Probabilistic Seismic Hazard Analysis Models Research Articles

Implementing Non-Poissonian Forecasts of Distributed Seismicity into the 2022 Aotearoa New Zealand National Seismic Hazard Model

ABSTRACT Seismicity usually exhibits a non-Poisson spatiotemporal distribution and could undergo nonstationary processes. However, the Poisson assumption is still deeply rooted in current probabilistic seismic hazard analysis models, especially when input catalogs must be declustered to obtain a Poisson background rate. In addition, nonstationary behavior and scarce earthquake records in regions of low seismicity can bias hazard estimates that use stationary or spatially precise forecasts. In this work, we implement hazard formulations using forecasts that trade-off spatial precision to account for overdispersion and nonstationarity of seismicity in the form of uniform rate zones (URZs), which describe rate variability using non-Poisson probabilistic distributions of earthquake numbers. The impact of these forecasts in the hazard space is investigated by implementing a negative-binomial formulation in the OpenQuake hazard software suite, which is adopted by the 2022 Aotearoa New Zealand National Seismic Hazard Model. For a 10% exceedance probability of peak ground acceleration (PGA) in 50 yr, forecasts that only reduce the spatial precision, that is, stationary Poisson URZ models, cause up to a twofold increase in hazard for low-seismicity regions compared to spatially precise forecasts. Furthermore, the inclusion of non-Poisson temporal processes in URZ models increases the expected PGA by up to three times in low-seismicity regions, whereas the effect on high-seismicity is minimal (∼5%). The hazard estimates presented here highlight the relevance, as well as the feasibility, of incorporating analytical formulations of seismicity that go beyond the inadequate stationary Poisson description of seismicity.

Exceedance of design seismic actions during the 2016-2017 central Italy seismic sequence: sensitivity to two seismic hazard models

In Italy, seismic ground motion intensity for structural design relies on probabilistic seismic hazard analysis (PSHA). Research has shown that, even in the case of a moderate magnitude earthquake, it is expected that ground motion intensities larger than the design counterparts are observed at some sites in the epicentral region. The area where such an exceedance has possibly occurred can be assessed via ShakeMap. In the case of seismic sequences, ShakeMap envelopes allow to identify the fraction of the region of interest subjected to at least one exceedance of design seismic actions. For the 2016-2017 central Italy sequence, ShakeMap envelopes reveal that exceedance of the design intensity enforced by the current building code occurred in an area covering thousands of square kilometers, depending on the spectral and exceedance return period of the design intensity. This study quantifies the exceedance area of ground motion intensity according to the PSHA results based on two alternative PSHA models for Italy. One is that at the basis of the official PSHA for Italy adopted by the current building code, known as MPS04; the other relies on a grid-seismicity source model deriving from a set of source models recently developed for a new PSHA study for the country (MPS19). Looking at ShakeMap envelopes, it is found that the estimated exceedance area is of the same order of magnitude for the two hazard models, at least for the spectral and exceedance return periods considered herein.

Probabilistic seismic hazard analysis in low-seismicity regions: an investigation of sensitivity with a focus on Finland

We investigate probabilistic seismic hazard analysis (PSHA) in low-seismicity regions in which epistemic uncertainties are largely due to the sparsity of data, with a focus on Finland, northern Europe. We investigate the sensitivity of site-specific PSHA outcomes to different choices of basic input parameters, starting from preexisting PSHA models of the nuclear licensees in the country, without producing a final hazard curve. The outcome shows that the parameters and models needed to estimate future seismicity rates from actual observations, in particular the b value, seismicity rates, and the largest possible magnitude, Mmax, as well as the median ground-motion prediction equation, play significant roles. The sensitivity also depends on the spectral frequency; for example, the effect of Mmax is significant especially for a low-frequency hazard at annual frequency of exceedance 10−5 but more moderate for peak ground acceleration. The delineation of seismic source zones (SSZs) remains ambiguous in regions of low seismicity. This, combined with the dominance of the host SSZ and its seismicity parameters, may have a substantial impact on the outcome. Our results are quantitatively applicable to Finland, but may also be of relevance to other low-seismicity regions in Europe and elsewhere. For future work we recommend the exploration of PSHA sensitivity with focus on the host SSZ with its immediate vicinity and the b value around the site of interest.

Residual drift risk of self-centering steel MRFs with novel steel column bases in near-fault regions

This paper evaluates the potential of novel steel column bases to reduce the residual drift risk of steel buildings located at near-fault regions when installed to post-tensioned self-centering moment-resisting frames (SC-MRFs). To this end, a prototype steel building is designed that consists of either conventional moment-resisting frames (MRFs) or SC-MRFs or SC-MRFs equipped with the novel steel column base (SC-MRF-CBs). The MRFs and SC-MRFs are used as benchmark frames. The frames are modelled in OpenSees where material and geometrical non-linearities are considered along with stiffness and strength degradation. A set of 91 near-fault ground motions with different pulse periods is used to perform incremental dynamic analysis (IDA), in which each ground motion is scaled appropriately until different residual storey drift limits are exceeded. The probability of exceedance of these limits is then computed as a function of the ground motion intensity and the period of the velocity pulse. Finally, the results of IDA are combined with probabilistic seismic hazard analysis models that account for near-fault directivity to evaluate and compare the residual drift risk of the frames used in this study. Results show that the predicted residual drift performance of the frames is influenced by the pulse period of the near-fault ground motions. The use of the novel steel column base significantly reduces the residual drift risk of the frames and the SC-MRF-CB exhibits the best residual drift performance. Finally, the paper highlights the effectiveness of combining post-tensioned beam-column connections with the novel steel column base, by showing that the SC-MRF-CB improves the residual drift performance of the MRF and SC-MRF by 80% and 50%, respectively.

