Challenges of Building Logic Trees for Probabilistic Seismic Hazard Analysis

2012 ◽  
Vol 28 (4) ◽  
pp. 1723-1735 ◽  
Author(s):  
Julian J. Bommer
Keyword(s):  
Seismic Hazard ◽  
Epistemic Uncertainty ◽  
Hazard Analysis ◽  
Continuous Distribution ◽  
Probabilistic Seismic Hazard Analysis ◽  
Seismic Hazard Analysis ◽  
Potential Range ◽  
Probabilistic Seismic Hazard ◽  
Logic Tree ◽  
Parameter Values

In the current practice of probabilistic seismic hazard analysis (PSHA), logic trees are widely used to represent and capture epistemic uncertainty in each element of the models for seismic sources and ground-motion prediction. Construction of a logic tree involves populating the branches with alternative models or parameter values, and then assigning weights, which together must represent the underlying continuous distribution. The logic tree must capture both the best estimates of what is known and the potential range of alternatives in light of what is currently not known. There are several scientific challenges involved in both populating the logic tree branches (for which new models often need to be developed) and in assigning weights to these branches. The most serious challenge facing this field now, however, may be a shortage of suitably qualified and experienced experts.

A SSHAC Level 3 Probabilistic Seismic Hazard Analysis for a New-Build Nuclear Site in South Africa

2015 ◽  
Vol 31 (2) ◽  
pp. 661-698 ◽  
Author(s):  
Julian J. Bommer ◽  
Kevin J. Coppersmith ◽  
Ryan T. Coppersmith ◽  
Kathryn L. Hanson ◽  
Azangi Mangongolo ◽  
...  
Keyword(s):  
South Africa ◽  
Seismic Hazard ◽  
Epistemic Uncertainty ◽  
Hazard Analysis ◽  
Probabilistic Seismic Hazard Analysis ◽  
Seismic Hazard Analysis ◽  
Probabilistic Seismic Hazard ◽  
Recurrence Rates ◽  
Moderate Seismicity ◽  
Level 3

A probabilistic seismic hazard analysis has been conducted for a potential nuclear power plant site on the coast of South Africa, a country of low-to-moderate seismicity. The hazard study was conducted as a SSHAC Level 3 process, the first application of this approach outside North America. Extensive geological investigations identified five fault sources with a non-zero probability of being seismogenic. Five area sources were defined for distributed seismicity, the least active being the host zone for which the low recurrence rates for earthquakes were substantiated through investigations of historical seismicity. Empirical ground-motion prediction equations were adjusted to a horizon within the bedrock at the site using kappa values inferred from weak-motion analyses. These adjusted models were then scaled to create new equations capturing the range of epistemic uncertainty in this region with no strong motion recordings. Surface motions were obtained by convolving the bedrock motions with site amplification functions calculated using measured shear-wave velocity profiles.

On Uncertainties in Probabilistic Seismic Hazard Analysis

2016 ◽  
Vol 32 (3) ◽  
pp. 1405-1418 ◽  
Author(s):  
Mario Ordaz ◽  
Danny Arroyo
Keyword(s):  
Seismic Hazard ◽  
Poisson Process ◽  
Epistemic Uncertainty ◽  
Hazard Analysis ◽  
Probabilistic Seismic Hazard Analysis ◽  
Poisson Processes ◽  
Seismic Hazard Analysis ◽  
Probabilistic Seismic Hazard ◽  
Aleatory Uncertainty ◽  
Time Frames

Probabilistic seismic hazard analysis (PSHA) is, in essence, a method to deal with uncertainty, the importance of which justifies the use of a formal and rigorous background for its study. Therefore, the purpose of this paper is to contribute to the reflections on how to correctly handle uncertainty in PSHA. We start by studying the simplest case, a Poisson process in which only “aleatory” uncertainty exists; then, we remove the Poisson hypothesis and find expressions for the occurrence probabilities of earthquakes in given time frames for general non-Poisson processes. Later, we include a simple variety of epistemic uncertainty and show that the resulting process is not Poissonian anymore, so computation of probabilities has to be made taking into account this fact. Next, we give a rigorous rule to combine uncertainties of aleatory and epistemic origin, which gives reasonable criteria to decide whether the epistemic uncertainty is large or not. Also, we propose unambiguous guidelines to decide whether a particular class of uncertainty has to be included in the hazard calculations as epistemic or as aleatory. Finally, we discuss the problem of how our estimates could differ if we wrongly considered that our epistemic uncertainty is of aleatory nature, or vice versa.

Accounting for Epistemic Uncertainty in Site Effects in Probabilistic Seismic Hazard Analysis

2021 ◽  
Author(s):  
Kristin J. Ulmer ◽  
Adrian Rodriguez-Marek ◽  
Russell A. Green
Keyword(s):  
Seismic Hazard ◽  
Site Effects ◽  
Epistemic Uncertainty ◽  
Hazard Analysis ◽  
Weighted Average ◽  
Probabilistic Seismic Hazard Analysis ◽  
Seismic Hazard Analysis ◽  
Probabilistic Seismic Hazard ◽  
Amplification Factors ◽  
Amplification Curve

ABSTRACT A probabilistic seismic hazard analysis performed for rock conditions and modified for soil conditions using deterministic site amplification factors does not account for uncertainty in site effects, which can be significant. One approach to account for such uncertainty is to compute a weighted average amplification curve using a logic tree that accounts for several possible scenarios with assigned weights corresponding to their relative likelihood or confidence. However, this approach can lead to statistical smoothing of the amplification curve and possibly to decreased computed hazard as epistemic uncertainty increases. This is against the expected trend that higher uncertainty leads to higher computed hazard, thus reducing the incentive for practitioners to characterize soil properties at a site. This study proposes a modified approach in which the epistemic uncertainty is captured in a plot of amplification factors versus period. Using a case history, the proposed method is shown to improve the issue with the weighted average method for at least two oscillator periods and to yield similar results for two other periods in which the highlighted issue is less significant.

Propagation of epistemic uncertainty in magnitude–frequency relations through PSHA using predictive distributions

2021 ◽  
pp. 875529302110552
Author(s):  
Mario Ordaz ◽  
Danny Arroyo
Keyword(s):  
Seismic Hazard ◽  
Current Practice ◽  
Epistemic Uncertainty ◽  
Hazard Analysis ◽  
Seismic Hazard Analysis ◽  
Probabilistic Seismic Hazard ◽  
Logic Tree ◽  
Predictive Distributions ◽  
Tree Approach ◽  
Different Parts

The current practice of Probabilistic Seismic Hazard Analysis (PSHA) considers the inclusion of epistemic uncertainties involved in different parts of the analysis via the logic-tree approach. Given the complexity of modern PSHA models, numerous branches are needed, which in some cases leads to concerns regarding performance issues. We introduce the use of a magnitude exceedance rate which, following Bayesian conventions, we call predictive exceedance rate. This rate is the original Gutenberg–Richter relation after having included the effect of the epistemic uncertainty in parameter β. The predictive exceedance rate was first proposed by Campbell but to our best knowledge is seldom used in current PSHA. We show that the predictive exceedance rate is as accurate as the typical logic-tree approach but allows for much faster computations, a very useful property given the complexity of some modern PSHA models.

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