Fleur O. Strasser

Style-of-faulting in ground-motion prediction equations

Equations for the prediction of response spectral ordinates invariably include magnitude, distance and site classification as independent variables. A few equations also include style-of-faulting as a fourth variable, although this has an almost negligible effect on the standard deviation of the equation. Nonetheless, style-of-faulting is a useful parameter to include in ground-motion prediction equations since the rupture mechanism of future earthquakes in a particular seismic source zone can usually be defined with some confidence. Current equations including style-of-faulting use different schemes to classify fault ruptures into various categories, which leads to uncertainty and ambiguity regarding the nature and extent of the effect of focal mechanism on ground motions. European equations for spectral ordinates do not currently include style-of-faulting factors, and seismic hazard assessments in Europe often combine, in logic-tree formulations, these equations with those from western North America that do include style-of-faulting coefficients. In this article, a simple scheme is provided to allow style-of-faulting adjustments to be made for those equations that do not include coefficients for rupture mechanism, so that style-of-faulting can be fully incorporated into the hazard calculations. This also considers the case of normal fault ruptures, not modelled in any of the current Californian equations, but which are the dominant mechanism in many parts of Europe. The scheme is validated by performing new regressions on a widely used European attenuation relationship with additional terms for style-of-faulting.

A Stochastic Earthquake Ground‐Motion Prediction Model for the United Kingdom

Low‐seismicity regions such as the United Kingdom (UK) pose a challenge for seismic hazard analysis in view of the limited amount of locally recorded data available. In particular, ground‐motion prediction is faced with the problem that most of the instrumental observations available have been recorded at large distances from small earthquakes. Direct extrapolation of the results of regression on these data to the range of magnitudes and distances relevant for the seismic hazard analysis of engineered structures generally leads to unsatisfactory predictions. The present study presents a new ground‐motion prediction equation (GMPE) for the UK in terms of peak ground acceleration (PGA), peak ground velocity (PGV), and 5% damped pseudospectral acceleration (PSA), based on the results of numerical simulations using a stochastic point‐source model calibrated with parameters derived from local weak‐motion data. The predictions from this model are compared with those of previous GMPEs based on UK data, other GMPEs derived for stable continental regions (SCRs), as well as recent GMPEs developed for the wider European area.

Review: Strong Ground Motions—Have We Seen the Worst?

Over the history of instrumental strong-motion recording, the largest amplitudes of ground motions recorded to date have had a significant impact on the perception of the largest amplitudes of ground motion considered physically realizable. However, the length of the instrumental recording history is comparatively short, and instrumental recording networks are relatively sparse, which raises the issue of whether the full range of ground motions has been captured in the current global holdings of strong-motion data. Because the answer to this question is quite obviously negative, a more difficult question then arises: How much greater than the largest currently available observation could future ground motions be? The present article explores this issue, drawing on empirical observations, results from numerical simulations, and a statistical exercise involving the sampling of spatially correlated stochastic ground-motion fields.

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

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.

A Comparative Study of European Earthquake Loss Estimation Tools for a Scenario in Istanbul

A damage estimation exercise has been carried out using the building stock inventory and population database of the Istanbul Metropolitan Municipality and selected European earthquake loss estimation packages: KOERILOSS, SELENA, ESCENARIS, SIGE, and DBELA. The input ground-motions, common to all models, correspond to a “credible worst case scenario” involving the rupture of the four segments of the Main Marmara Fault closest to Istanbul in a Mw 7.5 earthquake. The aim of the exercise is to assess the applicability of the selected software packages to earthquake loss estimation in the context of rapid post-earthquake response in European urban centers. The results in terms of predicted building damage and social losses are critically compared amongst each other, as well as with the results of previous scenario-based earthquake loss assessments carried out for the study area. The key methodological aspects and data needs for European rapid post-earthquake loss estimation are thus identified.

Earthquakes from 1820 to 1936 in Grahamstown and surroundings (Eastern Cape Province, South Africa)

The seismicity of Grahamstown, in the Eastern Cape Province of South Africa, for the years between 1820 and 1936 is investigated with recourse to contemporaneous documentary sources, leading to the development of a seismic history incorporating consideration of the broader geo-political context. Individual studies of five regional events that were felt in Grahamstown during that period, and of one that was not, are presented. Each study includes the development of a full set of intensity data points, which are used to determine reappraised epicentral locations and magnitudes, some of which differ significantly from previously listed parameters. The results thus obtained highlight the value of seeking out additional contemporary sources from different language groups when revisiting the source parameters of earthquakes for which no or only very limited instrumental information is available.

