Julian J. Bommer

AN IMPROVED METHOD OF MATCHING RESPONSE SPECTRA OF RECORDED EARTHQUAKE GROUND MOTION USING WAVELETS

Dynamic nonlinear analysis of structures requires the seismic input to be defined in the form of acceleration time-series, and these will generally be required to be compatible with the elastic response spectra representing the design seismic actions at the site. The advantages of using real accelerograms matched to the target response spectrum using wavelets for this purpose are discussed. The program RspMatch, which performs spectral matching using wavelets, is modified using new wavelets that obviate the need to subsequently apply a baseline correction. The new version of the program, RspMatch2005, enables the accelerograms to be matched to the pseudo-acceleration or displacement spectral ordinates as well as the spectrum of absolute acceleration, and additionally allows the matching to be performed simultaneously to a given spectrum at several damping ratios.

THE USE OF REAL EARTHQUAKE ACCELEROGRAMS AS INPUT TO DYNAMIC ANALYSIS

The increasing availability of strong-motion accelerograms, and the relative ease with which they can be obtained compared to synthetic or artificial records, makes the use of real records an ever more attractive option for defining the input to dynamic analyses in geotechnical and structural engineering. Guidelines on procedures for the selection of appropriate suites of acceleration time-series for this purpose are lacking, and seismic design codes are particularly poor in this respect. Criteria for selecting records in terms of earthquake scenarios and in terms of response spectral ordinates are presented, together with options and criteria for adjusting the selected accelerograms to match the elastic design spectrum. The application of both geophysical and response spectral search criteria is illustrated using compatible scenarios, and the selected records are analysed and adjusted to produce suites of acceleration time-series suitable for dynamic analyses. The paper concludes with suggestions for making use of real records in engineering analysis and design, and recommendations are given for improving the current guidelines provided in seismic design codes.

Earthquake-induced landslides: 1980–1997

Abstract A database of earthquake-induced landslides has been compiled which extends the work of Keefer (Keefer DK. Landslides caused by earthquakes. Bulletin of the Geological Society of America 1984;95:406–421) who covered the period 1811–1980 to 1997. A total of 36 earthquakes world-wide are included, the new database having about the same number of earthquakes as reported by Keefer. Correlations evolving from the new database are compared with those of Keefer. Generally the results are very similar, though the presence of extreme outliers in some of the correlations emphasises the need to be aware of special cases, particularly those involving quick clay landslides. Seismological features, including multiple earthquakes and simultaneous arrival of different phases of seismic waves, also influence the outliers. The correlations between earthquake magnitude and total landslide area, however, differ somewhat from Keefers. For the intermediate magnitude range 5.3–7.0, a modified correlation is suggested. The scatter of the data from which the correlations are derived is greater than found by Keefer. This is ascribed to the different geographic locations of the earthquakes in the two data sets.

Why Do Modern Probabilistic Seismic-Hazard Analyses Often Lead to Increased Hazard Estimates?

The basic elements of probabilistic seismic-hazard analysis (psha) were established almost four decades ago and psha has now become the most widely used approach for estimating seismic-design loads. Although the use of psha is widespread, considerable confusion remains regarding the details of how the calculations should be performed. This situation is largely a result of the way the discipline of psha evolved through a series of articles, reports, and software packages. This article demonstrates that the feature of psha about which there is perhaps the greatest degree of misunderstanding is the treatment of the aleatory variability in ground- motion prediction equations, which exerts a very pronounced influence on the calculated hazard. Probabilistic hazard studies performed in recent years have frequently resulted in appreciably higher design ground motions than had been obtained in previous assessments carried out in the 1970s and 1980s, often sparking controversial debate. Although several factors may contribute to the higher estimates of seismic hazard in modern studies, the main reason for these increases is that in the earlier studies the ground-motion variability was either completely neglected or treated in a way that artificially reduced its influence on the hazard estimates.

On the Use of Logic Trees for Ground-Motion Prediction Equations in Seismic-Hazard Analysis

Logic trees are widely used in probabilistic seismic hazard analysis as a tool to capture the epistemic uncertainty associated with the seismogenic sources and the ground-motion prediction models used in estimating the hazard. Combining two or more ground-motion relations within a logic tree will generally require several conversions to be made, because there are several definitions available for both the predicted ground-motion parameters and the explanatory parameters within the predictive ground-motion relations. Procedures for making conversions for each of these factors are presented, using a suite of predictive equations in current use for illustration. The sensitivity of the resulting ground-motion models to these conversions is shown to be pronounced for some of the parameters, especially the measure of source-to-site distance, highlighting the need to take into account any incompatibilities among the selected equations. Procedures are also presented for assigning weights to the branches in the ground-motion section of the logic tree in a transparent fashion, considering both intrinsic merits of the individual equations and their degree of applicability to the particular application.