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

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.

Long period ground motion simulation and its application to the seismic design of high-rise buildings

Increasing attention is being paid to the adverse effects of long period ground motion (LPGM) on high-rise buildings. However, it is hard to quantitatively define this special ground motion in seismic design, especially in the cases of regions in which sufficient seismic observations have not been made. To this end, this paper proposes an innovative LPGM simulation method. This method not only involves probabilistic seismic hazard analysis (PSHA), but also overcomes the limitations of the original empirical relation-based method. Firstly, a large number of data taken from typical sedimentary basins are utilized to determine the basin amplification factor model, by which an improved response spectra attenuation formula is developed based on the site amplification mechanisms in different periods. Then, based on the statistical regression analysis of the basin correction factors, the modified group delay time empirical formula is obtained. Furthermore, the conventional PSHA model is refined to determine the scenario earthquake. Finally, the design LPGM is generated by the phase difference spectrum method and its impact on high-rise buildings is embodied via a case study. The satisfactory result proves that the proposed method can effectively simulate LPGM and may have great potential in practice.

Probabilistic seismic hazard analysis model for the Philippines

The Philippine archipelago is tectonically complex and seismically hazardous, yet few seismic hazard assessments have provided national coverage. This article presents an updated probabilistic seismic hazard analysis for the nation. Active shallow crustal seismicity is modeled by faults and gridded point sources accounting for spatially variable occurrence rates. Subduction interfaces are modeled with faults of complex geometry. Intraslab seismicity is modeled by ruptures confined to the slab volume. Source geometries and earthquake rates are derived from seismicity catalogs, geophysical data sets, and historic-to-paleoseismic constraints on fault slip rates. The ground motion characterization includes models designed for global use, with partial constraint by residual analysis. Shallow crustal faulting near metropolitan Manila, Davao, and Cebu dominates shaking hazard. In a few places, peak ground acceleration with 10% probability of exceedance in 50 years on rock reaches 1.0 g. The results of this study may have utility for defining the design base shear in the National Structural Code of the Philippines.

Variabilities in probabilistic seismic hazard maps for natural and induced seismicity in the central and eastern United States

Probabilistic seismic hazard analysis (PSHA) characterizes ground-motion hazard from earthquakes. Typically, the time horizon of a PSHA forecast is long, but in response to induced seismicity related to hydrocarbon development, the USGS developed one-year PSHA models. In this paper, we present a display of the variability in USGS hazard curves due to epistemic uncertainty in its informed submodel using a simple bootstrapping approach. We find that variability is highest in low-seismicity areas. On the other hand, areas of high seismic hazard, such as the New Madrid seismic zone or Oklahoma, exhibit relatively lower variability simply because of more available data and a better understanding of the seismicity. Comparing areas of high hazard, New Madrid, which has a history of large naturally occurring earthquakes, has lower forecast variability than Oklahoma, where the hazard is driven mainly by suspected induced earthquakes since 2009. Overall, the mean hazard obtained from bootstrapping is close to the published model, and variability increased in the 2017 one-year model relative to the 2016 model. Comparing the relative variations caused by individual logic-tree branches, we find that the highest hazard variation (as measured by the 95% confidence interval of bootstrapping samples) in the final model is associated with different ground-motion models and maximum magnitudes used in the logic tree, while the variability due to the smoothing distance is minimal. It should be pointed out that this study is not looking at the uncertainty in the hazard in general, but only as it is represented in the USGS one-year models.

Collapse risk and residual drift performance of steel buildings using post-tensioned MRFs and viscous dampers in near-fault regions

The potential of post-tensioned self-centering moment-resisting frames (SC-MRFs) and viscous dampers to reduce the collapse risk and improve the residual drift performance of steel buildings in near-fault regions is evaluated. For this purpose, a prototype steel building is designed using different seismic-resistant frames, i.e.: moment-resisting frames (MRFs); MRFs with viscous dampers; SC-MRFs; and SC-MRFs with viscous dampers. The frames are modeled in OpenSees where material and geometrical nonlinearities are taken into account as well as stiffness and strength deterioration. A database of 91 near-fault, pulse-like ground motions with varying pulse periods is used to conduct incremental dynamic analysis (IDA), in which each ground motion is scaled until collapse occurs. The probability of collapse and the probability of exceeding different residual story drift threshold values are calculated as a function of the ground motion intensity and the period of the velocity pulse. The results of IDA are then combined with probabilistic seismic hazard analysis models that account for near-fault directivity to assess and compare the collapse risk and the residual drift performance of the frames. The paper highlights the benefit of combining the post-tensioning and supplemental viscous damping technologies in the near-source. In particular, the SC-MRF with viscous dampers is found to achieve significant reductions in collapse risk and probability of exceedance of residual story drift threshold values compared to the MRF.