Probabilistic seismic hazard assessment for a new-build nuclear power plant site in the UK

A probabilistic seismic hazard analysis (PSHA) has been conducted as part of the Safety Case justification for a new-build nuclear power plant in the UK. The study followed a cost-efficient methodology developed by CH2M and associates for safety-significant infrastructure where high-level regulatory assurance is required. Historical seismicity was re-evaluated from original sources. The seismicity model considered fourteen seismic sources which, when combined, formed six alternative seismic source models. Separate models for the median ground-motion and aleatory variability were considered. The median ground-motion model comprised a suite of ground-motion equations adjusted to the site-specific conditions using VS-kappa factors. A partially non-ergodic sigma model was adopted with separate components for the inter-event variability, and single-station intra-event variability, adjusted by a partially ergodic site-to-site variability term. Site response analysis was performed using equivalent-linear random vibration theory with explicit incorporation of the variability in the ground properties using Monte Carlo simulations. The final PSHA results were obtained by convolution of the hazard at the reference rock horizon with the site amplification factors. The overall epistemic uncertainty captured by the logic tree was assessed and compared against results from earlier PSHA studies for the same site.

Comment on "Influence of Focal Mechanism in Probabilistic Seismic Hazard Analysis" by Vincenzo Convertito and André Herrero

The influence of style-of-faulting on strong ground motions has been the subject of debate for some time. Although some controversy persists, the general consensus is that ground motions produced by reverse faults are higher than those produced by normal faults, whereas motions from strike-slip faults are somewhere in between. In a recent article, Convertito and Herrero (2004) derived a correction factor for focal mechanism to be applied to predictive equations. This issue was previously addressed by Bommer et al. (2003). Although this article is cited by Convertito and Herrero, it seems that its aims and scope were not well understood, and we would therefore like to clarify what the method presented therein entails, especially because we feel that Convertito and Herreros approach of characterizing focal mechanisms based solely on the radiation pattern is difficult to justify...

A streamlined approach for the seismic hazard assessment of a new nuclear power plant in the UK

This article presents a streamlined approach to seismic hazard assessment aimed at providing regulatory assurance, whilst acknowledging commercial and program constraints associated with the development of safety–critical facilities. The approach was developed based on international best practice and followed the spirit of the Senior Seismic Hazard Analysis Committee (SSHAC) Level 2 requirements, while incorporating the key features of the SSHAC Level 3 process aimed at achieving regulatory assurance, but with a more flexible implementation. It has also benefited from experience gained by others regarding the implementation of the SSHAC process in projects in the USA, Switzerland and South Africa. The approach has been successfully applied as part of the Safety Case for the new-build nuclear power plant at Hinkley Point, UK. The proposed approach can be considered as a cost-effective solution for the seismic hazard evaluation of safety-significant facilities where a high level of regulatory assurance is required.

Reply to “Comment on ‘Review: Strong Ground Motions—Have We Seen the Worst?’ by Fleur O. Strasser and Julian J. Bommer” by Heriberta Castaños and Cinna Lomnitz

We thank Heriberta Castanos and Cinna Lomnitz for their interest in our article and for their comment (Castanos and Lomnitz, 2010), although we cannot help but feel that the comment by these authors is not really about the topic of our article. Rather their comment mostly reflects their displeasure with standard probabilistic seismic hazard analysis (PSHA) in general, already amply documented elsewhere (Castanos and Lomnitz, 2002; Lomnitz, 2005). In our response, we focus primarily on the issue discussed in Strasser and Bommer (2009), namely the question of what inferences can be drawn from current strong-motion data holdings regarding maximum attainable amplitudes of strong ground motion parameters. However, we also feel obliged to respond to some of the arguments made against the use of PSHA. The comparison between motions recorded at one accelerograph station and flipping a coin is a misleading and unhelpful analogy, because the latter is a stochastic process with a finite number of outcomes (there are only two possible results), the possible values of which are moreover known in advance (the outcome has to be either heads or tails). The number of observations obtained from flipping the coin repeatedly very quickly exceeds the number of possible outcomes by orders of magnitude; hence, the law of large numbers can be invoked to infer a mean probability. Ground accelerations, on the other hand, follow a continuous distribution, the exact shape of which has not yet been determined. In particular, the upper limits of this distribution are not yet known, and hence can only be postulated in advance. Observations corresponding to the upper tail of this distribution …