THE EFFECTIVE DURATION OF EARTHQUAKE STRONG MOTION

The duration of strong ground shaking during earthquakes can play an important role in the response of foundation materials and structures, particularly when strength or stiffness degradation is encountered. A thorough seismic hazard assessment should therefore include an estimation of the expected duration of strong motion, which first requires criteria to define the part of an accelerogram considered to represent the duration of strong ground motion. Some 30 different definitions of strong motion duration are reviewed and classified into generic groups. Problems that arise with the use of these definitions for duration are highlighted. A new definition of duration is presented using a previously unexplored option which identifies the part of the record where the main energy is contained and constrains this strong shaking phase by absolute criteria. This new definition is shown to give consistently meaningful durations for strong earthquake accelerograms from an engineering viewpoint. The correlations be...

Relationships between Median Values and between Aleatory Variabilities for Different Definitions of the Horizontal Component of Motion

Ground-motion prediction equations (GMPE) for horizontal peaks of acceleration and velocity, and for horizontal response spectral ordinates, have employed a variety of definitions for the horizontal component of motion based on different treatments of the two horizontal traces from each accelerogram. New definitions have also recently been introduced and some of these will be used in future GMPEs. When equations using different horizontal-component definitions are combined in a logic-tree framework for seismic-hazard analysis, adjustments need to be made to both the median values of the predicted ground-motion parameter and to the associated aleatory variability to achieve compatibility among the equations. Because there is additional aleatory variability in the empirical ratios between the median values for different components, this uncertainty also needs to be propagated into the transformed logarithmic standard deviation of the adjusted equations. This study provides ratios of both medians and standard deviations for all existing component definitions with respect to the geometric mean of the two horizontal components, which is currently the most widely used in prediction equations. The standard deviations on the ratios of the medians are also reported. This article also discusses the issue of the ratios of different horizontal component definitions in relation to the specification of seismic input for dynamic structural analyses, highlighting the importance of consistency between the component definition used to derive the elastic design-response spectrum and the way that biaxial dynamic loading input is prepared.

Empirical Prediction Equations for Peak Ground Velocity Derived from Strong-Motion Records from Europe and the Middle East

Peak ground velocity (PGV) has many applications in earthquake engineering, but there are relatively few prediction equations for this parameter in comparison with the large numbers of equations for estimating peak ground accel- eration and response spectral ordinates. This lack of empirical equations for PGV has led to widespread use of the practice of scaling peak velocity from the 5%-damped response spectral ordinate at 1 sec, which is a poor substitute for direct prediction of the parameter. Responding to the need to provide equations for the prediction of PGV, this article derives new equations using the strong-motion database for the seismically active areas of Europe and the Middle East, following a new processing of all of the records. A total of 532 strong-motion accelerograms recorded at distances of up to 100 km from 131 earthquakes with moment magnitudes ranging from M 5 to 7.6 are used to derive equations for both the larger and the geometric mean of the horizontal components. The predictions are found to be broadly consistent with those from previous European equations, and also with preliminary results from the Next Generation of Attenuation (NGA) project, suggesting that systematic differences in ground motions from active crustal regions, if any, are sufficiently small not to prevent the combined use of strong-motion data from southern Europe, western North America, and other tectonically active areas of shallow crustal seismicity.

The Influence of Magnitude Range on Empirical Ground-Motion Prediction

A key issue in the assessment of seismic hazard in regions of low- to-moderate seismicity is the extent to which accelerograms obtained from small- magnitude earthquakes can be used as the basis for predicting ground motions due to the larger-magnitude events considered in seismic hazard analysis. In essence, the question is whether empirical ground-motion prediction equations can be applied outside their strict range of applicability as defined by the magnitude and distance ranges covered by the datasets from which they are derived. This question is explored by deriving new spectral prediction equations using an extended strong-motion da- taset from Europe and the Middle East covering the magnitude range Mw 3.0-7.6 and comparing the predictions with previous equations derived using data from only Mw 5.0 and above events. The comparisons show that despite their complex func- tional form, including quadratic magnitude-dependence and magnitude-dependent attenuation, the equations derived from larger-magnitude events should not be extra- polated to predict ground motions from earthquakes of small magnitude. Moreover, the results suggest not only that ground-motion prediction equations cannot be used outside the ranges of their underlying datasets but also that their applicability at the limits of these ranges may be questionable. Although only tested for smaller magni- tudes, the results could be interpreted to suggest that predictive equations also cannot be reliably extrapolated to higher magnitudes than those represented in the dataset from which they are derived, a finding that has important implications for seismic hazard analysis. The conclusion of the study is that empirical derivation of ground-motion pre- diction equations should be based on datasets extending at least one unit below the lower limit of magnitude considered in seismic hazard calculations. The inclusion of small-magnitude recordings results in a significant increase in the aleatory varia- bility of the equations, although it is yet to be established whether this is due to greater uncertainty in the associated metadata or whether ground-motion variability is gen- uinely dependent on earthquake magnitude.

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.