How to eliminate non-damaging earthquakes from the results of a probabilistic seismic hazard analysis (PSHA)—A comprehensive procedure with site-specific application

The results of probabilistic seismic hazard analyses are frequently presented in terms of uniform hazard spectra or hazard curves with spectral accelerations as the output parameter. The calculation process is based on the evaluation of the probability of exceedance of specified acceleration levels without consideration of the damaging effects of the causative earthquakes. The same applies to the empirical attenuation equations for spectral accelerations used in PSHA models. This makes interpreting and using the results in engineering or risk applications difficult. Uniform hazard spectra and the associated hazard curves may contain a significant amount of contributions of weak, low-energy earthquakes not able to damage the seismically designed structures of nuclear power plants. For the development of realistic engineering designs and for realistic seismic probabilistic risk assessments (seismic PRA) it is necessary to remove the contribution of non-damaging earthquakes from the results of a PSHA. A detailed procedure for the elimination of non-damaging earthquakes based on the CAV (Cumulative Absolute Velocity)-filtering approach was developed and applied to the results of the large-scale PEGASOS probabilistic seismic hazard study for the site of the Goesgen nuclear power plant. The procedure considers the full scope of epistemic uncertainty and aleatory variability present in the PEGASOS study. It involves the development of a set of empirical correlations for CAV and the subsequent development of a composite distribution for the probability of exceedance of the damaging threshold of 0.16 gs. Additionally, a method was developed to measure the difference in the damaging effects of earthquakes of different strengths by the ratio of a power function of ARIAS-intensity or, in the ideal case, by the ratio of the square roots of the associated strong motion durations. The procedure was applied for the update of the Goesgen seismic PRA and for the confirmation of a revised safe shutdown earthquake for the Goesgen nuclear power plant. The application of the procedure leads to results which are in reasonable compliance with evaluations based on the macroseismic method using European macroseismic intensities and associated vulnerabilities. The paper is an extended version of the paper #1142 presented at the 19th SMIRT conference in Toronto, 2007.

Comment on "Why Do Modern Probabilistic Seismic-Hazard Analyses Often Lead to Increased Hazard Estimates?" by Julian J. Bommer and Norman A. Abrahamson

This comment discusses some heuristic biases with respect to the correct treatment of uncertainty in probabilistic seismic-hazard analysis (PSHA) observed in the recent article titled “Why Do Modern Probabilistic Seismic-Hazard Analyses Often Lead to Increased Hazard Estimates” by Julian J. Bommer and Norman Abrahamson. I show that the distinction between aleatory variability and epistemic uncertainty in seismic-hazard analysis represents a think model rather than an objective property of earthquake occurrence. It is demonstrated that the separation between epistemic uncertainty and aleatory variability is model dependent. I show that a correct application of this think model does not lead to an increase of hazard estimates, because the total uncertainty to be incorporated into a PSHA model is bounded by the uncertainty observed in the real world. Ground-motion variability cannot be statistically treated as an inherent property of earthquake occurrence. Its statistical measures are model dependent. A refined definition of the terms epistemic uncertainty and aleatory variability is suggested. Furthermore, the paper addresses the observation that the methodology of traditional PSHA may lead to a violation of the energy conservation principle. Finally, a summary of some of the most problematic areas of current PSHA methodology is given.

Development of attenuation relation for the near fault ground motion from the characteristic earthquake

A composite source model has been used to simulate a broadband strong ground motion with an associated fault rupture process. A scenario earthquake fault model has been used to generate 1000 earthquake events with a magnitude of Mw8.0. The simulated results show that, for the characteristic event with a strike-slip faulting, the characteristics of near fault ground motion is strongly dependent on the rupture directivity. If the distance between the sites and fault was given, the ground motion in the forward direction (Site A) is much larger than that in the backward direction (Site C) and that close to the fault (Site B). The SH waves radiated from the fault, which corresponds to the fault-normal component plays a key role in the ground motion amplification. Corresponding to the sites A, B, and C, the statistical analysis shows that the ratio of their aPG is 2.15:1.5:1 and their standard deviations are about 0.12, 0.11, and 0.13, respectively. If these results are applied in the current probabilistic seismic hazard analysis (PSHA), then, for the lower annual frequency of exceedance of peak ground acceleration, the predicted aPG from the hazard curve could reduce by 30% or more compared with the current PSHA model used in the developing of seismic hazard map in the USA. Therefore, with a consideration of near fault ground motion caused by the rupture directivity, the regression model used in the development of the regional attenuation relation should be modified accordingly.