation first. The results of this research can facilitate future validations of the simulations against recorded ground motions by providing a regionally appropriate FAS-based Vs30 scaling relation.
Broader Impacts This project has supported the already strong collaboration of the group of scientists who work on and for the SCEC broadband platform, by contributing to the research goals and interacting on a regular basis with scientists (and engineers.) Possible benefits of the activity to society involve the improvement of earthquake simulations, which will eventually be used in seismic design, particularly for near fault ground motions.
Exemplary Figure Figure 5. Variation of within-event residuals from the rock condition EAS GMPE versus peak ground acceleration on rock, binned by V_s30 (rows) and for five frequencies (columns). The red line is the nonlinear fit to the residuals (Equation 2) and the blue and green dashed lines are the nonlinear models by Hashash et al., (2018) and Seyhan and Stewart (2014). The black diamonds represent the residual binned means with 95% confidence intervals.