Adrian Rodriguez-Marek

An Empirical Geotechnical Seismic Site Response Procedure

Abstract A simplified empirically based seismic site response evaluation procedure that includes measures of the dynamic stiffness of the surficial materials and the depth to bedrock as primary parameters is introduced. This geotechnical site classification scheme provides an alternative to geologic‐based and shear wave velocity‐based site classification schemes. The proposed scheme is used to analyze the ground motion data from the 1989 Loma Prieta and 1994 Northridge earthquakes. Period‐dependent and intensity‐dependent spectral acceleration amplification factors for different site conditions are presented. The proposed scheme results in a significant reduction in standard error when compared with a simpler “rock vs. soil” classification system. Moreover, results show that sites previously grouped as “rock” should be subdivided as competent rock sites and weathered soft rock/shallow stiff soil sites to reduce uncertainty in defining site‐dependent ground motions. Results also show that soil depth is an ...

Analysis of Single-Station Standard Deviation Using the KiK-net Data

Estimates of single-station standard deviation can be used as a lower bound to probabilistic seismic hazard analyses that remove the ergodic assumption on site response. This paper presents estimates of single-station standard deviation using data from the KiK-net network. The KiK-net network has a dense array of stations that recorded a large number of earthquakes over the period of study, both at the surface and at colocated borehole instruments. The large number of records implies that there are a large number of stations with recordings from multiple events; hence, site terms and single-station standard deviations can be properly estimated. Borehole instruments permit a breakdown of residuals, considering the effect of amplification in the shallow surface layers. Random-effects regression was first used to develop a ground-motion prediction equation (GMPE) using both the surface and borehole data. The GMPE was constrained such that event terms were the same at the surface and borehole. Residuals were then computed and the within-event (intraevent) residuals were separated into a repeatable site-term and a remaining residual, for both the ground motion itself and for the empirical amplification factor between surface and borehole. Results show that single-station standard deviations are considerably lower than standard deviations using the ergodic assumption, and these standard deviations are further reduced if only a small bracket of station-to-event azimuths is considered for each station such that path variability is minimized. Moreover, analyses of residuals indicate that most of the differences between ergodic standard deviations of surface and borehole data are the results of a poor parametrization of shallow site effects. However, the contribution of site-to-site variability in the empirical amplification factor is only limited. Finally, a comparison with results from other studies at different tectonic regions indicates that the values of single-station standard deviations are strikingly similar for all studies.

A Model for Single‐Station Standard Deviation Using Data from Various Tectonic Regions

Correctly accounting for the uncertainty in ground‐motion prediction is a critical component of probabilistic seismic‐hazard analysis (PSHA). This prediction is commonly achieved using empirical ground‐motion prediction equations. The differences between the observed and predicted ground‐motion parameters are generally assumed to follow a normal distribution with a mean of zero and a standard deviation sigma. Recent work has focused on the development of partially nonergodic PSHA, where the repeatable effects of site response on ground‐motion parameters are removed from their total standard deviation. The resulting value is known as single‐station standard deviation or single‐station sigma. If event‐to‐event variability is also removed from the single‐station standard deviation, the resulting value is referred to as the event‐corrected single‐station standard deviation (![Graphic][1] ). In this work, a large database of ground motions from multiple regions is used to obtain global estimates of these parameters. Results show that the event‐corrected single‐station standard deviation is remarkably stable across tectonic regions. Various models for this parameter are proposed accounting for potential magnitude and distance dependencies. The article also discusses requirements for using single‐station standard deviation in a PSHA. These include the need for an independent estimate of the site term (e.g., the repeatable component of the ground‐motion residual at a given station) and properly accounting for the epistemic uncertainty in both the site term and the site‐specific single‐station standard deviation. Values for the epistemic uncertainty on ![Graphic][2] are proposed based on the station‐to‐station variability of this parameter. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif

Application of Single-Station Sigma and Site-Response Characterization in a Probabilistic Seismic-Hazard Analysis for a New Nuclear Site

Abstract Aleatory variability in ground‐motion prediction, represented by the standard deviation (sigma) of a ground‐motion prediction equation, exerts a very strong influence on the results of probabilistic seismic‐hazard analysis (PSHA). This is especially so at the low annual exceedance frequencies considered for nuclear facilities; in these cases, even small reductions in sigma can have a marked effect on the hazard estimates. Proper separation and quantification of aleatory variability and epistemic uncertainty can lead to defensible reductions in sigma. One such approach is the single‐station sigma concept, which removes that part of sigma corresponding to repeatable site‐specific effects. However, the site‐to‐site component must then be constrained by site‐specific measurements or else modeled as epistemic uncertainty and incorporated into the modeling of site effects. The practical application of the single‐station sigma concept, including the characterization of the dynamic properties of the site and the incorporation of site‐response effects into the hazard calculations, is illustrated for a PSHA conducted at a rock site under consideration for the potential construction of a nuclear power plant.

