Capturing epistemic uncertainty in site response

Abstract

The incorporation of local amplification factors (AFs) determined through site response analyses has become standard practice in site-specific probabilistic seismic hazard analysis (PSHA). Another indispensable feature of the current state of practice in site-specific PSHA is the identification and quantification of all epistemic uncertainties that influence the final hazard estimates. Consequently, logic trees are constructed not only for seismic source characteristics and ground-motion models (GMMs) but also for the site AFs, the latter generally characterized by branches for alternative shear-wave velocity ( VS) profiles. However, in the same way that branch weights on alternative GMMs can give rise to unintentionally narrow distributions of predicted ground-motion amplitudes, the distribution of AFs obtained from a small number of weighted VS profiles will often be quite narrow at some oscillator frequencies. We propose an alternative approach to capturing epistemic uncertainty in site response in order to avoid such unintentionally constricted distributions of AFs using more complete logic trees for site response analyses. Nodes are included for all the factors that influence the calculated AFs, which may include shallow VS profiles, deeper VS profiles, depth of impedance contrasts, low-strain soil damping, and choice of modulus reduction and damping curves. Site response analyses are then executed for all branch combinations to generate a large number of frequency-dependent AFs. Finally, these are re-sampled as a discrete distribution with enough branches to capture the underlying distribution of AFs. While this approach improves the representation of epistemic uncertainty in the dynamic site response characteristics, modeling uncertainty in the AFs is not automatically captured in this way, for which reason it is also proposed that a minimum level of epistemic uncertainty should be imposed on the final distribution.

Ground-motion models for Vrancea intermediate-depth earthquakes

“…The reduction of the epistemic uncertainty from the total sigma can lead to significant reduction of the residuals computed for strong event scenarios, which in turn leads to a lower hazard level at small probabilities of exceedance (Villani and Abrahamson, 2015; Rodriguez-Marek et al, 2014, 2020).…”

“…It is particularly important to evaluate these repeatable effect components of ground-motion variability at a single site, due to their impact on site-specific hazard assessment studies (e.g. Bommer and Abrahamson, 2006; Rodriguez-Marek et al, 2014, 2020). This study represents a first attempt at incorporating realistic local site parameters in a GMM for intermediate to deep seismicity of the Vrancea region in Romania.…”

How well can we predict earthquake site response so far? Site-specific approaches

“…However, the modeling uncertainty in GRA is not explicitly considered (e.g. Rodriguez-Marek et al, 2021). The underestimation resulting from ignoring the modeling uncertainty may be balanced by the fact that the parametric uncertainty might be overestimated by considering the marginal distributions of parameters (e.g.…”

Reduction of Bias and Uncertainty in Regional Seismic Site Amplification Factors for Seismic Hazard and Risk Analysis

“…The latter corresponds to the uncertainty expected for single events and highlights the possibility of amplifications significantly larger than mean amplifications specified in design codes for specific earthquakes. Figures [13][14][15] show the amplification factors at the periods of 0.2 s, 0.5 s and 1.0 s as a function of Vs30 for RQDs of 1, 3, and 5, and PGA of 0.1 g. Amplifications for PGAs from 0.2 g to 0.5 g are provided in Talukder [47]. The amplification factors for the structural periods of 0.2 s, 0.5 s and 1.0 s increase as a function of RQD, which is consistent with an increase in the impedance contrast [9,11,12,54].…”

“…Site amplifications in Pistrino di Sotto in Italy were investigated by Sextos et al [13] to determine how site conditions, such as ancient riverbeds, are related to higher levels of damage observed during a recent earthquake. Rodriquez-Marek et al [14] used a logic tree approach to evaluate the uncertainties in amplifications. The nodes of the logic trees include shallow V s profiles, deeper V s profiles, depth of impedance contrasts, uncertainty on the depth of the rock surface, and uncertainty on the thickness of the weathered rock zone.…”