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.

Framework for a Ground-Motion Model for Induced Seismic Hazard and Risk Analysis in the Groningen Gas Field, The Netherlands

The potential for building damage and personal injury due to induced earthquakes in the Groningen gas field is being modeled in order to inform risk management decisions. To facilitate the quantitative estimation of the induced seismic hazard and risk, a ground motion prediction model has been developed for response spectral accelerations and duration due to these earthquakes that originate within the reservoir at 3 km depth. The model is consistent with the motions recorded from small-magnitude events and captures the epistemic uncertainty associated with extrapolation to larger magnitudes. In order to reflect the conditions in the field, the model first predicts accelerations at a rock horizon some 800 m below the surface and then convolves these motions with frequency-dependent nonlinear amplification factors assigned to zones across the study area. The variability of the ground motions is modeled in all of its constituent parts at the rock and surface levels.

Erratum to: Site effect assessment using KiK-net data: Part 1. A simple correction procedure for surface/downhole spectral ratios

When data is available, the estimation of site effects is usually performed using the “standard spectral ratio” (SSR) technique with respect to an outcropping, reference rock site. This study uses the Japanese KiK-net network, which has more than 600 pairs of surface-downhole stations allowing the computation of empirical borehole transfer functions, consisting of mean spectral ratios of surface over downhole recordings. The borehole transfer function deviates from the SSR in two respects: the reference is located at depth, and the downhole velocity varies from one site to another. These differences bias the estimation of the transfer function with reference to a standard outcrop rock site. The goal of this paper is to develop a simple and robust methodology to correct for such bias. The proposed correction procedure consists of two steps: a depth correction designed to account, in a simplified and physically acceptable way, for the existence at depth of destructive interferences and the absence of free-surface effects in the high-frequency range; and an impedance correction designed to normalize the shear wave velocity at depth. The depth correction involves a simple, frequency-dependent curve to be adapted for each site as a function of the first destructive interference frequency at depth. The impedance normalization combines the use of “generic” rock velocity profiles and a quarter-wavelength approach, resulting in a smooth frequency-dependent amplitude correction. The proposed methodology is applied on a large subset of KiK-net data in view of analysing the correlation between site amplification factors and site parameters in a companion paper.

Representation of near-fault pulse-type ground motions

Near-fault ground motions with long-period pulses have been identified as critical in the design of structures. To aid in the representation of this special type of motion, eight simple pulses that characterize the effects of either the fling-step or forward-directivity are considered. Relationships between pulse amplitudes and velocity pulse period for different pulses are discussed. Representative ratios and peak acceleration amplification can exhibit distinctive features depending on variations in pulse duration, amplitude and the selected acceleration pulse shape. Additionally, response spectral characteristics for the equivalent pulses are identified and compared in terms of fixed PGA and PGV, respectively. Response spectra are strongly affected by the duration of pulses and the shape of the basic pulses. Finally, dynamic time history response features of a damped SDOF system subjected to pulse excitations are examined. These special aspects of pulse waveforms and their response spectra should be taken into account in the estimation of ground motions for a project site close to a fault.

Sliding Displacement of Flexible Earth Slopes Subject to Near-Fault Ground Motions

AbstractA fully coupled simplified method that incorporates soil nonlinearity is used to conduct sliding-block analysis of slopes subjected to near-fault pulse-like and nonpulse-like ground motions. The effects of the ground motion pulse on the computed sliding displacements are investigated, and the efficiency of various ground motion intensity measures for predicting the sliding displacement of slopes is evaluated. It is shown that the slope is expected to have larger displacements over shorter time intervals when near-fault ground motions have pulse-like characteristics. Results also indicate that for cases in which the natural period of a slope is close to the period of the pulse of a recorded ground motion, an equivalent wavelet pulse appropriately represents the displacement response of slopes. Predictive models are developed for the sliding displacement of near-fault ground motions using spectral acceleration and peak ground velocity as predictive variables. In addition, it is shown that for certai...

Scenario dependence of linear site effect factors for short-period response spectral ordinates

Ground-motion models for response spectral ordinates commonly partition site-response effects into linear and nonlinear components. The nonlinear components depend upon the earthquake scenario being considered implicitly through the use of the expected level of excitation at some reference horizon. The linear components are always assumed to be independent of the earthquake scenario. This article presents empirical and numerical evidence as well as a theoretical explanation for why the linear component of site response depends upon the magnitude and distance of the earthquake scenario. Although the impact is most pronounced for small-magnitude scenarios, the finding has significant implications for a number of applications of more general interest including the development of site-response terms within ground-motion models, the estimation of ground-motion variability components φS2S and φSS, the construction of partially nonergodic models for site-specific hazard assessments, and the validity of the convolution approach for computing surface hazard curves from those at a reference horizon, among others. All of these implications are discussed in the present article.